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 vent ilator-dependent and/or severely obstructed flow-limited patients. The EVW delivers an inspiratory tidal volume of fresh gas with a flow pattern consi sting of multiple sinusoids from 0.156 to 8.1 Hz and permits a patient-driv en exhalation to the atmosphere or positive end-expiratory pressure. Weight ed least-squares estimates of the coefficients in a sinusoidal series appro ximation of the EVW inspirations yielded inspiratory Ii and E spectra. We f irst validated the EVW approach using simulated pressure and flow data unde r different physiological conditions, noise levels, and harmonic distortion s. 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 Ii was similar in both groups of patients, the inspiratory E of the emphysematous patients demonst rated a pronounced frequency-dependent increase consistent with severe peri pheral airway obstruction. We conclude that the EVW offers a potentially pr actical and efficient approach to monitor lung function in ventilator-depen dent patients, especially those with expiratory how limitation. (C) 1999 Bi omedical Engineering Society. [S0090-6964(99)02203-1].