The effect of intratracheal pulmonary ventilation on the decrease of dead space in rabbits with acute respiratory failure.

Abstract

OBJECTIVE: A technique that improves the efficiency of alveolar ventilation should decrease the pressure required and reduce the potential for lung injury during mechanical ventilation. High partial pressure of carbon dioxide (PaCO2) can be tolerated if associated with a lower airway pressure as in permissive hypercapnia (PH). Intratracheal pulmonary ventilation (ITPV) was developed to allow a decrease in physiological dead space during mechanical ventilation. We compared the effect of hybrid ventilation (HV) as a modification of ITPV with PH on the decrease of tidal volume and airway pressures in rabbits with acute respiratory failure. METHODOLOGY: Tracheostomy was performed in seven rabbits ventilated under volume-controlled mode in the supine position. Arterial blood gas analysis, airway pressures, and dead space ventilation were measured at respiratory rates of 20/min as control values. Oleic acid (OA) of 0.06 mL/kg was injected to induce acute respiratory failure. Tidal volume (V(T)) was elevated to maintain PaCO2 in the normal range. These same parameters were measured as OA values. Then V(T) was reduced to the control level to allow PH. Hybrid ventilation was initiated by inserting a reverse thrust catheter (RTC) into the endotracheal tube. Hybrid ventilation consists of a pressure-controlled mode of mechanical ventilation and ITPV while flushing fresh gas continuously via the RTC. Respiratory parameters were compared under control, OA, PH and HV conditions. RESULTS: Oleic acid injection decreased partial pressure of oxygen (PaCO2) from 401+/-35 mmHg to 129+/-39 mmHg, increased V(T) from 42+/-5 mL to 52+/-10 mL, and increased dead space (V(D))/V(T) ratio from 0.65+/-0.07 to 0.71+/-0.07. During PH, the increase in PaCO2 was accompanied by an increase in V(D)/V(T) ratio from 0.71+/-0.07 to 0.79+/-0.03 and by a decrease of peak inspiratory pressure (PIP) from 19.4+/-4.0 cmH2O to 16.8+/-3.1 cmH2O. PaCO2 was lowered from 50+/-5 mmHg in PH to 39+/-5 mmHg in HV with a lower V(T). V(D)/V(T) ratio in HV which was as low as that in control. CONCLUSIONS: Hybrid ventilation is an effective and easy-to-use ventilatory modality to reduce PaCO2 and airway pressures by the reduction in V(D)/V(T) ratio in acute respiratory failure model.