Inspiratory lung impedance in COPD: effects of PEEP and immediate impact of lung volume reduction surgery

Frequency-dependent characteristics of lung resistance (RL) and elastance ( EL) are sensitive to different patterns of airway obstruction. We used an e nhanced ventilator waveform (EVW) to measure inspiratory RL and EL spectra in ventilated patients during thoracic surgery. The EVW delivers an inspira tory flow waveform with enhanced spectral excitation from 0.156 to 8.1 Hz. Estimates of the coefficients in a trigonometric approximation of the EVW f low and transpulmonary pressure inspirations yielded inspiratory RL and EL spectra. We applied the EVW in a group with mild obstruction undergoing var ious thoracoscopic procedures (n = 6), and another group with severe chroni c obstructive pulmonary disease undergoing lung volume reduction surgery (n = 8). Measurements were made at positive end-expiratory pressure (PEEP) of 0, 3, and 6 cmH(2)O. Inspiratory RL was similar in both groups despite mar ked differences in spirometry. The chronic obstructive pulmonary disease pa tients demonstrated a pronounced frequency-dependent increase in inspirator y EL consistent with severe heterogeneous peripheral airway obstruction. PE EP appears to have beneficial effects by reducing peripheral airway resista nce. Lung volume reduction surgery resulted in increased inspiratory RL and EL at all frequencies and PEEPs, possibly due to loss of diseased lung tis sue, pulmonary edema, increased mechanical heterogeneity, and/or an improve ment in airway tethering.