Ventilation-Perfusion Distribution Related to Different Inspiratory Flow Patterns in Experimental Lung Injury

In acute lung injury (ALI), controlled mechanical ventilation with decelerating inspiratory flow (V_dec) has been suggested to improve oxygenation when compared with constant flow (V_con) by improving the distribution of ventilation and perfusion (V_A/Q). We performed the present study to test this hypothesis in an animal model of ALI. Furthermore, the effects of combined decelerating and constant flow (V_deco) were evaluated. Thus, 18 pigs with experimental ALI were randomized to receive mechanical ventilation with either V_con, V_dec or a fixed combination of both flow wave forms (V_deco) at the same tidal volume and positive end-expiratory pressure level for 6 h. Hemodynamics, gas exchange, and V_A/Q distribution were determined. The results revealed an improvement of oxygenation resulting from a decrease of pulmonary shunt within each group (P < 0.05). However, blood flow to lung areas with a normal V _A/Q distribution increased only during ventilation with V _con (P < 0.05). Accordingly, PaO₂ was higher with V_con than with V_dec and V_deco (P < 0.05). We conclude that contrary to the hypothesis, V_con provides a more favorable V_A/Q distribution, and hence better oxygenation, when compared with V_dec and V_deco in this model of ALI.