Treatment of ards r


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Treatment of ards r

  1. 1. Treatment of ARDS* Roy G. Brower, MD; Lorraine B. Ware, MD; Yves Berthiaume, MD; and Michael A. Matthay, MD, FCCP Improved understanding of the pathogenesis of acute lung injury (ALI)/ARDS has led to important advances in the treatment of ALI/ARDS, particularly in the area of ventilator- associated lung injury. Standard supportive care for ALI/ARDS should now include a protective ventilatory strategy with low tidal volume ventilation by the protocol developed by the National Institutes of Health ARDS Network. Further refinements of the protocol for mechanical ventilation will occur as current and future clinical trials are completed. In addition, novel modes of mechanical ventilation are being studied and may augment standard therapy in the future. Although results of anti-inflammatory strategies have been disappointing in clinical trials, further trials are underway to test the efficacy of late corticosteroids and other approaches to modulation of inflammation in ALI/ARDS. (CHEST 2001; 120:1347–1367) Key words: acute lung injury; mechanical ventilation; pulmonary edema; ventilator-associated lung injury Abbreviations: ALI acute lung injury; APRV airway pressure-release ventilation; ECco2R extracorporeal carbon dioxide removal; ECMO extracorporeal membrane oxygenation; Fio2 fraction of inspired oxygen; HFV high-frequency ventilation; I:E ratio of the duration of inspiration to the duration of expiration; IL interleukin; IMPRV intermittent mandatory pressure-release ventilation; IRV inverse-ratio ventilation; LFPPV low-frequency positive-pressure ventilation; NIH National Institutes of Health; NIPPV noninvasive positive-pressure ventilation; NO nitric oxide; PEEP positive end-expiratory pressure; PSB protected specimen brushing; TGI tracheal gas insufflation; TNF tumor necrosis factor T he syndrome of described in 1967. distress re- adults was first acute respiratory Until in 1 Standard Supportive Therapy Standard supportive therapy for ALI/ARDS is cently, most reported mortality rates exceeded 50%. directed toward identification and management of However, the mortality from acute lung injury (ALI) pulmonary and nonpulmonary organ dysfunction. and ARDS (ALI/ARDS) has decreased as laboratory and clinical studies have provided new evidence to Treatment of the Inciting Clinical Disorder in improve therapeutic strategies. This article reviews Patients With ARDS the results of these studies and summarizes current recommendations for standard supportive therapy. Identification and treatment of the inciting clinical New treatment strategies that are being evaluated in disorder is an important aspect of the initial manage- ongoing clinical trials are also reviewed. Information ment of a patient with ALI/ARDS. The most com- regarding clinical definitions, epidemiology, and mon disease processes associated with ALI include pathogenesis of ALI/ARDS is available in other sepsis, pneumonia, aspiration of gastric contents, reviews.2–7 trauma, multiple transfusions, and ischemia reperfu- sion (Table 1). In some circumstances, the underly- ing cause of ALI can be treated directly. For exam- ple, patients with pneumonia from bacterial or *From Johns Hopkins University (Dr. Brower), Baltimore, MD; opportunistic infections may respond to specific the University of Montreal (Dr. Berthiaume), Montreal, Canada; antimicrobial therapy. A careful search for a treat- and the Cardiovascular Research Institute (Drs. Ware and Mat- able cause of pulmonary infection, such as bacterial thay), The University of California at San Francisco, San Fran- cisco, CA. pneumonia, atypical pneumonia from Mycoplasma This article was supported by National Institutes of Health grants or Legionella, or an opportunistic infection from RO1-HL51856 (Drs. Matthay and Ware), NO1-HR46059 (Dr. fungi, tuberculosis, or Pneumocystis carinii is war- Matthay), NO1-HR46063 (Dr. Brower), and the Medical Re- search Council of Canada (Dr. Berthiaume). ranted. The diagnostic evaluation should be guided Manuscript received June 2, 2000; revision accepted January 30, by the clinical history. In other patients, an infectious 2001. cause of ALI may be related to an extrapulmonary Correspondence to: Michael A. Matthay, MD, FCCP, Moffitt Hospital, Room M-917, University of California, 505 Parnassus site of infection, such as the biliary tract, peritoneal Ave, San Francisco, CA 94143-0624; e-mail: cavity, or urinary tract. The diagnosis of intra-ab- CHEST / 120 / 4 / OCTOBER, 2001 1347
  2. 2. Table 1—Inciting Clinical Disorders Associated With pressure (PEEP). Ventilation can be supported with ALI and ARDS intermittent positive airway pressure. This section Pulmonary disorders addresses approaches to mechanical ventilation that Pneumonia are commonly used and accepted as standard sup- Bacterial portive care in patients with ALI/ARDS. Mechanical Fungal ventilation approaches that are not in common use or Parasitic have not yet been proven to be beneficial are Viral Aspiration of gastric contents reviewed in the subsequent section on “Potential Pulmonary contusion New Treatment Strategies.” Fat emboli Near-drowning Lung-Protective Ventilation With Small Tidal Vol- Inhalational injury umes: One of the clinical hallmarks of ALI/ARDS is Reperfusion pulmonary edema after lung transplantation Extrapulmonary disorders decreased respiratory system compliance.13 This is Sepsis caused by atelectasis and flooding of alveoli and by Trauma with multiple transfusions increased surface tension at air-fluid interfaces. Cardiopulmonary bypass Chest radiographs frequently suggest that the dis- Drug overdose ease is distributed homogeneously throughout the Acute pancreatitis Blood product transfusions lungs. However, CT images and physiologic studies demonstrate that the lung is affected in a patchy, heterogeneous manner.14,15 The lungs of ALI/ARDS patients can be modeled as consisting of three dominal sepsis should be considered early in patients different compartments: (1) regions of severe inflam- with sepsis syndrome and ALI of uncertain etiology. mation, alveolar filling, and atelectasis in which little Prompt surgical intervention to eradicate an intra- lung volume can be recruited with airway pressures abdominal source of sepsis is associated with better that are traditionally considered safe; (2) regions outcomes.8 Factors associated with positive findings with normal compliance and aeration, appearing to at exploratory laparotomy include objective findings be uninvolved with disease; and (3) intermediate on physical examination and ultrasound, or CT find- regions in which alveolar collapse and flooding are ings suggestive of an intra-abdominal focus of infec- apparent but where aeration can be restored by tion.9 Although the prognosis for recovery from raising airway pressures within a safe range. sepsis-induced lung injury is worse than from any When traditional tidal volumes of 10 to 15 mL/kg other cause,10,11 specific medical and surgical treat- are used in patients with ALI/ARDS receiving me- ment of the pulmonary or extrapulmonary source of chanical ventilation, the resulting airway pressures sepsis is indicated to enhance the chance of survival. are frequently elevated, reflecting overdistention of In many ALI/ARDS patients, the insult that caused the less-affected lung regions. In many laboratory lung injury, such as aspiration or multiple transfu- experiments,16 –21 ventilation with high airway pres- sions, cannot be treated except to prevent recur- sures caused increased pulmonary vascular perme- rence and provide optimal supportive care as out- ability, acute inflammation, alveolar hemorrhage, lined below. intrapulmonary shunt, and diffuse radiographic infil- trates. Most of these studies were conducted in normal animals with very large tidal volumes, and Mechanical Ventilation thus were not directly applicable to the experience in In many ALI/ARDS patients, intrapulmonary patients with ALI/ARDS. However, rats with exper- shunt and ventilation-perfusion imbalances cause imental ALI had significantly less edema when re- life-threatening hypoxemia. Moreover, high work of ceiving ventilation with smaller tidal volumes.22 breathing from increased alveolar dead space and Moreover, although the tidal volumes that caused reduced respiratory system compliance may cause injury in the animal models were approximately ventilatory failure with hypercapnia and respiratory three to four times greater than those used by most acidosis. The mainstay of supportive care of ALI/ clinicians, most of the tidal volume in ALI/ARDS ARDS is mechanical ventilation.12 By stabilizing patients is directed to a relatively small amount of respiration, mechanical ventilation allows time for aerated lung. These studies suggest that in some administration of treatment for the underlying cause patients with ALI/ARDS, traditional approaches to of ALI/ARDS (eg, infection) and for the evolution of mechanical ventilation exacerbate or perpetuate lung natural healing processes. Arterial oxygenation can injury by causing excessive stretch of aerated lung be supported by raising the fraction of inspired regions during inspiration. oxygen (Fio2) and applying positive end-expiratory Ventilation with small tidal volumes and limited 1348 Reviews
