Mechanical Ventilation

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  • Instead of the usual list, follow these rules: There is a tendency to delay intubation as long as possible in the hopes that it will be unnecessary. Elective intubation carries fewer dangers than emergent intubation. So, if the patient’s condition is severe enough that intubation is considered, then proceed without delay Remember you will never be faulted for establishing the control of airways ETT and ventilators do not create the need for mechanical ventilation: cardiopulmonary and neuromuscular disease do
  • At constant inflation volume the peak pressure is directly related to the airflow resistance and to the elastic recoil force of the lungs and the chest wall. Plateau pressure is measured by occluding the expiratory tubing at the end of inspiration. Because no airflow is present when the plateau pressure is created, the pressure is not a function of flow resistance. Instead Plateau pressure is proportional directly to the elastance of the lungs and the chest walls.
  • Very useful for determining the cause of sudden deterioration of a patient on a ventilator.
  • Connector tubing between the ventilator and the patient expands during lung inflations, some of the TV is lost in this tubing. Connector tubing compliance is 3ml/cm of water Meaning that 3ml of volume is lost for every 1cm increase in inflation pressure. Example: Inflation vol from vent is 700cc at Pk of 40cm water, then volume lost to expansion of ventilator tubing is 40X3=120ml. Therefor actually only 700-120=580ml reaches the patient, and this should be used as the Vt for compliance measurement.
  • Volume cycled means that each machine breath delivers a preselected lung inflation volume
  • IMV delivers periodic volume cycled breaths at a preselected rate but allows spontaneous breathing between machine breaths. Because each spontaneous breath does not trigger a machine breath, there is a reduced risk of respiratory alkalosis and hyperinflation Synchronized IMV: Machine breaths are synchronized to coincide with spontaneous lung inflations.
  • Increased work of breathing could lead to respiratory muscle fatigue and ventilator dependency
  • At the onset of each spontaneous breath, the negative pressure generated by the patient opens a valve that delivers the inspired gas at a preselected pressure (5-15cm water) The patient’s inspiratory flow rate is adjusted by ventilator as needed to keep inflation pressure constant. When inspiratory flow rate falls below 25% of the peak inspiratory flow, the augmented breath is terminated
  • Normally the alveolar pressure at the end of expiration is equal to the atmospheric pressure, called the zero reference point.
  • Valve exerts back pressure and exhalation proceeds until this back pressure is reached where upon, flow ceases. Something like placing the distal end of the expiratory tubing under water. Back pressure would be equal to the distance the tube is submerged.
  • Low levels of PEEP are as deleterious for cardiac output if the mean intrathoracic pressure is high Effects of PEEP on systemic oxygenation are determined by the change in cardiac output not by change in the arterial oxygenation Best PEEP curve?
  • In localized lobar pneumonia, PEEP may overdistend the normal alveoli and redirect blood back to the diseased areas Prevents the repeated opening and closing of small airways which is a source of further lung injury Small airways collapse in obstructive lung disease, PEEP can keep these open at the end of expiration. Level of peep should be enough to counterbalance the closing pressure of the small airways, but not be more than the intrinsic-PEEP, so as to not impair the inspiratory flow. The level of Extrinsic peep which first causes an increase in peak pressures is taken as the level of occult PEEP.
  • PEEP does not reduce lung edema in ARDS, but can increase water accumulation in the lungs by impaired lymphatic drainage from lungs Glottic closure at end exp causes low level of physiologic PEEP. No documented benefit in intubated patients PEEP is transmitted across the walls of the blood vessels, it may not reduce transmural pressure. No benefit.
  • In spontaneous PEEP, a negative pressure is required for inhalation. CPAP thus reduces work of breathing
  • Factors predisposing to auto peep fall in 2 categories. Occult peep increases work of breathing: hyperinflation places lungs in the flatter portion of the pressure volume curve. Take a deep breath and then try to breathe in further, higher pressures are needed to inhale the tidal volume. Level of Extrinsic peep that first causes a rise in peak inspiratory airway pressures is a quantitative measure of occult PEEP.
