Assisted mechanical ventilation using combined elastic and resistive unloading in cats with severe respiratory failure: effects on gas exchange and phrenic nerve activity

This study tests thr efficacy of respiratory mechanical unloading as a mode of assisted mechanical ventilation in cats with an intact breathing-contro l system but severe pulmonary parenchymal injury. Twelve anaesthetized, int ubated cats received multiple saline lung lavages so that their total respi ratory system compliance decreased from 56.1 +/- 10.4 to 26.8 +/- 6.8 ml/kP a (p < 0.001) and their PaO2 fell to 12.38 +/- 4.71 kPa when 100% O-2 was u sed as inspired gas. They were then exposed to three consecutive 15-min per iods of CPAP of 0.5 kPa, respiratory unloading and again CPAP of 0.5 kPa. U nloading was applied with end-expiratory pressure of 0.5 kPa, elastic assis tance of 0.03 kPa/ml and resistance compensation of 2.0 kPa/l/s, Arterial b lood gases for the CPAP baselines did not differ significantly before and a fter unloading: FH 7.14 +/- 0.03 vs. 7.16 +/- 0.06; PaCO2 8.99 +/- 2.07 vs. 8.33 +/- 2.01 kPa; PaO2 12.4 +/- 4.7 vs. 13.3 +/- 7.6 kPa. Nor did the bas elines differ in terms of tidal volume, respiratory rate and phrenic nerve activity. Unloading increased tidal volume substantially by about 50% and i ncreased respiratory rate slightly, while inspiratory time remained unchang ed, PaCO2 fell to 6.63 +/- 1.57 kPa and pH rose to 7.25 +/- 0.06. Phrenic n erve activity was significantly down-regulated in terms of total number of impulses and mean impulse frequency in the phrenic nerve burst. These resul ts suggest that combined elastic and resistive unloading may be an effectiv e means of assisted mechanical ventilation in severe respiratory failure of pulmonary parenchymal origin.