1. Anesthesia & Analgesia 2006, Issue 102, p. 298-305 Departments of Anesthesiology and Surgery, Division of Pulmonary and Critical Care Medicine, and Department of Health Sciences Research, Mayo Clinic College of Medicine, Mayo Clinic, Rochester, Minnesota Francis X. Whalen, MD, OgnjenGajic, MD, Geoffrey B. Thompson, MD, Michael L. Kendrick, MD, Florencia L. Que, MD, Brent A. Williams, MS, Michael J. Joyner, MD, Rolf D. Hubmayr, MD, David O. Warner, MD, and Juraj Sprung, MD, PhD The Effects of the Alveolar Recruitment Maneuver and Positive End-Expiratory Pressure on Arterial Oxygenation During Laparoscopic Bariatric Surgery
2. Anesthesia and Pulmonary Function What is atelectasis? 3 Basic mechanisms of atelectasis: - Compression - Absorption - Loss of surfactant
4. The Effects of the Alveolar Recruitment Maneuver and Positive End-Expiratory Pressure on Arterial Oxygenation During Laparoscopic Bariatric Surgery METHODS Prospective randomized study ASA II-III, Ages 25-65, BMI > 40 kg/m2 No significant pre-op. pulmonary disease, active asthma, or home 0xygen therapy.
7. Results of the Study No significant differences in demographics, surgical times, opioids, IV fluids. Pa02/Fi02 significantly higher in recruitment group. Recruitment group and compliance. No significant difference in Pa02/Fi02 after extubation.
10. Discussion Recruitment group experienced a higher Pa02 intra-operatively. Higher PaC02 and lower Pa02 in recruitment group post-operatively. Mode of ventilation used. Small sample size.
The article that I will be presenting today is titled, The effects of the alveolar recruitment maneuver and positive end-expiratory pressure on arterial oxygenation during laparascopic bariatric surgery. The study was published in Anesthesia & Analgesia 2006, Issue 102, Pgs. 298-305. It was performed at the Mayo Clinic in Rochester, Minnesota and the authors include Whalen, Gajic, Thompson, Kendrick, Que, Williams, Joyner, Hubmayr, Warner and Sprung.
Before I begin talking about my journal article, I would like to quickly review the effects of anesthesia on pulmonary function. Pulmonary gas exchange is regularly impaired during anesthesia and general anesthesia with mechanical ventilation. The major cause of decreased oxygenation of blood during anesthesia is due to collapse of lung tissue, also known as atelectasis. Atelectasis during anesthesia causes a decrease in arterial oxygenation, functional residual capacity and respiratory compliance. Pulmonary atelectasis may be caused by a variety of factors, which have been classified into three basic mechanisms. Compression atelectasis occurs when the transmural pressure distending the alveolus is reduced. Compression atelectasis may be due to a number of factors such as positioning or obesity. It can also be seen due to increased abdominal pressure transmitted into the thoracic cavity when the diaphragm has a reduced tone or is paralyzed, as in general anesthesia. Absorption atelectasis occurs when less gas enters the alveolus than is removed by uptake by the blood. Absorption atelectasis may be due to a mucous plug. A mucous plug may cause a proximal obstruction in the airway causing the gases in the alveoli to gradually empty into the blood along its concentration gradient. When these gases are not replenished due to the proximal obstruction, the alveoli will collapse. Another cause of absorptionatelectasis that is most relevant to us as anesthesia practitioners, may be attributed to the use of 100% Fi02 or a combination of 02 and N20. We all know that the composition of nitrogen in room air is roughly 79% and that nitrogen diffuses across alveoli and into the blood very sluggishly. When we flush this large concentration of nitrogen out of the alveoli by breathing high concentrations of 02 or combining 02 with more soluble N20, the process of absorption atelectasis is accelerated. The third and last form of atelectasis is called loss of surfactant atelectasis. This occurs when the surface tension of an alveolus increases because of reduced surfactant action. Atelectasis, once formed will impede surfactant function and these affected regions are prone to collapse again after having been re-opened. Any of these factors may contribute to atelectasis during anesthesia and the post-operative period. Atelectasis secondary to the administration of anesthesia has been demonstrated by obtaining CT scans of the thorax both pre and post-induction.