  3. 3. airway pressures can reduce ventilator-associated ment of acidosis was also different in the ARDS lung injury from overdistention. However, small tidal Network trial,30 which required high respiratory volume ventilation may cause complications from rates and allowed sodium bicarbonate infusion to acute respiratory acidosis.23–26 Thus, to achieve the compensate for respiratory acidosis. beneficial effect of this approach requires some The ARDS Network trial30 excluded patients with compromise of traditional objectives with respect to elevated intracranial pressure and with sickle hemoglo- gas exchange and acid-base balance. Clinical evi- bin because hypercapnia and acidosis could have ad- dence supporting this strategy came initially from verse effects in these conditions. However, the lower two observational studies24,25 in which mortality rates tidal volume approach is recommended for most other of ARDS patients treated with small tidal volumes patients with ALI/ARDS. The complete methodology and permissive hypercapnia were compared to mor- for the trial procedures is available at tality rates predicted from historical control subjects. and from the National Auxiliary Publications Service These studies were not conclusive because they (c/o Microfiche Publications, 248 Hempstead Turn- lacked concurrent control groups treated with a pike, West Hempstead, NY 11552; document 05542). traditional ventilation approach. The lower tidal volume strategy is summarized in Table Three small prospective, randomized trials27–29 of 2. Except for the lower tidal volumes with permissive traditional vs lower tidal volume ventilation in pa- hypercapnia, this approach is consistent with previously tients with or at risk for ALI/ARDS did not demon- accepted standard supportive treatment for ALI/ strate beneficial effects of the lower tidal volume ARDS. With the substantial improvements in impor- approach. However, mortality was reduced substan- tant clinical outcomes demonstrated in the ARDS tially from 40% (traditional strategy) to 31% (lower Network trial,30 the lower tidal volume strategy may tidal volume strategy) in a larger trial by the National now be considered standard supportive treatment for Institutes of Health (NIH) ARDS Network30 (Fig 1). patients with ALI/ARDS, until another mechanical There were also more ventilator-free and organ ventilation strategy is demonstrated to be superior. failure-free days in patients who received the lower tidal volume strategy. In the lower tidal volume Support of Arterial Oxygenation (PEEP vs FIO2): group, the target tidal volume was 6 mL/kg of predicted body weight. This was reduced further to Most ALI/ARDS patients require support for arterial 5 mL/kg or 4 mL/kg if necessary to maintain the oxygenation with a combination of increased Fio2 end-inspiratory plateau pressure (0.5-s pause) 30 and PEEP. Both of these treatments have potential cm H2O. An important difference between the adverse effects that must be carefully considered in ARDS Network trial and the previous studies is that individual patients. In laboratory animals, high levels the tidal volumes in the lower tidal volume strategy of inspired oxygen cause physiologic and pathologic of the ARDS Network trial were smaller. Manage- changes that are similar to other forms of ALI.31–35 Table 2—NIH ARDS Network Lower Tidal Volume Ventilation for ALI/ARDS Protocol Summary* Variables Protocol Ventilator mode Volume assist-control Tidal volume 6 mL/kg predicted body weight† Plateau pressure 30 cm H2O Ventilation set rate/ 6–35/min, adjusted to achieve arterial pH goal pH 7.30 if possible Inspiratory flow, I:E Adjust flow to achieve I:E of 1:1–1:3 Oxygenation goal 55 Pao2 mm Hg or 88 Spo2 95% Fio2/PEEP 0.3/5, 0.4/5, 0.4/8, 0.5/8, 0.5/10, 0.6/10, (mm Hg) 0.7/10, 0.7/12, 0.7/14, 0.8/14, 0.9/14, combinations‡ 0.9/16, 0.9/18, 1.0/18, 1.0/22, 1.0/24 Weaning Attempts to wean by pressure support required when Fio2/PEEP .40/8 *Spo2 oxyhemoglobin saturation by pulse oximetry. Figure 1. Proportions of patients surviving and achieving dis- †Predicted body weight for male subjects 50 (2.3 [height in charge home in traditional (12 mL/kg) and lower tidal volume (6 mL/kg) study groups. Mortality before discharge home with inches 60]) or 50 (0.91 [height in centimeters 152.4]); unassisted breathing was significantly lower in the lower tidal predicted body weight for female subjects 4.5 (2.3 [height in volume study group (39.8% vs 31.0%). Reprinted with permission inches 60]) or 4.5 (0.91 [height in centimeters 152.4]). of the ARDS Network.30 ‡Further increases in PEEP to 34 cm H2O allowed but not required. CHEST / 120 / 4 / OCTOBER, 2001 1349
  4. 4. In humans, no detectable oxygen toxicity occurred in volume-cycled modes in relation to risks of baro- normal subjects when the Fio2 was 50%,36 but trauma or stretch-induced lung injury. Some have impaired gas exchange was apparent after breathing suggested that the rapid inspiratory airflow that 100% oxygen at sea level for approximately 40 h.37 occurs with pressure-controlled modes is more fa- Diseased lungs may be more susceptible to injury vorable for gas exchange. However, there were no from moderate hyperoxia.38 However, initial expo- differences in Pao2 or Paco2 when ALI/ARDS pa- sures to a moderate Fio2 or to endotoxin may confer tients received ventilation with volume-cycled vs some protection from hyperoxic lung injury.39 – 42 pressure-controlled modes at constant tidal volume, Also, plasma proteins that leak into the pulmonary end-expiratory alveolar pressure, and ratio of the airspaces may have antioxidant properties.43 Al- duration of inspiration to the duration of expiration though the relationship of Fio2 to oxygen-induced (I:E).61,62 Some patients may be more comfortable lung injury has not been clearly defined in ALI/ receiving pressure-support ventilation, especially ARDS patients, an Fio2 0.6 is usually considered when there are substantial respiratory efforts. How- to be safe.44 ever, volume-cycled modes provide greater control PEEP reduces intrapulmonary shunt and im- over tidal volume, which is an important determinant proves arterial oxygenation,1,12 thus allowing ade- of ventilator-associated lung injury.56 quate arterial oxygenation at a lower Fio2, which Sufficient gas exchange can usually be achieved may reduce pulmonary oxygen toxicity. However, with conventional mechanical ventilation. However, adverse effects of PEEP include decreased cardiac this may not be possible in some ALI/ARDS patients output,45–51 increased pulmonary edema forma- without causing ventilator-associated lung injury or tion,52–54 increased dead space, increased resistance oxygen toxicity. Numerous additional treatments to of the bronchial circulation,55 and increased lung improve gas exchange or reduce ventilation or hy- volume and stretch during inspiration, which may peroxia-associated lung injury are currently under cause further lung injury or barotrauma.19,20,56 These investigation. Some new treatments utilize novel adverse effects of PEEP may be more pronounced in methods of mechanical ventilation. Others utilize patients with direct lung injury (pneumonia and pharmacologic mechanisms to improve gas exchange aspiration pneumonitis), in whom PEEP is not as and lung mechanics. These approaches are discussed effective at recruiting airspaces. Thus, beneficial in the subsequent section on “Potential New Treat- effects of PEEP on arterial oxygenation must be ments Strategies.” weighed carefully in relation to potential adverse effects. Some investigators have suggested using Hemodynamic Management: Fluids, Vasopressors, higher PEEP to minimize Fio2,57 or to protect the and Oxygen Delivery lung from injurious mechanical forces that occur from ventilation with atelectasis at end-expiration.58 Optimal fluid management in patients with ALI/ The best strategy for using PEEP and Fio2 in ARDS is a controversial issue. Substantial data from individual patients has not yet been defined. The animal experimentation suggest that fluid restriction levels of PEEP and Fio2 shown in Table 2 represent may reduce pulmonary edema in patients with a consensus among investigators and clinicians work- increased pulmonary vascular permeability, as in ing in the NIH ARDS Network centers since 1995 ALI/ARDS. However, other experimental data63,64 and were used in the recent clinical trial that was suggest that ALI/ARDS patients may benefit from a associated with a 22% reduction in mortality in hemodynamic management strategy that increases ALI/ARDS patients. This approach is recommended oxygen delivery, which may require increased vascu- for most patients as standard therapy pending evi- lar volume. dence for a better approach. Edema formation occurs at lower pulmonary cap- illary pressures when pulmonary vascular permeabil- Volume-Cycled vs Pressure-Controlled Ventila- ity is increased.63– 67 The experimental data that tion: Volume-cycled modes (volume-assist/control support fluid restriction in patients with ALI are and intermittent mandatory ventilation) are used supported by some observational clinical studies. most frequently in ALI/ARDS patients,59,60 but pres- Treatment of ARDS patients with diuretics or dial- sure-cycled modes can provide similar levels of ysis has been shown68 to improve oxygenation and ventilatory support. Inspiratory increments in trans- respiratory system lung compliance. One study69 mural alveolar pressure and volume vary directly reported that survival in ARDS patients was related with each other according to the pressure-volume to negative fluid balance, while another study70 characteristics of the lung, regardless of ventilator reported that survival was greater in patients in mode. Hence, for a given tidal volume, there is no whom there was a 25% reduction in pulmonary advantage or disadvantage of pressure-controlled vs arterial wedge pressure. In a third study,71 patients 1350 Reviews
