Pulmonary Mechanics and Graphics during Mechanical Ventilation

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

Objectives Evaluate lung function Assess response to therapy Optimize mechanical support

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

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

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

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

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

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

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

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

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

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

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

Peak Alveolar Pressure (Pplat) Uses Prevention of overinflation Pplat  34 cmH2O Compliance calculation CStat = VT / (PPlat – PEEP) Resistance calculation RI = (PIP – PPlat) / VI

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

Auto-PEEP Effects Overinflation Failure to trigger Barotrauma

Measurement technique Auto-PEEP Measurement technique

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

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

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

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

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

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

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

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

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

Volume Measurement: Integration of expiratory flow waveform

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

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

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

Chest wall compliance Increase Flail Chest Muscle paralysis

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

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

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

Airway resistance Increased Bronchospasm Secretions Small ID tracheal tube Mucosal edema

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

Mean Airway Pressure  14 cmH2O

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

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

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

Normal PV Loop

PV Loop in Acute Lung Injury UIP LIP

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

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

Volume Ventilation Parameters Interaction Run VVPI Program