Technique to determine inspiratory impedance during mechanical ventilation: implications for flow limited patients

We present the design of an enhanced ventilator waveform (EVW) for routine measurement of inspiratory resistance (R) and elastance (E) spectra in ventilator-dependent and/or severely obstructed flow-limited patients. The EVW delivers an inspiratory tidal volume of fresh gas with a flow pattern consisting of multiple sinusoids from 0.156 to 8.1 Hz and permits a patient-driven exhalation to the atmosphere or positive end-expiratory pressure. Weighted least-squares estimates of the coefficients in a sinusoidal series approximation of the EVW inspirations yielded inspiratory R and E spectra. We first validated the EVW approach using simulated pressure and flow data under different physiological conditions, noise levels, and harmonic distortions. We then applied the EVW in four intubated patients during anesthesia and paralysis: two with mild airway obstruction and two with severe emphysema and flow limitation. While the level of inspiratory R was similar in both groups of patients, the inspiratory E of the emphysematous patients demonstrated a pronounced frequency-dependent increase consistent with severe peripheral airway obstruction. We conclude that the EVW offers a potentially practical and efficient approach to monitor lung function in ventilator-dependent patients, especially those with expiratory flow limitation.