  5. 5. who gained 1 L of fluid after 36 h of being be considered when hemoglobin concentration is recruited into a study of ALI had a better survival 10 g/dL. However, a higher threshold may be rate (74%) than the others (50%). However, these better in patients without cardiovascular disease.92 observations do not prove that fluid restriction is Reduction in oxygen demand should be achieved efficacious. Fluid accumulation may have been a first with sedation and analgesia. Neuromuscular marker of the severity of systemic and pulmonary blocking agents are occasionally useful when seda- capillary permeability. tion and analgesia are ineffective at reducing exces- This issue was addressed in a prospective, random- sive muscular activity. However, use of neuromuscu- ized trial in which diuresis, fluid restriction, and lar blocking agents in critically ill patients may hemodynamic management were directed either by contribute to neuromuscular complications such as measuring the extravascular lung water using a dou- myopathy and neuropathy. Judicious and sparing use ble-indication technique71,72 or with standard clinical of these drugs is recommended.93 Hyperpyrexia information, which included pulmonary arterial cath- should also be treated, but excessive active cooling eter data.72 After 24 h of treatment, lung water was may increase oxygen demands if it causes shivering. significantly lower in the extravascular lung water Mechanical ventilation of patients in shock can re- management group.72 These patients also required a duce oxygen requirements from the high work of shorter duration of mechanical ventilation and a breathing. The consensus committee91 on tissue shorter stay in the ICU, but survival was not signif- hypoxia concluded that “. . . aggressive attempts to icantly different between the groups. Furthermore, increase oxygen delivery to supranormal values in all the study population included patients with hydro- critically ill patients are unwarranted.” static pulmonary edema, who would be expected to Vasopressors are needed to support systemic BP benefit from aggressive fluid restriction. or to increase cardiac output in patients with shock. Fluid restriction may reduce cardiac output and There is no clear evidence that any vasopressor or tissue perfusion, which could cause or worsen non- combination of vasopressors is superior. In general, a pulmonary organ dysfunction. In many ALI/ARDS prudent approach in ALI/ARDS patients is to restore patients, dysfunction of multiple organs and sys- intravascular volume to euvolemic levels (central tems occurs from a systemic inflammatory re- venous pressure of approximately 4 to 12 mm Hg or sponse.10,11,73–76 A related explanation for multiple pulmonary capillary wedge pressure of approxi- organ dysfunction is that tissue oxygen delivery is mately 6 to 14 mm Hg) and then to use a vasopressor inadequate in some systemic inflammatory condi- such as dopamine to achieve a mean arterial pressure tions such as sepsis or severe trauma, even when of 55 to 65 mm Hg (perhaps higher in patients with cardiac output and oxygen delivery are normal.77,78 chronic systemic hypertension). However, both fluid Some investigators78,79 have suggested that organ and vasopressor therapy must be guided by clinical function and clinical outcomes in ALI/ARDS pa- indexes of organ perfusion. Urine output, blood pH, tients would improve if supranormal levels of oxygen and base deficit are helpful to assess the adequacy of delivery were achieved with vigorous volume reple- organ perfusion. In some patients, a pulmonary tion, transfusions of packed RBCs, or inotropic arterial catheter may provide useful additional infor- medications. Several clinical trials addressed this mation (cardiac output and pulmonary arterial question, but the results were disparate. In postop- wedge pressure), especially when there is left ven- erative or posttrauma patients, there were trends tricular dysfunction or pulmonary hypertension, toward decreased mortality with supranormal oxygen which are common in patients with ALI/ARDS.63 delivery.79 – 87 However, there were no beneficial Dobutamine may be useful as a positive inotropic effects of this strategy in ALI/ARDS patients.88,89 agent and, in some patients, to reduce systemic Furthermore, one randomized trial90 reported in- vascular resistance. More details regarding use of creased mortality in patients who received a su- vasopressors in ALI/ARDS patients are available in pranormal oxygen delivery strategy. several sources.63,94,95 New information on the issue A recent international consensus conference91 on of fluid management and the value of a central tissue hypoxia provided guidelines for management venous vs a pulmonary arterial catheter will be of oxygen delivery and for reduction of oxygen forthcoming from a large prospective NIH ARDS demand in critically ill patients. The consensus com- Network trial that is currently underway. mittee concluded that “. . . timely resuscitation and achievement of normal hemodynamics is essential.” Vasodilators To promote oxygen delivery, initial management should ensure adequate vascular volume. There was Most ALI/ARDS patients have mild-to-moderate no clear evidence favoring colloid vs crystalloid pulmonary arterial hypertension. A progressive rise solutions for this purpose. Blood transfusion should in pulmonary vascular resistance has been observed CHEST / 120 / 4 / OCTOBER, 2001 1351
  6. 6. in patients who die from ALI.46 The cause of pul- Management of Infection in the ALI/ARDS Patient monary arterial hypertension is multifactorial, and may include hypoxic vasoconstriction, destruction Patients with ALI/ARDS frequently die from un- and/or obstruction of the pulmonary vascular bed, controlled infection. The infection may have been and high levels of PEEP.63 In some patients, pulmo- the initial cause of ALI/ARDS, as in nonpulmonary nary arterial hypertension can lead to cardiac dys- sepsis (see section on “Treatment of the Inciting function from right ventricular overload.63 In several Clinical Disorder”). There is also a high risk of studies, investigators have attempted to improve developing nosocomial infections, such as pneumo- ALI/ARDS management by lowering pulmonary ar- nia and catheter-related sepsis. Since uncontrolled terial pressure with pulmonary vasodilators. For infection of any cause is associated with the devel- example, hydralazine appears to be more efficacious opment of multiple organ dysfunction, a major ob- in increasing cardiac output than nitroprusside with- jective of standard supportive care in patients with out increasing the shunt fraction,96 probably because ALI/ARDS is to identify, treat, and prevent infec- it does not influence hypoxic vasoconstriction.97 tions. The remainder of this section will give an However, hydralazine has not been evaluated in overview of the incidence, diagnosis, treatment, and randomized, controlled trials. Preliminary studies98 prevention of nosocomial pneumonia in patients suggested that a continuous infusion of prostaglandin with ALI/ARDS. The diagnosis and treatment of E1 could improve survival in addition to cardiac other infections such as catheter-related sepsis are output and oxygen delivery, but a randomized, dou- not substantially different in ALI/ARDS than in ble-blind, multicenter study99 did not confirm these other critically ill patients. results. IV prostacyclin was also promising, but its Almost all aspects of the management of nosoco- vasodilator effect caused adverse effects in systemic mial pneumonia in ALI/ARDS are controversial, hemodynamics.100 including the incidence. Several prospective studies Nitric oxide (NO) is a powerful endogenous vaso- have attempted to quantify the incidence prospec- dilator.101,102 Because it is rapidly inactivated, its tively, with varied results. In a study109 of scheduled vasodilatory effects are restricted to the blood vessels BAL and protected specimen brushing (PSB) in 105 at the site of generation or administration. NO patients with ALI/ARDS in Seattle, WA, the inci- inhalation dilates pulmonary vessels perfusing aer- dence of nosocomial pneumonia diagnosed by