MECHANICAL VENTILATIONlThings “I” wish I knew when I was an InternAmit Gupta, MD Internal Medicine North Mississippi Medical CenterMechanical VentilationlIndications for Intubation and VentilationlPrinciples of Mechanical VentilationlPatterns of Assisted VentilationlVentilator Dependence: Complications lLiberation from Mechanical Ventilation: WeaninglTroubleshootinglArterial Blood GasesIndications for Mechanical Ventilationl“.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.”-Andreas Vesalius (1555)Indications for Mechanical Ventilation1. “Thinking” of Intubation: elective v/s emergent 2. “Act of weakness?” 3. Endotracheal tubes are not a disease and ventilators are not an addiction 4. And the usual elective and emergent indications that you all know!Objectives of Mechanical VentilationImprove pulmonary gas exchange Reverse hypoxemia and Relieve acute respiratory acidosis Relieve respiratory Distress Decrease oxygen cost of breathing and reverse respiratory muscle fatigue Alter pressure-volume relations Prevent and reverse atelectasis Improve Compliance Prevent further injury Permit lung and airway healing Avoid complicationsStrategies for Mechanical VentilationVentilatory ParameterTraditionalLung-ProtectiveInflation Volume10-15 ml/kg5-10 ml/kgEnd-insp. pressurePeak PrLead to overventilation and severe respiratory alkalosis Hyperinflation and Auto-PEEP Lead to Electromechanical dissociationIntermittent Mandatory VentilationlDelivers volume cycled breaths at a preselected rate with spontaneous breathing between machine breathslLess Alkalosis and HyperinflationlSynchronized IMVIntermittent Mandatory VentilationDisadvantages: Increased work of Breathing: Spontaneous breathing through a high resistance circuit Solution: Add Pressure support Cardiac Output Changes: C O decreased by decreasing ventricular filling C O increased by reducing ventricular afterload More significant decrease in patients with LV dysfunctionIMV vs. ACVSwitch to IMV for: Rapid breathers with alkalosis and over- Inflation Switch to ACV for: Patients with respiratory muscle weakness andLV dysfunctionPressure Controlled VentilationlPressure cycled breathing, fully ventilator controlledlInspiratory flow rate decreases exponentially during lung inflationl(+)Reduces peak airway pressure and improves gas exchangel(-)Inflation volume varies with changes in mechanical properties of the lungs.lSuited for patients with neuromuscular diseases and normal lung mechanicsInverse ratio VentilationlPCV combined with prolonged inflation timelInspiratory flow rate is decreasedlI:E ratio reversed to 2:1lHelps prevent alveolar collapsel(-) Hyperinflation, Auto-PEEP and decreased cardiac outputlUse: ARDS with refractory hypoxemia or hypercapnia ?mortality benefitPressure Support VentilationlPressure augmented breathinglAllows patient to determine the inflation volume and respiratory cycle durationlUses: augment inflation during spontaneous breathing or overcome resistance of breathing through ventilator circuits (during weaning)lPopular an a non-invasive mode of ventilation via nasal or face masksPositive end-expiratory pressurelAlveolar pressure at end-expiration is above atmospheric pressure : PEEPlExtrinsic PEEPlAuto PEEPPositive end-expiratory pressurelEXTRINSIC PEEPlApplied by placing pressure limiting valve in the expiratory limb of ventilator circuitlPrevents end-expiratory alveolar collapse and recruits collapsed alveolilThis decreases intrapulmonary shunting, improves gas exchange and improves lung compliance, allowing the FiO2 to be reduced to less toxic levelsPositive end-expiratory pressureCardiac Performance: Greater reduction in cardiac filling and cardiac output (Q),irrespective of level of PEEP! It is a function of PEEP induced increase in mean intrathoracic pressure Oxygen transport Do2: Do2 = Q X 1.3 X Hb X SaO2 Systemic O2 delivery may vary with the effect of PEEP on the Cardiac Output.Positive end-expiratory pressurelBest PEEP: Monitor Cardiac OutputlAnother measure: Venous Oxygen SaturationlIf VOS decreases after PEEP applied= Drop COlSwan-Ganz catheter may be indicated in most patients on PEEPPositive end-expiratory pressurelCLINICAL USES:lReduce toxic levels of FiO2 (ARDS not pneumonia)lLow-volume ventilationlObstructive lung disease (Extrinsic=Occult PEEP)Positive end-expiratory pressurelCLINICAL MISUSES:lReducing Lung EdemalRoutine PEEPlMediastinal Bleeding after CABGContinuous positive Airway PressurelSpontaneous breathinglPatient does not need to generate negative pressure to receive inhaled gaslCPAP replaced spontaneous PEEPlUse: Non-intubated patients (OSA, COPD)Occult PEEPlIntrinsic or Auto-PEEP or HyperinflationlIncomplete alveolar emptying during expirationlVentilator Factors: High inflation volumes, rapid rate, low exhalation timelDisea