A pulse method of measuring respiratory system compliance in ventilated patients

We describe a method of measuring total static respiratory system compliance (Crs) in ventilated patients during inflation, which appears to detect relaxation of respiratory muscles and does not require an end-inspiratory pause or disconnection of a constant-flow intermittent mandatory ventilation (IMV) circuit. Flow is measured with a pneumotachometer attached to the endotracheal tube. Transthoracic pressure is taken as the difference between mouth pressure measured at the proximal pneumotachometer port and body surface (atmospheric) pressure. Flow and transthoracic pressure are displayed on separate channels of a strip chart recorder. The ventilator is adjusted to deliver a constant rule of air flow. When inflation begins, the pressure tracing shows an initial step rise related to the flow resistance of the subject followed by a section with a slower rise and a constant slope. Respiratory system compliance is calculated by dividing the flow rate in L/sec by the slope of the pressure tracing in cm H2O/sec. Pulse Crs was compared with static Crs measured with an end-inspiratory pause in nine subjects receiving mechanical ventilation. Correlation between pulse Crs and static Crs in nine ventilated patients was highly significant (4 = .997, pulse Crs = 1.00 static Crs + 0.001). We conclude that with the pulse method, one can measure static Crs during inflation without an inspiratory pause and without disconnecting an IMV circuit.