quan- ated lung units, diverting blood flow from poorly titative BAL or PSB cultures was only 15%. How- ventilated or shunt regions. Because of these phar- ever, antibiotic use may have inhibited bacterial macologic and physiologic effects, gaseous NO is growth in culture in this study, leading to underdi- potentially an ideal agent to treat pulmonary hyper- agnosis of pneumonia. Two prospective French stud- tension and severe hypoxemia in ALI/ARDS pa- ies of ALI/ARDS patients with suspected ventilator- tients. Encouraging results in some animal mod- associated pneumonia used either BAL110 or BAL els103–105 led to the evaluation of the therapeutic and plugged telescoping catheter sampling111 for potential of NO in ALI/ARDS patents. In 9 of 10 quantitative cultures and reported a much higher consecutive ALI/ARDS patients, inhaled NO at a incidence, 55 to 60%. Sampling of distal airway concentration of 18 ppm reduced mean pulmonary secretions was done prior to any changes in antibiotic artery pressure from a mean of 37 to 30 mm Hg. This therapy in both studies, probably accounting for the was associated with a decrease in intrapulmonary much higher yield from quantitative cultures. Most shunt from 36 to 31% and an increase in Pao2/Fio2 pneumonias occurred late in the course of ALI/ of 47.106 Important clinical outcomes were not as- ARDS, after the first 7 days. Interestingly, in all sessed in this study. In a randomized, double-blind three studies, the presence or absence of ventilator- study of different doses of inhaled NO (1.25 to 80 associated pneumonia had little or no effect on ppm) in ALI/ARDS patients, improvements in oxy- mortality. genation were modest and not sustained after the The diagnosis of nosocomial pneumonia in pa- first day of treatment.107 Interestingly, the results of tients with ALI/ARDS is particularly difficult. The a recent unpublished, prospective, double-blinded, usual clinical criteria for pneumonia such as a new randomized French phase III study of inhaled NO radiographic infiltrate, fever, and leukocytosis are for ARDS in 208 patients also demonstrated no commonly present in ALI/ARDS patients, even effect on mortality or the duration of mechanical when infection is absent.112 However, many ALI/ ventilation.108 The results of these recent trials sug- ARDS patients have evidence of pneumonia at au- gest that NO will not become part of standard topsy that was not recognized before death.113–115 therapy for ALI/ARDS. There may a role for NO in Culture of endotracheal aspirates may be misleading, some ALI/ARDS patients with severe refractory since most patients receiving prolonged ventilatory hypoxemia and pulmonary arterial hypertension. support develop colonization of the upper airway and 1352 Reviews
  7. 7. trachea. Several attempts have been made to assess nutritional support. It is worth noting that the ben- the value of bronchoscopy with PSB or lavage to efits of nutritional support in critically ill patients sample distal airway secretions in patients with sus- have not been conclusively demonstrated by com- pected lung infections. The results have been vari- parison to a control group which did not receive able and controversial. Only one study116 has at- nutritional support. The lack of controlled clinical tempted to study the effect of different diagnostic trials in this area has led at least one expert119 to techniques on morbidity and mortality. In this tri- recommend that nutritional supplementation be al,116 413 patients receiving mechanical ventilation withheld from critically ill patients. Nevertheless, the with suspected ventilator-associated pneumonia authors believe that the available evidence supports were randomized to antibiotic management strate- the administration of nutritional support in ALI/ gies using endotracheal aspirates or bronchoscopy ARDS patients. with protected specimens. Mortality at 14 days was The route of administration of nutrition in ALI/ significantly lower in the bronchoscopy group. How- ARDS will be influenced by the individual patient’s ever, only a minority of patients in this study116 had condition and ability to tolerate enteral feeding. ALI/ARDS, and management of the noninvasive arm Parenteral nutrition has been used frequently in of the study may have been suboptimal. ALI/ARDS patients, but experimental and clinical Regardless of whether bronchoscopic or more trials suggest that enteral nutrition may be superi- conservative techniques are used for diagnosis, the or.118 In animal models, lack of enteral nutrition prompt initiation of appropriate empiric therapy promoted bacterial translocation from the gut.120 while awaiting the results of cultures is critically Normal human volunteers who received parenteral important. Empiric therapy should be guided by nutrition had higher levels of systemic and hepatic local patterns of microbial incidence and resistance. vein tumor necrosis factor (TNF), arterial glucagon It is also important to remember that administration and epinephrine, and increased febrile responses to of adequate antibiotics does not always improve endotoxin compared to subjects who received en- outcome.114 It is beyond the scope of this review to teral nutrition.121 Enteral nutrition is also associated present an in-depth discussion of antibiotic treat- with a lower incidence of infectious complications ment for ventilator-associated pneumonia. The than parenteral nutrition,122 and is less costly. Thus, reader is referred to the recent consensus statement there is enough evidence to support the use of from the American Thoracic Society for detailed enteral feeding over parenteral nutrition when pos- recommendations.117 sible. However, since enteral nutrition is sometimes Given the high incidence of nosocomial pneumo- not tolerated in critically ill patients,123 parenteral nia in patients with ALI/ARDS receiving ventilation, nutrition will frequently be needed. It is reassuring strategies for the prevention of nosocomial pneumo- to note that in a meta-analysis124 of studies compar- nia are an important area of investigation.117 Hand ing total parenteral nutrition to enteral nutrition washing by medical personnel is of proven value but after major surgery or critical illness, there was no is often overlooked. Other areas that are currently difference in mortality between the two groups. In being studied in clinical trials include the continuous addition, when Cerra et al125 examined the impact of suctioning of subglottic secretions to prevent their parenteral vs enteral nutrition in 66 patients with aspiration, and the development of new endotracheal sepsis at high risk for organ failure, they found no tubes that resist the formation of a bacterial biofilm difference in the incidence of organ failure or mor- that can be embolized distally with suctioning. tality in the two groups. The composition of nutritional supplementation in patients with ALI/ARDS is an area of ongoing Nutrition research. One study126 has reported that a high-fat, The provision of adequate nutrition via the enteral low-carbohydrate diet can reduce the duration of or parenteral routes has become the standard of care ventilation in patients receiving mechanical ventila- for critically ill patients, including those with ALI/ tion, presumably by reducing the respiratory quo- ARDS, and is recommended by the authors. Guide- tient and the level of carbon dioxide production. lines for nutrition in ICU patients have recently been However, the most common cause of a high respi- summarized by a consensus group of the American ratory quotient in critically ill patients is simple College of Chest Physicians.118 The goals of nutri- overfeeding.118 Another approach has been to sup- tional support include the provision of adequate plement feeding with immunomodulatory nutrients nutrients for the patient’s level of metabolism, and including amino acids such as arginine and glu- the prevention and treatment of deficiencies of tamine, ribonucleotides, and omega-3 fatty acids. A macronutrients and micronutrients while attempting meta-analysis127 of immunomodulatory nutritional to minimize complications related to the mode of supplementation in patients with critical illness CHEST / 120 / 4 / OCTOBER, 2001 1353