  • Excluding the complications if intubation, sedation etc. One liter of saliva a day with a billion bugs per microliter.
  • These two have a greater predictive value than the ones mentioned before which have a very poor predictive value.
  • Show figure
  • PSV to overcome resistance of the circuit and valve
  • False sense of security, patient often left unattended more than those with T-piece weaning
  • ELECTROLYTES:phosphorus, K, Mg OVERFEEDING: Excess calories causes excess CO2 production, can impair ability to wean
  • Rapid breathing could be the result of anxiety, which leads to hyperventilation and high tidal volume. Where as wean failure muscle fatigue and cardiopulmonary disease usually causes rapid shallow breathing.
  • CSA of tracheal tube is 50 cu. mm and that of the glottis is 66 cu. mm
  • Mechanical Ventilation

    1. 1. MECHANICAL VENTILATION <ul><li>Things “I” wish I knew when I was an Intern </li></ul><ul><li>Amit Gupta, MD </li></ul><ul><li>Internal Medicine </li></ul><ul><li>North Mississippi Medical Center </li></ul>
    2. 2. Mechanical Ventilation <ul><li>Indications for Intubation and Ventilation </li></ul><ul><li>Principles of Mechanical Ventilation </li></ul><ul><li>Patterns of Assisted Ventilation </li></ul><ul><li>Ventilator Dependence: Complications </li></ul><ul><li>Liberation from Mechanical Ventilation: Weaning </li></ul><ul><li>Troubleshooting </li></ul><ul><li>Arterial Blood Gases </li></ul>
    3. 3. Indications for Mechanical Ventilation <ul><li>“… .An opening must be attempted in the trunk of the trachea, into which a tube or cane should be put; You will then blow into this so that lung may rise again….And the heart becomes strong….” </li></ul><ul><li>-Andreas Vesalius (1555) </li></ul>
    4. 4. Indications for Mechanical Ventilation <ul><li>1. “Thinking” of Intubation: elective v/s emergent </li></ul><ul><li>2. “Act of weakness?” </li></ul><ul><li>3. Endotracheal tubes are not a disease and ventilators are not an addiction </li></ul><ul><li>4. And the usual elective and emergent indications that you all know! </li></ul>
    5. 5. Objectives of Mechanical Ventilation <ul><li>Improve pulmonary gas exchange </li></ul><ul><li>Reverse hypoxemia and Relieve acute respiratory acidosis </li></ul><ul><li>Relieve respiratory Distress </li></ul><ul><li>Decrease oxygen cost of breathing and reverse respiratory muscle fatigue </li></ul><ul><li>Alter pressure-volume relations </li></ul><ul><li>Prevent and reverse atelectasis </li></ul><ul><li>Improve Compliance </li></ul><ul><li>Prevent further injury </li></ul><ul><li>Permit lung and airway healing </li></ul><ul><li>Avoid complications </li></ul>
    6. 6. Strategies for Mechanical Ventilation Hypercapnia allowed, pH 7.2-7.4 Normal, pH 7.36-7.44 ABG 5-15 cm of water PRN to keep FiO2<0.6 PEEP Plateau Pr<35 Peak Pr<50cm water End-insp. pressure 5-10 ml/kg 10-15 ml/kg Inflation Volume Lung-Protective Traditional Ventilatory Parameter
    7. 7. Monitoring Lung Mechanics <ul><li>Proximal Airway Pressures (end-inspiratory) </li></ul><ul><li>1. Peak Pressure </li></ul><ul><li>Function of: Inflation volume, recoil force of </li></ul><ul><li>lungs and chest wall, airway resistance </li></ul><ul><li>2. Plateau Pressure </li></ul><ul><li>Occlude expiratory tubing at end-inspiration </li></ul><ul><li>Function of elastance alone </li></ul>
    8. 8. Use of Airway Pressures <ul><li>Peak Pressure increased Plateau Pressure unchanged: </li></ul><ul><li>Tracheal tube obstruction </li></ul><ul><li>Airway obstruction from secretions </li></ul><ul><li>Acute bronchospasm </li></ul><ul><li>Rx: Suctioning and Bronchodilators </li></ul>
