1. Pulmonary Mechanics and Graphics during Mechanical Ventilation

2. Definition 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

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 ()  = (0.05 to 0.1) • 10 = 0.5 – 1 sec
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 Motion Paw = V(t) / C + R . V(t) + PEEP + PEEPi •

8. Airway Pressure Sites of Measurement
Directly at proximal airway
At the inspiratory valve
At the expiratory valve

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

10. Airway Pressure Sites of Measurement
Directly at proximal airway
At the inspiratory valve
To approximate airway pressure during expiration

11. Airway Pressure Sites of Measurement
Directly at proximal airway
At the inspiratory valve
At the expiratory valve
To approximate airway pressure during inspiration

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 overinflation Pplat  34 cmH2O
Compliance calculation CStat = VT / (PPlat – PEEP)
Resistance calculation RI = (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-PEEP Effects
Overinflation
Failure to trigger
Barotrauma

18. Measurement technique
Auto-PEEP
Measurement technique

19. Auto-PEEP Influencing 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 Pressure Auto-PEEP Measurement
Airway flow & esophageal pressure trace
Auto-PEEP = Change in esophageal pressure to reverse flow direction
Passive exhalation

22. Auto-PEEP Measurement
Esophageal Pressure
Auto-PEEP Measurement
Flow
Peso

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

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

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

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

27. Flow waveform application
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 compliance Decrease
Abdominal distension
Chest wall edema
Chest wall burn
Thoracic deformities
Muscle tone

32. Chest wall compliance Increase
Flail Chest
Muscle paralysis

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

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

35. Airway resistance
Volume ventilation RI = (PIP – PPlat) / VI RE = (Pplat – PEEP) / VEXP
Intubated mechanically ventilated RI  10 cmH2O/L/sec RE > RI

36. Airway resistance Increased
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 Pressure Typical 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 Loop Role 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