  8. 8. showed a decrease in infectious complications and tilation with higher PEEP levels compared to those duration of hospital stay, but no difference in mortality. who received traditional PEEP levels. However, in In the only study128 to date (and to our knowledge) of this study,58 higher PEEP was used in conjunction patients with ALI/ARDS, a diet high in fish oil, -lin- with lower tidal volumes and other measures to olenic acid, and antioxidants shortened the duration of reduce ventilator-associated lung injury. Because of mechanical ventilation and reduced new organ failures the many potential adverse effects of PEEP, it is but had no effect on mortality. Until larger multicenter important to confirm that mechanical ventilation trials of immunomodulatory nutritional supplementa- with higher PEEP levels, independent of other tion in patients with ALI/ARDS are available, standard lung-protective strategies, will improve important nutritional formulations are recommended with avoid- clinical outcomes in ALI/ARDS patients. The NIH ance of overfeeding. ARDS Network is currently conducting a trial to test the value of higher levels of PEEP. Potential New Treatment Strategies for Noninvasive Positive-Pressure Ventilation: Endo- ALI/ARDS tracheal intubation is required for most applications of positive-pressure ventilation. Complications of Several promising new approaches for improving endotracheal intubation include upper-airway inju- pulmonary gas exchange are currently being assessed ries, tracheomalacia, tracheal stenosis, sinusitis, and in clinical trials and could contribute further to ventilator-associated pneumonia. Noninvasive posi- improved outcomes in patients with ALI/ARDS. It is tive-pressure ventilation (NIPPV) uses a tight-fitting important to realize, however, that mortality in pa- face mask as an alternative interface between the tients with ALI/ARDS is closely related to factors patient and ventilator to avoid these complica- such as hepatic failure and severe infec- tions.136 NIPPV has additional advantages of allow- tions.10,11,73,129 Reduction of mortality in these pa- ing some verbal communication by patients, and tients may require improved management of the some patients can eat during short respites from the conditions that cause or contribute to the dysfunc- face mask. Studies137–139 in ALI/ARDS patients tion of nonpulmonary organ systems. demonstrated fewer cases of nosocomial pneumonia and shorter requirements for ventilator assistance in New Approaches to Mechanical Ventilation patients who received NIPPV as compared to those who received ventilation via endotracheal tubes. Lung-Protective Ventilation With Higher PEEP: However, NIPPV is not feasible in delirious or PEEP is traditionally used to achieve adequate obtunded patients.140 Moreover, air leaks from the arterial oxygenation without resorting to potentially face mask may prevent adequate ventilatory assis- toxic oxygen concentrations.59 However, there may tance in patients who require high inspiratory airway also be lung-protective effects of PEEP. Several pressures. Additional time commitments by nurses animal studies19,130,131 suggest that PEEP may pre- or respiratory therapists may be needed during the vent lung injury from repeated opening and closing initial period of support with NIPPV.141 of small bronchioles and alveoli, or from excessive stress at margins between atelectatic and aerated High-Frequency Ventilation: High-frequency ven- lung units. This mechanism of ventilator-associated tilation (HFV) utilizes very small tidal volumes with lung injury may be more likely in patients with very high respiratory rates.142,143 It is an attractive indirect causes of ALI/ARDS, as in sepsis and approach to mechanical ventilation in patients with trauma, in which elevations in airway pressure typi- ALI/ARDS because it achieves the two main lung- cally cause substantial airspace recruitment.132,133 protective objectives (avoiding both overdistention Some investigators58 have suggested that PEEP and ventilation with atelectasis at end-expiration) should be customized in individual patients after while maintaining normal Paco2 as well as arterial assessments of the pressure-volume characteristics oxygenation.144 A trial145 of HFV in premature in- of the respiratory system or lungs. Studies with fants with respiratory distress did not demonstrate a experimental ALI134 and humans with ALI/ARDS135 significant effect on morbidity or mortality. How- demonstrated reductions in inflammatory cytokines ever, the ventilation procedures in this study145 did in the alveolar lavage fluid and plasma when higher not use high mean airway pressures to achieve high PEEP was used. This protective effect may require levels of alveolar recruitment, as is currently recom- PEEP levels that are substantially higher than those mended.146 More recent studies147–149 of HFV in typically used to support arterial oxygenation. In a patients with neonatal respiratory distress demon- prospective, randomized trial,58 clinical outcomes strated reduced chronic lung disease in survivors and improved in patients who received mechanical ven- other encouraging trends toward improved outcomes. 1354 Reviews
  9. 9. The results of a large randomized, controlled trial150 of mode is most favorable for breathing comfort and for HFV in adults with acute respiratory failure were reducing unnecessary work of breathing. It may be disappointing, but this study included a heterogeneous the best mode to use with NIPPV.159 group of patients. Moreover, the HFV procedures in this trial150 were not designed to avoid ventilation with Inverse-Ratio Ventilation and Airway Pressure- atelectasis at end-expiration. Uncontrolled studies151,152 Release Ventilation: Some investigators160,161 have reported that gas exchange could be maintained at suggested that atelectatic alveoli may be recruited acceptable levels with HFV in patients with severe and stabilized by extending the duration of inspira- ARDS. Randomized trials will be necessary to deter- tion and shortening the duration of expiration. If so, mine if important clinical outcomes improve with HFV then shunt could be reduced and arterial oxygen- when compared to conventional ventilator-based lung- ation improved without increasing PEEP, inspiratory protective strategies. airway pressures, tidal volume, or lung stretch. Inverse-ratio ventilation (IRV) is associated with Tracheal Gas Insufflation: Physiologic dead space shunt reduction and improved arterial oxygenation in is elevated in patients with ALI/ARDS, and small patients with ALI/ARDS.161–163 However, the short tidal volume ventilation frequently causes hypercap- exhalation times of IRV probably cause some auto- nia and acute acidosis. Tracheal gas insufflation PEEP.164,165 Thus, improved gas exchange in previ- (TGI) is an adjunct to mechanical ventilation that ous studies with IRV may have occurred because of reduces dead space.153–157 It is therefore attractive an increase in end-expiratory alveolar pressure. In for use with small tidal volume ventilation in ALI/ three studies61,62,166 in ARDS patients, effects of IRV ARDS patients to attenuate the resulting hypercap- on shunt and oxygenation were compared with ef- nia and acidosis. fects of PEEP without IRV. When end-expiratory Without TGI, the bronchi and trachea are filled alveolar pressures or thoracic volumes were matched with alveolar gas at the end of exhalation. This during IRV and conventional ventilation, arterial carbon dioxide-laden gas is forced back into the oxygenation and shunt were similar. These studies alveoli during the next inspiration. With TGI, a suggest that the mechanism by which IRV improves stream of fresh gas (approximately 4 to 8 L/min) is oxygenation is the same as with externally applied insufflated through a small catheter or through small PEEP: that shunt reduction does not occur with IRV channels in the wall of the endotracheal tube into the unless there is increased end-expiratory alveolar lower trachea, flushing the carbon dioxide-laden gas pressure.167 Because IRV is very uncomfortable, out prior to the next inspiration. TGI may cause most patients will require heavy sedation, and many desiccation of secretions and airway mucosal injury, will require neuromuscular blockade. There is grow- and the TGI catheter may become a nidus for ing awareness of complications from sedation and accumulation of secretions. TGI may also cause paralysis in critically ill patients.93,168 auto-PEEP from the expiratory flow and resistance Airway pressure-release ventilation (APRV) is sim- of the ventilator-exhalation tubes and valve. The ilar to IRV, but patients can breathe spontaneously development of special equipment and explicit during the prolonged periods of elevated airway guidelines may allow clinicians to use TGI in the pressure.169 –171 Thus, APRV may be considered a near future to manage patients with severe hyper- hybrid of pressure-controlled IRV and intermittent capnia and acidosis. mandatory ventilation. A related mode, intermittent mandatory pressure-release ventilation (IMPRV), Proportional-Assist Ventilation: Like other modes provides an inspiratory pressure support to some or of positive-pressure ventilation, proportional-assist all of the spontaneous efforts that occur independent ventilation elevates airway pressure during inspira- of the IRV-like cycle of the ventilator.172 This can tion. Unlike other modes, the inspiratory airway further reduce work of breathing and oxygen cost of pressure assistance varies directly with patient ef- breathing and enhance alveolar ventilation while fort.158 This allows breath-to-breath variations in retaining some potential lung-protective effects of inspiratory airflow and tidal volume, as with pres- IRV. Arterial oxygenation may improve with APRV sure-support ventilation, but the magnitude of the and IMPRV, but as with IRV, air trapping may occur pressure assistance increases with patient effort. from the very short periods of exhalation. If im- Moreover, the inspiratory assistance can be custom- proved oxygenation requires air trapping, then it is ized to the elastance and resistance properties of not clear that lung protection can be achieved with each patient’s respiratory system. Proportional-assist these modes. To our knowledge, there are no con- ventilation can also be adjusted to provide more or trolled studies demonstrating improvements in key less positive-pressure assistance, depending on a clinical outcomes in patients who received IRV, patient’s ability to sustain some ventilation. This APRV, or IMPRV. CHEST / 120 / 4 / OCTOBER, 2001 1355