    9. 9. Use of Airway Pressures <ul><li>Peak Pressure and Plateau Pressure are both increased: </li></ul><ul><li>Pneumothorax </li></ul><ul><li>Lobar atelectasis </li></ul><ul><li>Acute pulmonary edema </li></ul><ul><li>Worsening pneumonia </li></ul><ul><li>COPD with tachypnea </li></ul><ul><li>Increased abdominal pressure </li></ul><ul><li>Asynchronous breathing </li></ul>
    10. 10. Use of Airway Pressures <ul><li>Decreased Peak Pressure : </li></ul><ul><li>System air leak: Tubing disconnection, cuff leak </li></ul><ul><li>Rx: Manual inflation, listen for leak </li></ul><ul><li>Hyperventilation: Enough negative intrathoracic </li></ul><ul><li>pressure to pull air into lungs may drop Pk. </li></ul>
    11. 11. Compliance <ul><li>Static Compliance (Cstat): </li></ul><ul><li>Distensibility of Lungs and Chest wall </li></ul><ul><li>Cstat = Vt/Pl </li></ul><ul><li>Normal C stat: 50-80 ml/cm of water </li></ul><ul><li>Provides objective measure of severity of illness in a </li></ul><ul><li>pulmonary disorder </li></ul><ul><li>Dynamic Compliance: </li></ul><ul><li>Cdyn: Vt/Pk </li></ul><ul><li>*Subtract PEEP from Pl or Pk for compliance </li></ul><ul><li>measurement </li></ul><ul><li>Use Exhaled tidal volume for calculations </li></ul>
    12. 12. Patterns of Assisted Ventilation <ul><li>Assist Control </li></ul><ul><li>Intermittent Mandatory Ventilation </li></ul><ul><li>Pressure Controlled Ventilation </li></ul><ul><li>Pressure Support Ventilation </li></ul><ul><li>Positive end-expiratory ventilation </li></ul><ul><li>Continuous Positive Airway Pressure </li></ul>
    13. 13. Assist Control Ventilation <ul><li>Volume-cycled lung inflation </li></ul><ul><li>Patient can initiate each mechanical breath or Ventilator </li></ul><ul><li>provides machine breaths at a preselected rate </li></ul><ul><li>Maintain I:E ratio to 1:2 to 1:4. An increase in Peak flow </li></ul><ul><li>decreases the time for lung inflation and increases the I:E </li></ul><ul><li>Ratio </li></ul><ul><li>I:E ratio of <1:2 can cause hyperinflation by air trapping </li></ul><ul><li>Diaphragmatic contraction continues during Assist Control Ventilation and increases the work of breathing. </li></ul>
    14. 14. Assist Control Ventilation <ul><li>Adverse effects: </li></ul><ul><li>In a tachypneic patient>>Lead to overventilation and </li></ul><ul><li>severe respiratory alkalosis>> Hyperinflation and </li></ul><ul><li>Auto-PEEP>> Lead to Electromechanical </li></ul><ul><li>dissociation </li></ul>
    15. 15. Intermittent Mandatory Ventilation <ul><li>Delivers volume cycled breaths at a preselected rate with spontaneous breathing between machine breaths </li></ul><ul><li>Less Alkalosis and Hyperinflation </li></ul><ul><li>Synchronized Intermittent Mandatory Ventilation </li></ul>
    16. 16. Intermittent Mandatory Ventilation <ul><li>Disadvantages: </li></ul><ul><li>Increased work of Breathing: </li></ul><ul><li>Spontaneous breathing through a high resistance circuit </li></ul><ul><li>Solution: Add Pressure support </li></ul><ul><li>Cardiac Output Changes: </li></ul><ul><li>Cardiac O decreased by decreasing ventricular filling </li></ul><ul><li>Cardiac O increased by reducing ventricular afterload </li></ul><ul><li>More significant decrease in patients with Left Ventricular dysfunction </li></ul>
