Comparison of high-frequency negative-pressure oscillation with conventional mechanical ventilation in normal and saline-lavaged cats.

We modified a negative-pressure respirator to produce high-frequency, subatmospheric pressure oscillations around the chest. The effects of negative-pressure oscillation (NPO) on gas exchange, lung volume, compliance and cardiovascular variables were compared to those of conventional intermittent positive-pressure ventilation (IPPV) at 30 breath/min, using cats with normal and surfactant-depleted lungs. For frequencies in both normal lungs (1, 3, 4, 5, and 7 Hz) and saline-lavaged lungs (3, 5, and 7 Hz), peak inflating pressures were lower during NPO. Oxygenation was similar for both modes of ventilation. In cats with normal lungs, PaCO2 was significantly lower during NPO at 3 and 4 Hz (mean 24 torr) than during IPPV (mean 30 torr); normocapnia was maintained at the other frequencies. In damaged lungs, NPO and IPPV at 3 and 5 Hz resulted in similar CO2 removal, but PaCO2 was significantly higher during NPO at 7 Hz. Oscillatory tidal volumes decreased with increasing frequencies: in normal lungs, mean oscillatory tidal volume was 4.4 ml/kg at 1 Hz and 2.3 ml/kg at 7 Hz; in damaged lungs it was 6.5 ml/kg at 3 Hz and 3.2 ml/kg at 7 Hz. At 3 Hz and above, NPO was associated with a larger functional residual capacity than during sequences of IPPV matched for end-expiratory transthoracic pressure. There were no significant differences in respiratory system compliance, cardiac output, and pulmonary vascular resistance between both modes of ventilation. Further studies are warranted to investigate the potential clinical usefulness of NPO.