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Pulmonary Mechanics and Graphics during Mechanical Ventilation.

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Presentation on theme: "Pulmonary Mechanics and Graphics during Mechanical Ventilation."— Presentation transcript:


2 Pulmonary Mechanics and Graphics during Mechanical Ventilation

3 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

4 Objectives Evaluate lung function Assess response to therapy Optimize mechanical support

5 Exponential Decay y = y 0. e (-t / TC) y TC

6 y = y f. (1 - e (-t / TC) ) y TC Exponential Rise

7 Time Constant ( ) Time required for rise to 63% Time required for fall to 37% In Pul. System = C ompliance Resistance = (0.05 to 0.1) 10 = 0.5 – 1 sec

8 Equation of Motion P aw = V(t) / C + R. V(t) + PEEP + PEEP i Airway Pressure

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

10 Airway Pressure Sites of Measurement Directly at proximal airway 1)The best approximation 2)Technical difficulty 3)Hostile environment

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

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

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

14 Peak Alveolar Pressure (P plat ) P alv can not be measured directly If flow is present, during inspiration: P aw > P plat Measurement by end-inspiratory hold

15 Peak Inspiratory Pressure (PIP) PZPZ Pressure at Zero Flow P Plat

16 Peak Alveolar Pressure (P plat ) Uses Prevention of overinflation P plat 34 cmH2O Compliance calculation C Stat = V T / (P Plat – PEEP) Resistance calculation R I = (PIP – P Plat ) / V I

17 Auto-PEEP Short T E air entrapment Auto-PEEP = The averaged pressure by trapped gas in different lung units T E shorter than 3 expiratory time constant So it is a potential cause of hyperinflation

18 Auto-PEEP Effects Overinflation Failure to trigger Barotrauma

19 Auto-PEEP Measurement technique

20 Auto-PEEP Influencing factors Ventilator settings: RR – V T – T Plat – I:E – T E Lung function: Resistance – Compliance auto-PEEP = V T / (C · (e Te/ – 1)) Te = Exp. Time, = Exp. Time constant, C = Compliance

21 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

22 Esophageal Pressure Auto-PEEP Measurement Airway flow & esophageal pressure trace Auto-PEEP = Change in esophageal pressure to reverse flow direction Passive exhalation

23 Flow P eso Esophageal Pressure Auto-PEEP Measurement

24 Flow Inspiratory Volume ventilation Value by Peak Flow Rate button Waveform by Waveform select button

25 Pressure ventilation Value : V = ( P / R) · (e -t / ) Waveform: Flow Inspiratory ·

26 P alv, R A, V = –(P alv / R) · (e -t / ) Flow Expiratory ·

27 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

28 Flow waveform application Dips in exp. flow during assisted ventilation or PSV: Insufficient trigger effort Inspiratory effort Auto- PEEP

29 Volume Measurement: Integration of expiratory flow waveform

30 Compliance V T divided by the pressure required to produce that volume: C = V / P = V T / (P plat – PEEP) Range in mechanically ventilated patients: 50 – 100 ml/cmH2O 1 / C T = 1 / C cw + 1 / C L

31 Changes in P eso during passive inflation Normal range: 100 – 200 ml/cmH2O Chest wall compliance (C cw ) 400 ml

32 Chest wall compliance Decrease Abdominal distension Chest wall edema Chest wall burn Thoracic deformities Muscle tone

33 Chest wall compliance Increase Flail Chest Muscle paralysis

34 Lung compliance V T divided by transpulmonary pressure (P TP ) P TP = P plat – P eso Normal range : 100 – 200 ml/cmH2O 30 cmH2O 17 cmH2O P TP = P plat – P eso = 30 – 17 = 13

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

36 Volume ventilation RI RI = (PIP – P Plat ) / VIVI RE RE = (P plat – PEEP) / V EXP Intubated mechanically ventilated RI RI 10 cmH2O/L/sec RE RE > RIRI Airway resistance · ·

37 Airway resistance Increased Bronchospasm Secretions Small ID tracheal tube Mucosal edema

38 Beneficial and detrimental effects of IPPV Direct relationship to oxygenation Time average of pressures in a cycle Pressure ventilation (PIP – PEEP) · (T I / T tot ) + PEEP Volume ventilation 0.5 · (PIP – PEEP) · (T I / T tot ) + PEEP Mean Airway Pressure

39 Mean Airway Pressure 14 cmH2O

40 Mean Airway Pressure Typical values Normal lung : 5 – 10 cmH2O ARDS : 15 – 30 cmH2O COPD : 10 – 20 cmH2O

41 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. P eso Lung PV: Volume vs. P TP

42 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, P plat < UIP PV Loop

43 Normal PV Loop

44 LIP UIP PV Loop in Acute Lung Injury

45 PEEP > LIP, P plat < UIP Reduce ventilator associated lung injury Prevention of overinflation Increased recruitment of collapsed units Lower incidence of barotrauma Higher weaning rate Higher survival rate

46 PV Loop Role of chest wall component Effect on LIP and UIP PV loop for lung alone: Use of P eso LIP underestimates the necessary PEEP Better results with PEEP set above LIP on deflation PV loop rather inflation

47 Volume Ventilation Parameters Interaction Run VVPI Program


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