  10. 10. Surfactant Replacement Therapy ence refractory hypoxemia, leading some investiga- tors to suggest that extracorporeal membrane oxy- Surfactant, which is normally produced by type II genation (ECMO) would be useful in these pneumocytes, decreases surface tension at the air- patients.179 A prospective, multicenter, randomized fluid interface of small airways and alveoli. Without trial180 was conducted to compare ECMO to con- the beneficial effect of surfactant, alveoli may col- ventional ventilation alone; mortality in both groups lapse and resist opening, even with high airway of patients was approximately 90%. pressures. In respiratory distress syndrome of pre- Since the initial experience with ECMO, extracor- mature infants, surfactant production by the imma- poreal gas exchange technology has been improved ture lung is deficient and surfactant replacement to decrease complications and improve outcomes. In therapy is beneficial.173 In ALI/ARDS, injured type the early ECMO trial, oxygenation was the primary II pneumocytes produce less surfactant, and plasma objective. To achieve effective arterial oxygenation, proteins that leak into the alveolar airspaces inacti- blood flow through the extracorporeal device had to vate existing surfactant. Moreover, a change in the be 50% of cardiac output. Extracorporeal carbon lipid composition of surfactant contributes to poor dioxide removal (ECco2R) has now been developed surfactant function.174 The resulting increase in sur- with the primary objective of reducing the high face tension leads to atelectasis and decreased lung respiratory rates and tidal volumes required to compliance174 and may also increase edema forma- achieve normal Paco2, thereby decreasing ventila- tion.175 Several experimental studies in ALI models tor-associated lung injury. This goal can be achieved demonstrated improved pulmonary function, includ- with lower extracorporeal blood flow rates, but ing lung compliance and oxygenation, when exoge- achieves only 20 to 30% of total oxygen require- nous surfactant was administered.174 ments.181 In ECco2R, most oxygenation is still Initial clinical studies176 of exogenous surfactant achieved through the lungs, but this requires much therapy in patients with ARDS were encouraging. less mechanical ventilation support than mechanical However, in a multicenter, randomized, placebo- ventilation without ECco2R. controlled trial177 in 725 patients with sepsis-induced In 1986, Gattinoni et al182 reported mortality of ARDS, an artificial protein-free surfactant given by 50% in 47 patients treated with low-frequency pos- aerosol did not affect arterial oxygenation, duration itive-pressure ventilation (LFPPV) and ECco2R. of mechanical ventilation, or survival. There are This was a striking reduction compared to the 90% several possible explanations for these results. First, mortality in a historical control group.180 Brunet et surfactant delivery to the alveoli may have been al183,184 also reported mortality of about 50% in their inadequate. It is estimated that only 5% of the 23 patients treated with ECco2R, and mortality in a aerosolized dose administered in this trial reached larger group of patients treated with ECco2R was the distal airspaces.178 Second, artificial protein-free 53%. These results were encouraging, but many surfactants may not be as effective as natural surfac- factors in addition to extracorporeal gas exchange tants or protein-containing artificial surfactant.174 may have contributed to the lower mortality rates. A Third, the inflammatory injury in patients with prospective, randomized trial185 compared important ARDS often progresses to fibrotic destruction of the clinical outcomes in 40 patients with severe ARDS lung. This may not be ameliorated by surfactant who received either conventional mechanical venti- replacement. Fourth, most patients with ALI/ARDS lation or LFPPV with ECco2R. There was no signif- do not die from respiratory failure but instead from icant difference in mortality between the two treat- dysfunction or failure of multiple nonpulmonary ment groups. Perhaps the beneficial effects from organ systems.10,11,74 Surfactant therapy, even if op- LFPPV were counteracted by complications from timally effective in reducing surface tension, alveolar ECco2R, such as bleeding with increased transfu- collapse, and shunt, would not have a direct effect on sion requirements. These findings suggest that the uncontrolled infections and nonpulmonary organ improved mortality in the earlier, uncontrolled tri- dysfunction. Some newer surfactant preparations als182–184 was not from LFPPV with ECco2R, but with recombinant surfactant proteins are in current instead from improvements in other aspects of crit- clinical trials in ALI/ARDS patients. In these studies, ical care. the surfactant preparations are being delivered into the lung through the endotracheal tube or by bron- choscopic instillation. Prone Positioning Prone positioning leads to substantial improve- Extracorporeal Gas Exchange ments in arterial oxygenation in approximately 65% Despite maximal supportive care with mechanical of ARDS patients.186 –189 There is little information ventilation, some patients with ALI/ARDS experi- to predict which patients will respond positively to 1356 Reviews
  11. 11. prone positioning. However, the improvements in beneficial effects. Moreover, there are no clear some patients are quite striking, allowing substantial guidelines regarding when prone positioning should reduction in Fio2 and PEEP. be initiated or discontinued. Some investigators rec- The mechanism by which the prone position ommend using prone positioning early in the course improves oxygenation has been investigated experi- of ALI/ARDS, to improve lung recruitment, mini- mentally. In a pig model of ALI, Lamm et al190 mize ventilator-associated lung injury, and reduce demonstrated improved ventilation to previously de- requirements for PEEP and Fio2.192 An aggressive pendent (dorsal) regions in the prone position. In the approach maintains prone positioning for 20 h/d, supine position, pleural pressures were higher near allowing relatively brief periods of supine positioning the more dependent dorsal regions due to hydro- for bathing, servicing of vascular catheters, and for static gradients. Higher pleural pressures reduced relief of pressure on ventral surfaces. This schedule transmural pressures of dependent bronchioles and may be maintained until requirements for ventilator alveoli, contributing to the tendency for atelectasis in assistance diminish and weaning appears feasible. these lung zones. In the prone position, pleural pressures appeared more uniform, allowing some Fluorocarbon Liquid-Assisted Gas Exchange dorsal regions to open and participate in ventilation and gas exchange. This suggests that prone position- As previously discussed, reduced surfactant func- ing could prevent ventilator-associated lung injury by tion and increased surface tension cause collapse of promoting more uniform distribution of tidal volume small airways and alveoli in ARDS patients. Surface and by recruiting dorsal lung regions, preventing tension can be eliminated by filling the lungs with a repeated opening and closing of small airways or liquid such as saline solution. However, because of excessive stress at margins between aerated and the low carrying capacity of saline solution for oxygen atelectatic dorsal lung units. and carbon dioxide, it is impossible to maintain Pelosi et al188 assessed lung mechanics and ana- adequate gas exchange with saline solution ventila- lyzed CT images of ARDS patients in the supine and tion. Organic fluorocarbon liquids can dissolve 17 prone positions. Chest wall compliance tended to times more oxygen than water,192 have low surface decrease in the prone position, and tidal volume tension, and spread quickly over the respiratory tended to redistribute toward previously atelectatic epithelium. They appear to be nontoxic, are mini- dorsal regions. Thus, in the prone position, the mally absorbed, and are eliminated by evaporation anterior chest wall may be constricted between the through the lungs. Reduced surface tension may bed surface and the weight of the body above it, improve alveolar recruitment, arterial oxygenation, resulting in some redistribution of tidal volume to and increase lung compliance, even with small dorsal lung units close to the diaphragm, recruiting amounts of the substance instilled into the lung, as atelectatic lung units in this region, with an improve- with surfactant