Ventilation with biphasic positive airway pressure in experimental lung injury: Influence of transpulmonary pressure on gas exchange and haemodynamics

Objective: We investigated whether improvement in ventilation perfusion (V_A/Q̇) distribution during mechanical ventilation using biphasic positive airway pressure (BIPAP) with spontaneous breathing may be attributed to an effectively increased transpulmonary pressure (P_TP) and can also be achieved by increasing P_TP during controlled ventilation. Design: In 12 pigs with saline lavage-induced lung injury we compared the effects of BIPAP to pressure-controlled ventilation with equal airway pressure (PCV_Aw) or equal transpulmonary pressure (PCV_TP) on V̇_A/Q̇ distribution assessed by the multiple inert gas elimination technique (MIGET). Setting: Animal laboratory study. Measurements and results: Intrapulmonary shunt was 33±11% during BIPAP, 36±10% during PCV_AW and 33±15% during PCV_TP (p= n.s.). BIPAP resulted in higher PaO₂ than PCV_AW (188±83 versus 147±82 mmHg, p<0.05), but not than PCV_TP (187±139 mmHg). Oxygen delivery was significantly higher during BIPAP (530±109 ml/min) versus 374±113 ml/min during PCV_Aw and 353±93 ml/min during PCV_TP (p<0.005). Tidal volume with PCV_TP increased to 11.9±2.3 ml/kg, compared to 8.5±0.8 with BIPAP and 7.6±1.4 with PCV_Aw (p<0.001) and cardiac output decreased to 3.5±0.61/min (BIPAP 4.9±0.8 and PCV _Aw 3.9±0.8, p<0.006). Conclusions: In experimental lung injury, BIPAP with preserved spontaneous breathing was effective in increasing regional P_TP, since pressure-controlled ventilation with the same PTP resulted in similar gas exchange effects. However, PCV_TP caused increased airway pressures and tidal volumes, whereby, with BIPAP, less depression of oxygen delivery and cardiac output were observed. BIPAP could be useful in maintaining pulmonary gas exchange and slightly improving oxygenation without interfering with circulation as strongly as PCV does.