    17. 17. IMV vs. ACV <ul><li>Switch to IMV for: </li></ul><ul><li>Rapid breathers with alkalosis and over- </li></ul><ul><li>Inflation </li></ul><ul><li>Switch to ACV for: </li></ul><ul><li>Patients with respiratory muscle weakness and </li></ul><ul><li>LV dysfunction </li></ul>
    18. 18. Pressure Controlled Ventilation <ul><li>Pressure cycled breathing, fully ventilator controlled </li></ul><ul><li>Inspiratory flow rate decreases exponentially during lung inflation </li></ul><ul><li>(+)Reduces peak airway pressure and improves gas exchange </li></ul><ul><li>(-)Inflation volume varies with changes in mechanical properties of the lungs. </li></ul><ul><li>Suited for patients with neuromuscular diseases and normal lung mechanics </li></ul>
    19. 19. Inverse ratio Ventilation <ul><li>PCV combined with prolonged inflation time </li></ul><ul><li>Inspiratory flow rate is decreased </li></ul><ul><li>I:E ratio reversed to 2:1 </li></ul><ul><li>Helps prevent alveolar collapse </li></ul><ul><li>(-) Hyperinflation, Auto-PEEP and decreased cardiac output </li></ul><ul><li>Use: ARDS with refractory hypoxemia or hypercapnia ?mortality benefit </li></ul>
    20. 20. Pressure Support Ventilation <ul><li>Pressure augmented breathing </li></ul><ul><li>Allows patient to determine the inflation volume and respiratory cycle duration </li></ul><ul><li>Uses: augment inflation during spontaneous breathing or overcome resistance of breathing through ventilator circuits (during weaning) </li></ul><ul><li>Popular an a non-invasive mode of ventilation via nasal or face masks </li></ul>
    21. 21. Positive end-expiratory pressure <ul><li>Alveolar pressure at end-expiration is above atmospheric pressure : PEEP </li></ul><ul><li>Extrinsic PEEP </li></ul><ul><li>Auto PEEP </li></ul>
    22. 22. Positive end-expiratory pressure <ul><li>EXTRINSIC PEEP </li></ul><ul><li>Applied by placing pressure limiting valve in the expiratory limb of ventilator circuit </li></ul><ul><li>Prevents end-expiratory alveolar collapse and recruits collapsed alveoli </li></ul><ul><li>This decreases intrapulmonary shunting, improves gas exchange and improves lung compliance, allowing the FiO2 to be reduced to less toxic levels </li></ul>
    23. 23. Positive end-expiratory pressure <ul><li>Cardiac Performance: </li></ul><ul><li>Greater reduction in cardiac filling and cardiac output (Q), </li></ul><ul><li>irrespective of level of PEEP! </li></ul><ul><li>It is a function of PEEP induced increase in mean </li></ul><ul><li>intrathoracic pressure </li></ul><ul><li>Oxygen transport Do2: </li></ul><ul><li>Do2 = Q X 1.3 X Hb X SaO2 </li></ul><ul><li>Systemic O2 delivery may vary with the effect of PEEP on </li></ul><ul><li>the Cardiac Output. </li></ul>
    24. 24. Positive end-expiratory pressure <ul><li>Best PEEP: Monitor Cardiac Output </li></ul><ul><li>Another measure: Venous Oxygen Saturation </li></ul><ul><li>If VOS decreases after PEEP applied= Drop CO </li></ul><ul><li>Swan-Ganz catheter may be indicated in most patients on PEEP </li></ul>
    25. 25. Positive end-expiratory pressure <ul><li>CLINICAL USES: </li></ul><ul><li>Reduce toxic levels of FiO2 (ARDS not pneumonia) </li></ul><ul><li>Low-volume ventilation </li></ul><ul><li>Obstructive lung disease (Extrinsic=Occult PEEP) </li></ul>
    26. 26. Positive end-expiratory pressure <ul><li>CLINICAL MISUSES: </li></ul><ul><li>Reducing Lung Edema </li></ul><ul><li>Routine PEEP </li></ul><ul><li>Mediastinal Bleeding after CABG </li></ul>
