Pulmonary Mechanics and Graphics during Mechanical Ventilation Definition

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 y = y 0. e (-t / TC) TC

Exponential Rise y 95 86. 5 63 y = yf . (1 -e (-t / TC)) TC

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

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

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

Airway Pressure Sites of Measurement • 1) 2) 3) 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 Linear increase Initial rise PPlat End-exp. Pause (Auto-PEEP)

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 cm. H 2 O • 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

Auto-PEEP Measurement technique

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

Esophageal Pressure • • In the lower third(35– 40 cm, 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

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 endexpiration

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/cm. H 2 O • 1 / CT = 1 / Ccw + 1 / CL

Chest wall compliance (Ccw) • Changes in Peso during passive inflation • Normal range: 100 – 200 ml/cm. H 2 O 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/cm. H 2 O 30 cm. H 2 O PTP = Pplat – Peso= 30 – 17 = 13 17 cm. H 2 O

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

Airway resistance • Volume ventilation · RI = (PIP – PPlat) / VI · RE = (Pplat – PEEP) / VEXP • Intubated mechanically ventilated RI 10 cm. H 2 O/L/sec RE > R I

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 cm. H 2 O

Mean Airway Pressure Typical values • Normal lung : 5 – 10 cm. H 2 O • ARDS : 15 – 30 cm. H 2 O • COPD : 10 – 20 cm. H 2 O

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