therapy. ment in arterial oxygenation. There could also be Fluorocarbons have been used in animals with lung-protective effects of prone positioning from the total liquid ventilation.193 This approach requires a overall decrease in atelectasis at end-expiration. liquid ventilator-gas exchange device to oxygenate Several ICU personnel are required to safely the liquid, deliver the tidal volume, and remove implement prone positioning. One person must en- carbon dioxide. An alternative approach is partial sure stability of the airway during the position liquid ventilation, in which the lungs are filled change, since dislodgment of the endotracheal tube approximately to functional residual capacity. Gas may not be immediately apparent and is difficult to ventilation is then continued with a conventional manage in the prone position. Others must manipu- ventilator.194 –198 In these various animal models of late chest tubes, IV catheters, and monitoring de- lung injury, total and partial liquid ventilation im- vices. Once patients are in the prone position, pro- proved gas exchange when compared to conven- cedures for routine care, such as bathing and daily tional ventilation. The improvement in gas exchange assessments of IV catheter sites, must be adjusted is probably explained by alveolar recruitment. Stud- and are frequently compromised. In a recent trial, ies199,200 in humans with ARDS also showed prom- clinical outcomes did not improve in ARDS patients ising improvements in gas exchange. Atelectasis and randomized to prone positioning for at least 6 h/d vs alveolar filling are frequently worse in dependent patients randomized to remain supine.191 More pro- lung regions,14 and the dense fluorocarbon tends to longed periods of prone positioning may be neces- “gravitate” to these regions, where it is of potentially sary to achieve lung protection and survival benefits. greatest value for alveolar recruitment. Moreover, There are no clinical studies to guide clinicians the weight and resulting pressure of the liquid in regarding the length of time each day that prone dependent regions may divert blood flow to nonde- positioning should be maintained to achieve maximal pendent, better-ventilated regions. CHEST / 120 / 4 / OCTOBER, 2001 1357
  12. 12. The use of mechanical ventilation with high airway ARDS have persistent inflammation, fibroprolifera- pressures may still be injurious to the lung paren- tion, and inflammatory cytokine release in the chyma during liquid ventilation, as during gas venti- airspaces of the lung, glucocorticoids at this late lation. In total liquid ventilation, there is also the risk stage could modulate these processes and facilitate of mechanical interference with venous return. recovery. However, glucocorticoids could also in- There was minimal hemodynamic instability with crease risks of nosocomial infections, which would partial liquid ventilation at a dose of 20 mL/kg.194 diminish chances for recovery. Several case series Instillation of greater volumes of fluorocarbon may reports208,209 suggested that glucocorticoids could decrease cardiac output by a similar mechanism as lower mortality in some patients with severe ALI/ high PEEP.198 There are some encouraging reports ARDS when administered several days after ALI/ of the safety and efficacy of partial liquid ventilation ARDS onset. In a small, randomized, placebo-con- in adults199 and pediatric patients200 with ARDS, as trolled trial,210 important clinical outcomes were well as in neonates with respiratory distress.201 How- better in patients randomized to receive methylpred- ever, more investigation is needed to demonstrate nisolone in the late phase of ALI/ARDS. This was a improvements in key clinical outcomes before this small trial (16 patients randomized to receive meth- novel technique can be adopted for routine clinical ylprednisolone and 8 patients to receive placebo), use in ALI/ARDS patients. and several patients crossed over between study groups. The NIH ARDS Network is conducting a Anti-inflammatory Strategies larger prospective, randomized, double-blind trial to confirm these results. The inflammatory response in ALI is associated with recruitment of large numbers of neutrophils Antioxidant Therapy: There is convincing evi- and monocytes to the distal airspaces of the lung and dence that reactive oxygen species play a major role the release of proinflammatory molecules, including in mediating injury to the endothelial barrier of the cytokines, oxygen radicals, and proteases.202 Exces- lung in the presence of endotoxin, bacterial sepsis, or sive inflammation may worsen ALI/ARDS. As dis- hyperoxic lung injury. Antioxidant therapy has been cussed below, some recent studies suggested that useful in the prevention and the treatment of ALI in important clinical outcomes in ALI/ARDS patients some animal models.211 Patients with ALI/ARDS would improve with modulation of lung inflamma- experience oxidative stress from neutrophil activa- tion. Other studies were disappointing. tion and from high levels of inspired oxygen.212 Work by Quinlan et al213 indicates that patients who do not Therapeutic Strategies to Reduce Sepsis-Induced survive ARDS sustain much greater levels of oxida- ARDS: Patients with ALI/ARDS from sepsis have tive molecular damage, suggesting that their antiox- higher mortality than patients with ALI/ARDS from idant defense mechanisms are weakened. most other causes.10,73 Treatment of sepsis before or N-acetylcysteine and procysteine, oxygen free- in the early phase of ALI/ARDS could improve radical scavengers and precursors for glutathione, outcomes in these patients. Unfortunately, the re- were efficacious in some experimental studies.211 In sults of trials of high doses of glucocorticoids,203–205 phase II clinical studies214,215 in ALI/ARDS and antiendotoxin monoclonal antibody, anti–TNF- sepsis, there were encouraging trends in important therapy, and anti–interleukin (IL)-1 therapy were clinical outcomes in patients who received these disappointing. However, recently, activated protein agents. However, the results of a large, randomized, C has been shown to reduce mortality in sepsis placebo-controlled trial failed to show beneficial patients206 by novel anti-inflammatory and antico- effects of procysteine in patients with ALI/ARDS.2 agulent mechanisms.207 Prostaglandin Agonists/Inhibitors: Prostaglandin Glucocorticoid Therapy: As discussed in the pre- E1 is a vasodilator that blocks platelet aggregation ceding section, high doses of glucocorticoids do not and decreases neutrophil activation. This agent prevent the development of ARDS in patients with showed promise in experimental and preliminary sepsis. In addition, randomized, controlled clinical clinical studies of lung injury.98 However, a multi- trials203–205 did not show beneficial effects when high center study99 of 100 ALI/ARDS patients reported doses of glucocorticoids were administered to ALI/ no evidence of reduced mortality in those treated ARDS patients early in their course. Interestingly, in with IV prostaglandin E1. Liposomal delivery of one of these studies,204 serum complement levels prostaglandin E1 was also not beneficial in a phase II were not lowered in patients with sepsis-induced study.216 ARDS who were treated with high-dose methylpred- The synthesis of cyclooxygenase products of the nisolone. Since some patients with late-phase ALI/ prostaglandin pathway, particularly thromboxane, 1358 Reviews
  13. 13. has been linked with abnormal airway mechanics, an extravascular location. Monoclonal antibodies that hypoxemia, systemic hypotension, and multiple or- neutralize IL-8 reduced acid-induced lung injury in gan dysfunction in animal models of lung injury. rabbits.228 Several clinical studies229 –233 indicate that Therefore, a prospective, double-blind, randomized substantial quantities of IL-8 are present in the BAL trial207 tested the ability of ibuprofen, an inhibitor of fluid or the pulmonary edema fluid of patients in the the cyclooxygenase pathway, to reduce morbidity early phase of ARDS. Additional studies are needed, and mortality in 455 patients with sepsis who were at especially because of a concern for increased risk of risk of multiple organ failure, including ARDS. infection with anti–IL-8 therapy. Clinical trials of Despite an 89% reduction in prostanoid levels, mor- anti–IL-8 therapy for prevention in high-risk patients tality rates in the placebo group (40%) and the or in early ALI/ARDS may soon be warranted. ibuprofen group (37%) were similar, and there were Other potentially useful strategies for modulating no significant effects on the duration of shock or in inflammation in patients with ALI/ARDS include organ failure-free days.207 platelet-activating factor inhibitors, antiproteases, Ketoconazole, a potent inhibitor of thromboxane anticytokine therapies, and agents designed to in- and leukotriene synthesis,217 was reported to prevent hibit the coagulation cascade. To our knowledge the development of ALI/ARDS in high-risk surgical none of these strategies have been tested in clinical patients.218 However, when this agent was studied in trials