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Pulmonary Mechanics and Graphics during Mechanical Ventilation

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Pulmonary Mechanics and Graphics during Mechanical Ventilation

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  1. Pulmonary Mechanics and Graphics during Mechanical Ventilation

  2. Mechanics: Expression of lung function through measures of pressure and flow: Derived parameters: volume, compliance, resistance, work Graphics: Plotting one parameter as a function of time or as a function of another parameter P - T , F - T , V – T F - V , P - V Definition

  3. Objectives • Evaluate lung function • Assess response to therapy • Optimize mechanical support

  4. Exponential Decay y 37 13.5 5 TC y = y0 . e (-t / TC)

  5. Exponential Rise y 95 86.5 63 TC y = yf . (1 - e (-t / TC))

  6. Time Constant () • Time required for rise to 63% • Time required for fall to 37% • In Pul. System= Compliance• Resistance  = (0.05 to 0.1) • 10 = 0.5 – 1 sec

  7. Airway Pressure • Equation of MotionPaw = V(t) / C + R . V(t) + PEEP + PEEPi •

  8. Airway PressureSites of Measurement • Directly at proximal airway • At the inspiratory valve • At the expiratory valve

  9. Airway PressureSites of Measurement • Directly at proximal airway • The best approximation • Technical difficulty • Hostile environment

  10. Airway PressureSites of Measurement • Directly at proximal airway • At the inspiratory valve To approximate airway pressure duringexpiration

  11. Airway PressureSites of Measurement • Directly at proximal airway • At the inspiratory valve • At the expiratory valve To approximate airway pressure duringinspiration

  12. A typical airway pressure waveform Volume ventilation PIP PPlat Linear increase End-exp. Pause (Auto-PEEP) Initial rise

  13. Peak Alveolar Pressure (Pplat) • Palv can not be measured directly • If flow is present, during inspiration:Paw > Pplat Measurement by end-inspiratory hold

  14. Peak Inspiratory Pressure (PIP) PPlat PZ Pressure at Zero Flow

  15. Peak Alveolar Pressure (Pplat)Uses • Prevention of overinflationPplat  34 cmH2O • Compliance calculationCStat = VT / (PPlat – PEEP) • Resistance calculationRI = (PIP – PPlat) / VI

  16. Auto-PEEP • Short TE air entrapment • Auto-PEEP = The averaged pressure by trapped gas in different lung units • TE shorter than 3 expiratory time constant • So it is a potential cause of hyperinflation

  17. Auto-PEEPEffects • Overinflation • Failure to trigger • Barotrauma

  18. Auto-PEEP Measurement technique

  19. Auto-PEEPInfluencing factors • Ventilator settings: RR – VT – TPlat – I:E – TE • Lung function: Resistance – Compliance • auto-PEEP = VT / (C · (eTe/ – 1))Te = Exp. Time ,  = Exp. Time constant , C = Compliance

  20. Esophageal Pressure • In the lower third(35– 40cm, nares) • Fill then remove all but 0.5 – 1 ml • Baydur maneuver, cardiac oscillation • Pleural pressure changes • Work of breathing • Chest wall compliance • Auto-PEEP

  21. Esophageal PressureAuto-PEEP Measurement • Airway flow & esophageal pressure trace • Auto-PEEP = Change in esophageal pressure to reverse flow direction • Passive exhalation

  22. Esophageal Pressure Auto-PEEP Measurement Flow Peso

  23. FlowInspiratory Volume ventilation • Value by Peak Flow Rate button • Waveform by Waveform select button

  24. FlowInspiratory Pressure ventilation • Value : V = (P / R) · (e-t / ) • Waveform: ·

  25. FlowExpiratory • Palv , RA ,  • V = –(Palv / R) · (e-t / ) ·

  26. Flow waveformapplication • Detection of Auto-PEEP 1) Expiratory waveform not return to baseline (no quantification) 2) May be falsely negative Flow at end-expiration

  27. Flow waveformapplication • Dips in exp. flow during assisted ventilation or PSV: Insufficient trigger effort Auto-PEEP Inspiratory effort

  28. Volume • Measurement: Integration of expiratory flow waveform

  29. Compliance • VT divided by the pressure required to produce that volume:C = V / P = VT / (Pplat – PEEP) • Range in mechanically ventilated patients:50 – 100 ml/cmH2O • 1 / CT = 1 / Ccw + 1 / CL

  30. Chest wall compliance(Ccw) • Changes in Peso during passive inflation • Normal range: 100 – 200 ml/cmH2O 400 ml

  31. Chest wall complianceDecrease • Abdominal distension • Chest wall edema • Chest wall burn • Thoracic deformities • Muscle tone

  32. Chest wall complianceIncrease • Flail Chest • Muscle paralysis

  33. Lung compliance • VTdivided by transpulmonary pressure (PTP) • PTP = Pplat – Peso • Normal range : 100 – 200 ml/cmH2O 30 cmH2O PTP = Pplat – Peso= 30 – 17 = 13 17 cmH2O

  34. Pulmonary edema ARDS Pneumothorax Consolidation Atelectasis Pulmonary fibrosis Pneumonectomy Bronchial intubation Hyperinflation Pleural effusion Abdominal distension Chest wall deformity Lung complianceDecrease

  35. Airway resistance • Volume ventilationRI = (PIP – PPlat) / VIRE = (Pplat – PEEP) / VEXP • Intubated mechanically ventilatedRI 10 cmH2O/L/secRE > RI · ·

  36. Airway resistanceIncreased • Bronchospasm • Secretions • Small ID tracheal tube • Mucosal edema

  37. Mean Airway Pressure • Beneficial and detrimental effects of IPPV • Direct relationship to oxygenation • Time average of pressures in a cycle • Pressure ventilation (PIP – PEEP) · (TI / Ttot) + PEEP • Volume ventilation 0.5 · (PIP – PEEP) · (TI / Ttot) + PEEP

  38. Mean Airway Pressure  14 cmH2O

  39. Mean Airway PressureTypical values • Normal lung : 5 – 10 cmH2O • ARDS : 15 – 30 cmH2O • COPD : 10 – 20 cmH2O

  40. Pressure-Volume Loop • Static elastic forces of the respiratory system independent of the dynamic and viscoelastic properties • Super-syringe technique • Constant flow inflation • Lung and chest wall component • Chest wall PV: Volume vs. Peso • Lung PV: Volume vs. PTP

  41. PV Loop • Normal shape: Sigmoidal • Hysteresis: Inflation vs. deflation • In acute lung injury:Initial flat segment – LIP – Linear portion – UIP • LIP = Closing volume in normal subjects • UIP = Overdistension • Best use of PV loop: To guide ventilator management PEEP > LIP , Pplat < UIP

  42. Normal PV Loop

  43. PV Loop in Acute Lung Injury UIP LIP

  44. PEEP > LIP , Pplat < UIP • Reduce ventilator associated lung injury • Prevention of overinflation • Increased recruitment of collapsed units • Lower incidence of barotrauma • Higher weaning rate • Higher survival rate

  45. PV LoopRole of chest wall component • Effect on LIP and UIP • PV loop for lung alone: Use of Peso • LIP underestimates the necessary PEEP • Better results with PEEP set above LIP on deflation PV loop rather inflation

  46. Volume Ventilation Parameters Interaction Run VVPI Program