    27. 27. Continuous positive Airway Pressure <ul><li>Spontaneous breathing </li></ul><ul><li>Patient does not need to generate negative pressure to receive inhaled gas </li></ul><ul><li>CPAP replaced spontaneous PEEP </li></ul><ul><li>Use: Non-intubated patients (OSA, COPD) </li></ul>
    28. 28. Occult PEEP <ul><li>Intrinsic or Auto-PEEP or Hyperinflation </li></ul><ul><li>Incomplete alveolar emptying during expiration </li></ul><ul><li>Ventilator Factors: High inflation volumes, rapid rate, low exhalation time </li></ul><ul><li>Disease factors: Asthma, COPD </li></ul><ul><li>Consequences: Decreased CO/EMD, Alveolar rupture, Underestimation of thoracic compliance, increased work of breathing. </li></ul><ul><li>If extrinsic PEEP does not increase Pk, then occult PEEP is present </li></ul>
    29. 29. Complications of Mechanical Ventilation <ul><li>Toxic effects of Oxygen </li></ul><ul><li>Decreased cardiac output </li></ul><ul><li>Pneumonia and sepsis </li></ul><ul><li>Psychological problems </li></ul><ul><li>Ventilator dependence </li></ul>
    30. 30. Complications of Mechanical Ventilation <ul><li>Purulent sinusitis </li></ul><ul><li>Laryngeal Damage </li></ul><ul><li>Aspiration :Value of routine tracheal suctioning </li></ul><ul><li>Tracheal Necrosis (pressure below 20mm water) </li></ul><ul><li>Alveolar rupture: Pneumothorax, pneumomediastinum, subQ emphysema, pneumoperitoneum </li></ul><ul><li>Basilar and sub-pulmonic air collections in the supine position, as seen on X-ray </li></ul>
    31. 31. Liberation from Mechanical Ventilation: Weaning <ul><li>Weaning: Gradual withdrawal of mechanical ventilation </li></ul><ul><li>Misconceptions: </li></ul><ul><li>Duration- longer duration, harder to wean </li></ul><ul><li>Method of weaning determines ability to wean </li></ul><ul><li>Diaphragm weakness is a common cause of failed weaning </li></ul><ul><li>Aggressive nutrition support improves ability to wean </li></ul><ul><li>Removal of ET tube reduces work of breathing </li></ul>
    32. 32. Bedside Weaning Parameters 10ml/kg 65-75ml/kg Vital capacity <10L/min 5-7L/min Minute Ventl. <40/min 14-18/min Resp. Rate 5ml/kg 5-7ml/kg Tidal Volume 200 >400 PaO2/FiO2 Threshold for weaning Normal Adult range Parameter
    33. 33. Bedside Weaning Parameters <100/min/L <50/min/L Rate/Tidal Volume -25cm of water >-90 cm Water (F) >-120 cm water (M) Maximal Inspiratory Pressure
    34. 34. Maximal Inspiratory Pressure <ul><li>Pmax: Excellent negative predictive value if less than –20 (in one study 100% failure to wean at this value) </li></ul><ul><li>An acceptable Pmax however has a poor positive predictive value (40% failure to wean in this study with a Pmax more than –20) </li></ul>
    35. 35. Frequency/Volume ratio <ul><li>Index of rapid and shallow breathing RR/Vt </li></ul><ul><li>Single study results: </li></ul><ul><li>RR/Vt>105 95% wean attempts unsuccessful </li></ul><ul><li>RR/Vt<105 80% successful </li></ul><ul><li>One of the most predictive bedside parameters. </li></ul>
    36. 36. T-Piece Weaning <ul><li>On-off toggle switch that circulates between on and off the ventilator </li></ul><ul><li>Inhaled gas is delivered at a high flow rate </li></ul><ul><li>Varied protocols: like 30min-2hr on and off, or keep as long as possible and if tolerated for >2-4hr…. Deemed successsful (RR, TV, HR, diaphoresis, sat) </li></ul><ul><li>Failed T piece: Resume Vent support till comfortable, 24h </li></ul>vent Airflow with CPAP patient