in patients with established ALI/ARDS. an NIH-sponsored multicenter phase III trial219 to test its efficacy for decreasing mortality and the Enhanced Resolution of Alveolar Edema: Until duration of assisted ventilation in 234 patients with recently, attention was focused on pulmonary endo- ALI/ARDS, there was no decrease in mortality for thelial function during ALI/ARDS. It is now clear ketoconazole treatment (35%) vs the placebo group that the structure and function of the alveolar epi- (34%), and the median number of ventilator-free thelium are also important determinants of lung days was 9 in the placebo group vs 10 days in the injury.234,235 The epithelium is the site of alveolar ketoconazole group. fluid reabsorption,236 an essential step in the resolu- tion of ALI/ARDS. Alveolar fluid clearance depends Lisofylline and Pentoxifylline: Pentoxifylline is a primarily on active sodium transport across the phosphodiesterase inhibitor that inhibits neutrophil alveolar epithelium.235 Substantial experimental chemotaxis and activation in animal models of work has elucidated the mechanisms that modulate ARDS.220 –222 Limited clinical experience in humans sodium transport and water movement. suggests some beneficial effects,223 but there is not Several pharmacologic agents have been identified enough information to allow definite recommenda- that can increase alveolar fluid clearance experimen- tions for clinical use. Lisofylline is chemically related tally either by acting on the epithelial sodium chan- to pentoxifylline, but its anti-inflammatory mecha- nel or the sodium/potassium adenosine triphos- nism is through inhibition of the release of free-fatty phatase pumps. 2-Adrenergic stimulation markedly acids from cell membranes under oxidative increases alveolar fluid clearance in the normal lung stress.224,225 In animal studies,226 lisofylline inhibited of several species236 and in the ex vivo human release of TNF, IL-1, and IL-6, attenuated shock- lung.237 In most of these studies, the 2-agonist was induced lung injury in mice, and had favorable administered into the airspaces. 2-Agonists admin- effects on the course of murine endotoxin shock. istered IV and endogenous epinephrine released Unfortunately, a recently completed phase III tri- from the adrenal gland also markedly increase alve- al227 by the NIH ARDS Network in 220 ALI/ARDS olar epithelial sodium and fluid clearance.236 Data patients showed no beneficial effects of lisofylline. from a 1997 study237 indicate that salmeterol, a lipid-soluble 2-agonist, can maximally upregulate Anti–IL-8 Therapy and Other Potential Anti-in- alveolar fluid clearance in the ex vivo human lung at flammatory Strategies: Other anti-inflammatory a dose of only 10-6 mol/L. This is the same concen- strategies could be effective in attenuating lung tration that was achieved in the alveolar compart- injury or preventing its development in high-risk ment in sheep studies in which salmeterol was patients. One approach is to reduce the number of aerosolized in a clinically relevant dosage of 5 mg/ neutrophils that migrate into the extravascular space h.238 These studies suggest that 2-agonists can be of the lung by interfering with neutrophil adhesion to delivered by aerosol in intubated patients receiving the lung endothelium, or by reducing the release of mechanical ventilation and can achieve concentra- chemotactic factors in the extravascular space. There tions in the distal airspaces of the lung that will is strong experimental evidence for inhibiting the enhance alveolar fluid clearance. release of IL-8, an important chemotactic stimulus Can sodium and fluid transport be stimulated with for migration of neutrophils from an intravascular to 2-agonists in the presence of lung injury? In three CHEST / 120 / 4 / OCTOBER, 2001 1359
  14. 14. recent studies239 –241 in hyperoxic lung injury models ALI/ARDS. Thus, the severe fibroproliferative re- in rats, intra-alveolar terbutaline administration sponse in some patients in the late-phase of ALI/ markedly increased alveolar fluid clearance. In these ARDS may be determined early in the course of lung studies, the edema was probably confined predomi- injury. nantly to the interstitium. However, the results The provision of a new epithelial barrier with type established that exogenous 2-agonist therapy could II cells may have beneficial effects in addition to increase alveolar and lung fluid clearance in the restoration of the air-liquid interface. For example, injured lung. In other studies,236 alveolar fluid clear- re-epithelialization of the air-lung interface is asso- ance was markedly increased by endogenous epi- ciated with a gradual regression of intra-alveolar nephrine release in the presence of endotoxemia or granulation tissue.247 Also, the rate of alveolar epi- bacteremia. However, following prolonged hemor- thelial fluid clearance in the subacute phase of rhagic shock in rats, oxidant mechanisms decreased bleomycin-induced ALI in rats was increased by the response of the alveolar epithelium to 2-agonist 100% over baseline levels.250 Enhanced alveolar stimulation.242 Thus, under some circumstances, the fluid clearance depends in part on extensive prolif- epithelium may not respond to 2-agonist stimula- eration of alveolar epithelial type II cells. tion because of extensive injury and loss of alveolar Studies251–253 suggest that hepatocyte growth fac- type II cells or because of downregulation of the tor and keratinocyte growth factor are major mito- response to 2-agonists. Controlled clinical trials are gens for alveolar epithelial type II cells, and intratra- needed to evaluate aerosolized -adrenergic agonist cheal pretreatment of rats with keratinocyte growth therapy in patients with ALI/ARDS. factor (5 mg/kg) prior to induction of lung injury with In addition to aerosolized 2-agonists, alveolar hyperoxia, acid instillation, bleomycin, or radiation epithelial fluid clearance could be increased with decreased severity of injury. The mechanism of systemically delivered 2-agonists. Dobutamine, a protection may be due to increased alveolar fluid commonly used 2-adrenergic agonist, markedly in- transport secondary to the increased numbers of creased alveolar and lung fluid clearance in an alveolar type II cells and by other mechanisms, experimental rat model of pulmonary edema when including increased release of surface-active material administered IV at a clinically relevant dosage of at 5 or more resistance of the alveolar epithelium to g/kg/min.243 Dopamine, when administered at 5 injury. g/kg/min IV, increased alveolar fluid clearance in an isolated perfused rat model by increasing the activity of sodium/potassium adenosine triphos- Conclusion phatase pumps.244 Thus, clinically available vasoac- tive agents could be useful in some patients with The decrease in ALI/ARDS mortality reported pulmonary edema to increase rates of alveolar fluid since 1991254,255 is attributable to improvements in clearance. many aspects of care, such as ventilator manage- ment, diagnosis and treatment of infections, and Enhanced Repair of the Alveolar Epithelial Bar- nutritional support. However, mortality is still high, rier: One of the hallmarks of ALI/ARDS is disrup- and some survivors suffer with various sequelae for tion of the alveolar epithelium with necrosis or months after recovery from critical illness.256,257 apoptosis of alveolar type I cells. Effective recovery Thus, further improvements in treatment are of lung function depends on reconstitution of the needed. alveolar structure in the injured lung areas. As part of Improved understanding of the pathogenesis of the repair process, alveolar epithelial type II cells ALI/ARDS has led to important advances in the proliferate and provide a provisional new epithelial treatment of ALI/ARDS, particularly in the area of barrier.245 Ideally, alveolar epithelial proliferation ventilator-associated lung injury.2 Standard support- would occur with a minimal fibrotic response. How- ive care for patients with ALI/ARDS should now ever, in some patients, activated myofibroblasts from include a protective ventilatory strategy with low the interstitium migrate into the alveoli through gaps tidal volume ventilation by the protocol developed by in the basement membrane and attach to the luminal the NIH ARDS Network.30 Further refinements of surface of damaged alveolar membranes. Myofibro- the protocol for mechanical ventilation will occur as blast replication at the air-lung interface may cause additional clinical trials are completed. In addition, fibrosing alveolitis and obliteration of gas exchange novel modes of mechanical ventilation are being units.246,247 This process is controlled by endogenous studied and may augment standard therapy in the mediators such as platelet-derived growth factor and future. Although most anti-inflammatory strategies other peptides.247,248 Clinical evidence249 suggests have been disappointing in clinical trials, further that collagen synthesis occurs in the early phase of trials are underway to test the efficacy of late corti- 1360 Reviews
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