The following slide shows CT scans both pre and post induction of a pt. being ventilated without PEEP above and with PEEP below. These CT slices were obtained 1 cm above the diaphragm of each pt. Can anyone tell me where the atelectasis is present on the above CT scan? The study that I will be talking about today looks at the effects of an alveolar recruitment maneuver and positive end-expiratory pressure on arterial oxygenation during laparoscopic bariatric surgery. Without getting into too much detail, I would like to quickly explain what an alveolar recruitment maneuver is before we begin with the study so that we are all on the same page when I explain this study to you. Alveolar recruitment is a strategy aimed at re-expanding collapsed or atelectic lung tissue, and then maintaining high PEEP after the maneuver is performed in order to prevent subsequent 'de-recruitment'. In order to recruit collapsed lung tissue, sufficient pressure must be imposed to exceed the critical opening pressure of the affected lung. By combining the pressure of PEEP and the pressure exerted by the ventilator with each subsequent tidal volume, an overall pressure of at least 40 and in some severe cases 60 cm H20 must be delivered to a pt with each breath in order to open or recruit already atelecticareas of the lung. Keep in mind that ventilating pts. with airway pressures at 40-60 cm H20 is only done for about 5-10 breaths and then the PEEP is reduced as well as the TV for adequate ventilation that does not put a pt. at risk for baro and volutrauma. One last important thing that one must understand is that after completion of this maneuver, PEEP must be used at higher than physiologic levels to prevent derecruitment of alveoli and subsequent atelectasis. Okay now that we have gotten that out of the way, lets move on to the study.
The aim of the study was to quantitate the effects of alveolar recruitment maneuvers followed by PEEP on oxygenation, pulmonary mechanics and hemodynamics during laparoscopic bariatric Roux-en-Y surgery. The investigators constructed a prospective randomized study to quantify the effect of a lung recruitment maneuver on Pa02. After IRB approval was obtained as well as informed consent, the pts. were grouped according to a computerized random number generator. Pts. recruited were between the ages of 25 and 65 years old, ASA classes II-III with BMI > 40 kg/m2. Exclusion criteria included significant pre-op pulmonary disease defined as an FEV1:FVC of < 50% or a FVC < 50% as well as any active asthma, or the use of home oxygen therapy.
Anesthesia was standardized using Propofol, Succinylcholine, Vecuronium, Desflurane, a 50/50 mixture of air and oxygen and Morphine. Tracheal intubation was achieved with a 8.0 mm OET. The use of vasopressors, which included Ephedrine and Neosynephrine, was left to the discretion of the attending anesthesiologist. In addition to the standard monitors used, an arterial line, either brachial or radial, was placed and an ultra-thin sensor, the Paratrend 7 monitoring system subsequently inserted. This sensor provides continuous Pa02, PaC02, and pH values. A non-invasive CO monitor was also used to measure CO. In addition, this CO monitor also measured mean and peak airway pressures, MV, expiratory TV, respiratory system dynamic compliance, inspired airway resistance, and physiologic deadspace to TV ratios. Mechanical ventilation was conducted with the DatexOhmedaAestiva 5 ventilator as pictured above. Pts. were randomized to receive one of the following ventilatory regimens. The control group, which consisted of 10 subjects, were ventilated with 8 mL/kg IBW, 4 PEEP, a 1:2 IE with an initial RR of 8 bpm. Ventilation was adjusted first by increasing or decreasing the frequency followed by adjusting the TV in 50 mL increments if required. These adjustments were made to maintain a mild permissive PaC02 between 45-50 while the PEEP was maintained at a physiologic level of 4 throughout the entire anesthetic. When I first read this study, I thought to myself, “wow, a PaC02 of 45-50 is a bit high.” The authors stated later in the discussion section that their rationale for this mild hypercapnia was to avoid aggressive ventilation which could have added mechanical stress to already stretched lungs in the recruitment group. Pts. in the recruitment group were initially ventilated with the identical respiratory variables and after the creation of pneumoperitoneum (Maintained at 15 cm of pressure) a lung recruitement maneuver was performed as follows. TV of 8 mL/kg, 1:2, and in increase in PEEP in a stepwise fashion starting with 10 cm H20 for 3 breaths, followed by 15 for 3 breaths, and then 20 for 3 breaths. If the Pa02 reached a plateau during a 10 second monitoring interval following the recruitment maneuver, only a single maneuver was to be performed. If the Pa02 continued to show an increasing trend after the 1st recruitment, a series of up to 4 sequential maneuvers were to be performed in order to maximize Pa02. Repeat maneuvers were to be performed during the surgery whenever the Pa02 decreased more than 25 mmHg below maximal Pa02 value achieved after recruitment. It should be noted that the maneuver was to be aborted if the SBP decreased more than 20% the pre-recruitment value. Measurements were obtained at 6 different intervals during the anesthetic. The Pa02 and PaC02 were also recorded 30 mins. after extubation in the RR. Other intra-op variables recorded included were the use of opioids (in Morphine equivalents) fluids and vaospressorsadminstered. Finally post-op medical records were reviewed for any pulmonary complications following hospital discharge.
