Objective In surfactant depleted cats, proportional assist ventilation (PAV) has been shown to increase the tidal volume (VT) and decrease the breathing effort, in terms of reduced phrenic nerve activity (PNA), in comparison to continuous positive airway pressure (CPAP). Our aim was to study PNA during PAV in the transition from assist control ventilation (A/C) to CPAP in healthy cats, thereby simulating a clinical setting of weaning.

Methods Five anesthetized, endotracheally intubated healthy cats were exposed to interposed periods of CPAP, A/C (peak-inspiratory pressure 0.73±0.14 kPa) and PAV (set to compensate for 60% of the elastic recoil) with the same end-expiratory pressure (0.2 kPa). The PIP during A/C was adjusted to obtain the same VT as during CPAP.

Results With comparable blood gases and VT (A/C 14.75±2.27 vs PAV 13.00±1.66 ml/kg; NS), both a higher PNA duration and amplitude was observed during PAV (1.44±0.11 sec; 0.41±0.13 AU) than during A/C ( 1.20±0.09 sec; 0.22±0.06 AU; p = 0.0003 and p = 0.03 respectively). Compared to both A/C and PAV, VT was slightly lower during CPAP (12.13±1.63 ml/kg; p<0.05) in the presence of higher PNA (1.61±0.10 sec; 0.63±0.21 AU; p<0.035). No difference in transpulmonary pressure (PTP) was detected between the three modes.

Conclusion With similar VT, PTP, and blood gases as during A/C and CPAP, PAV stimulates higher breathing activity than A/C but lower breathing activity than CPAP, suggesting that PAV might be an optimal mode of weaning from A/C to CPAP in normal lung conditions.