A volume-controlled liquid ventilator with pressure-limit mode: Imperative expiratory control

Liquid ventilation with perfluorocarbon (PFC) has been considered to offer advantages over gas ventilation to respiratory distress syndrome patients. We developed a volume-controlled liquid ventilator with pressure-limit mode; inspiration is performed mechanically with an actuator under the preset limit of the intratracheal pressure (Paw): expiration is performed by gravity assistance. Oxygenation and CO2 removal of PFC are done with a membrane oxygenator. An endotracheal tube with a Paw monitor line was placed in 5 rabbits weighing 2.7 +/- 0.6 kg, and liquid Ventilation was conducted with the condition that the upper and lower limits of Paw were 30 and -20 mm HE, respectively. The best arterial pH and gas tension were examined. The averaged arterial pH, PaO2, PaCO2, and SaO(2) were 7.45 mm Hg, 369 mm Hg, 46.2 mm Hg, and 100% at the best values, respectively. Ventilatory conditions at the best values were as follows: ventilation rates, tidal volume peak Paw, average Paw, and trough Paw were 5-15 (11 +/- 4) times/min, 13.3-17.3 (15.6 +/- 1.4) ml/kg, 5-18 (12 +/- 5) mm Hg, -7-4 (-1 +/- 4) mm Hg, and -20--6 (-13 +/- 5) mm Hg, respectively, Pressure-limit control of the system worked well, but in the initial 3 animals, fluorothrax, that is the leakage of PFC into thoracic cavity, was recognized at the Paw from 20 to 25 mm Hg after the upper pressure limit was raised to 25 mm Hg to improve PaCO2. The fluorothrax seemed to be caused by excess end-expiratory residual volume. An expiratory control mechanism appears to be imperative for further improvement of our liquid ventilator.