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

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 (P(a)CO₂) 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 (VT) was elevated to maintain P(a)CO₂ 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 (P(a)O₂) 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 P(a)CO₂ 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. P(a)CO₂ 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 P(a)CO₂ and airway pressures by the reduction in V(D)/V(T) ratio in acute respiratory failure model.