Inhibition of breathing after surfactant depletion is achieved at a higher arterial PCO2 during ventilation with liquid than with gas.

Background: Inhibition of phrenic nerve activity (PNA) can be achieved when alveolar ventilation is adequate and when stretching of lung tissue stimulates mechanoreceptors to inhibit inspiratory activity. During mechanical ventilation under different lung conditions, inhibition of PNA can provide a physiological setting at which ventilatory parameters can be compared and related to arterial blood gases and pH.

Objective: To study lung mechanics and gas exchange at inhibition of PNA during controlled gas ventilation (GV) and during partial liquid ventilation (PLV) before and after lung lavage.

Methods: Nine anaesthetised, mechanically ventilated young cats (age 3.8 +/- 0.5 months, weight 2.3 +/- 0.1 kg) (mean +/- SD) were studied with stepwise increases in peak inspiratory pressure (PIP) until total inhibition of PNA was attained before lavage (with GV) and after lavage (GV and PLV). Tidal volume (Vt), PIP, oesophageal pressure and arterial blood gases were measured at inhibition of PNA. One way repeated measures analysis of variance and Student Newman Keuls-tests were used for statistical analysis.

Results: During GV, inhibition of PNA occurred at lower PIP, transpulmonary pressure (Ptp) and Vt before than after lung lavage. After lavage, inhibition of inspiratory activity was achieved at the same PIP, Ptp and Vt during GV and PLV, but occurred at a higher PaCO2 during PLV. After lavage compliance at inhibition was almost the same during GV and PLV and resistance was lower during GV than during PLV.

Conclusion: Inhibition of inspiratory activity occurs at a higher PaCO2 during PLV than during GV in cats with surfactant-depleted lungs. This could indicate that PLV induces better recruitment of mechanoreceptors than GV.