It should be noted that demographics, administration of IV fluids, morphine and vasopressor equivalents, and other outcomes were reported and compared across procedure groups using Fisher’s exact tests for categorical variables and t-tests or Wilcoxon’s ranked sum tests for continuous variables. A P value of <0.05 was considered statistically significant. Now moving on to the results of the study….There were no significant differences in demographic characteristics, duration of operation or the intraoperative use of opioids or fluids between the two groups. Pts. in the recruitment group had a statistically significant higher intra-op Pa02/Fi02, even after adjusting for baseline Pa02/Fi02, BMI, age and sex with P <0.01. Recruitment maneuvers significantly increased respiratory system dynamic compliance (by about 40% 5 mins post recruitment with P <0.05) however, compliance in these pts. subsequently declined and 30 mins post recruitment, the values approached pre-recruitment values. After release of pneumoperitoneum, dynamic compliance increased in both groups but was significantly higher in the recruitment group with a P < 0.01. At 30 mins after tracheal extubation, there was no significant difference in Pa02/Fi02 between the recruitment and control pts. The PaC02 measured 30 mins after extubation was significantly higher in the recruitment group. Baseline CI was larger in the recruitment group (4.6 vs 3.8) however, after adjusting it for differences in baseline value, age, sex and BMI, pts. in the recruitment group had a smaller CI during surgery compared with control pts. (P 0.02) Pts. in the recruitment group did receive a larger total dose of vasopressors (P0.04). Peak inspiratory and mean airway pressures were higher in the recruitment group (P<0.01). The length of hospitalization and incidence of pulmonary complications were not statistically significant, although the small number of the latter events precluded meaningful statistical comparison. One pt. in the recruitment group had a pulmonary embolism, and two had respiratory failure and were tracheallyintubated more than 24 hours. Whereas the control group only experienced one pt. with respiratory failure and one pt. with atelectasis requiring intervention and bronchoscopy. 4 out of 20 pts. in this study experienced significant pulmonary complications. This is a good example of risks that can be encountered in the morbidly obese population. The next few slides focus on the respiratory values derived from the study for all of you visual learners out there.
The graph at the top of this slide depicts the Pa02 values for both the control group and the recruitment group. The 5 min. marker denotes measurements after the recruitment maneuver was performed. As you can see the recruitment group had significantly greater Pa02’s compared to the control group after initiation of a recruitment maneuver. The graph on the bottom shows dynamic respiratory system compliance in both the control and recruitment group. Compliance increased after recruitment but continued to decrease throughout surgery despite the added baseline PEEP. Compliance was not substantially different from pre-recruitment baseline by 30 mins.
The following graphs show the various hemodynamic parameters measured throughout the study. There were no changes over time or between groups in mean arterial blood pressures and heart rates. The cardiac index graph below appears to show higher indexes in the recruitment group but after adjusting for pt. demographics, it was actually lower in the recruitment group.
This study shows that recruitment maneuvers followed by PEEP effectively increases intra-op Pa02 in pts. undergoing laparoscopic bariatric surgery. However, this improvement was only sustained in most pts. as long as endotracheal intubation and PEEP were maintained. Unfortunately, these increased Pa02’s disappeared within 30 mins of tracheal extubation. Also it should be noted that the recruitment group had higher PaC02 values and lower Pa02 values after surgery. These findings raise an important question: Does the recruitment therapy interfere with post-op pulmonary function? The author suggested that a larger number of pts. studied were required to answer this question. One thing that was not mentioned anywhere in the study was the form of ventilation that was being used. We all know that the ventilator used in this study offers pressure and volume control but the authors failed to mention which mode was used to ventilate the pts. This is a very important thing that was left out of the study. I can only assume that this study was performed using pressure control ventilation. If these pts. who had BMI’s > 40 were ventilated using a volume control setting, their peak pressures would have been significantly higher compared to a pressure control setting. From my recent studies that I have been reading regarding alveolar recruitment, it is well known that pressure control ventilation is the most effective way of ventilating pts. with multiple respiratory co-morbidities. Also the study used a 1:2 ratio to ventilate both groups of pts. Prior studies have also shown that using a 1:1 I:E ratio is optimal when ventilating pts. and performing alveolar recruitment maneuvers. This is especially true with the pt. population that was used in this study. These pts. were obese with BMI’s > 40 kg/m2 which means that most of these pts. suffered from a restrictive lung pathology. Ventilating these pts. with a 1:1 ratio would have been optimal because more time would be spent in the inspiratory phase. Greater time spent in the inspiratory phase leads to better ventilation and gas exchange. And lastly, the study used a very small sample size; there were only 20 participants used in the study. The authors had even suggested that had there been more subjects, the study may have revealed whether or not recruitment therapy interferes with post-operative pulmonary function.
