Pressure support compared with controlled mechanical ventilation in experimental lung injury

It has been suggested that, in acute lung injury (ALI), spontaneous breathing activity may increase oxygenation because of an improvement of ventilation-perfusion distribution. Pressure support ventilation (PSV) is one of the assisted spontaneous breathing modes often used in critical care medicine. We sought to determine the prolonged effects of PSV on gas exchange in experimental ALI. We hypothesized that PSV may increase oxygenation because of an improvement in ventilation-perfusion distribution. Thus, ALI was induced in 20 pigs by using repetitive lung lavage. Thereafter, the animals were randomized to receive either PSV with a pressure level set to achieve a tidal volume >4 mL/kg and a respiratory rate <40 min(-1) (n = 10) or controlled mechanical ventilation (CMV) with a tidal volume of 10 mL/kg and a respiratory rate of 20 min(-1) (n = 10). Positive end-expiratory pressure was set at 10 cm H2O in both groups. Blood gas analyses and determination of ventilation-perfusion (V-A/Q) distribution were performed at the onset of ALI and after 2,4, 8, and 12 h. The main result was an improvement of oxygenation because of a decrease of pulmonary shunt and an increase of areas with normal V-A/Q ratios during PSV (P < 0.005). However, during CMV, a more pronounced reduction of shunt was observed compared with PSV (P < 0.005). We conclude that, in this model of ALI, PSV improves gas exchange because of a reduction Of V-A/Q inequality. However, improvements in V-A/Q distribution may 1 e more effective with CMV than with PSV.