    37. 37. T-Piece with Ventilator <ul><li>Drawback: increased resistance due to vent tubing and actuator valve in circuit </li></ul><ul><li>Provide minimum pressure support (PSV) :Pmin </li></ul><ul><li>Pmin= PIFR X R </li></ul><ul><li>PIFR is during spontaneous breathing </li></ul><ul><li>R is airflow resistance during mech ventilation </li></ul><ul><li>R= Pk-Pl/Vinsp </li></ul><ul><li>(Vinsp:inspiratory flow rate delivered by the vent) </li></ul>
    38. 38. IMV Weaning <ul><li>Gradual decrease in no of machine breaths in between the spontaneous breaths </li></ul><ul><li>False security: It does not adjust to patient’s ventilatory demands to maintain constant MV </li></ul><ul><li>End point in IMV weaning is the T-piece trial </li></ul><ul><li>Most important to recognize when a patient is capable of spontaneous unassisted breathing </li></ul><ul><li>T-piece more rapid than IMV </li></ul>
    39. 39. Complicating Factors <ul><li>DYSPNEA </li></ul><ul><li>Anxiety and dyspnea are detrimental (low dose haloperidol or morphine) </li></ul><ul><li>CARDIAC OUTPUT </li></ul><ul><li>Increased LV afterload can reduce CO, impair diaphragm function, promote pulmonary edema </li></ul><ul><li>(Use Swan to monitor CO, may use dobutamine) </li></ul><ul><li>ELECTROLYTE DEPLETION </li></ul><ul><li>OVERFEEDING </li></ul>
    40. 40. The Problem Wean <ul><li>RAPID BREATHING : Check TV </li></ul><ul><li>Low TV>> Resume vent support </li></ul><ul><li>TV not low…….. Check arterial pCO2 </li></ul><ul><li>Arterial pCO2 decreased>sedate (anxiety) </li></ul><ul><li>Arterial pCO2 not decreased> Resume vent </li></ul>
    41. 41. The Problem Wean <ul><li>ABDOMINAL PARADOX </li></ul><ul><li>Inward displacement of the diaphragm during inspiration is a sign of diaphragmatic muscle fatigue </li></ul><ul><li>HYPOXEMIA </li></ul><ul><li>May be due to low CO and MVO2 </li></ul><ul><li>HYPERCAPNIA </li></ul><ul><li>Increase in PaCO2-PetCO2: increase dead space ventilation </li></ul><ul><li>Unchanged gradient: Respiratory muscle fatigue or enhanced CO2 production </li></ul>
    42. 42. Tracheal Decannulation <ul><li>Successful weaning is not synonymous with tracheal decannulation </li></ul><ul><li>If weaned and not fully awake or unable to clear secretions, leave ETT in place </li></ul><ul><li>Contrary to popular belief, tracheal decannulation increases the work of breathing due to laryngeal edema and secretions </li></ul><ul><li>Do not perform tracheal decannulation to reduce work of breathing </li></ul>
    43. 43. Inspiratory Stridor <ul><li>Post extubation inspiratory stridor is a sign of severe obstruction and should prompt reintubation </li></ul><ul><li>Laryngeal edema (post-ext) may respond to aerosolized epinephrine in children </li></ul><ul><li>Steroids have no role </li></ul><ul><li>Most need reintubation followed by tracheostomy </li></ul>
    44. 45. ARDS and Low Volume Ventilation <ul><li>ARDS Network trial : NEJM May 4, 2000 p1301-08 </li></ul><ul><li>Traditional: TV 10-15ml/kg, keep plateau<50cm water </li></ul><ul><li>Low TV ventilation: TV 6ml/kg, keep plateau<30cm water </li></ul><ul><li>Need high RR in Low TV group to prevent acidosis </li></ul><ul><li>Permissive hypercapnia tolerated well, if needed, use IV bicarb to maintain pH </li></ul><ul><li>May add PEEP in addition to the low TV group to prevent atelectrauma (open-close alveoli>> alveolar fracture) </li></ul><ul><li>Results: Lower mortality in the Low TV group (31% v/s 39.8% p<0.007); Higher days without vent use and lower average plateau pressures in low TV group. </li></ul>

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