EFFECT OF INVERSE I E RATIO VENTILATION ON PULMONARY GAS-EXCHANGE IN ACUTE RESPIRATORY-DISTRESS SYNDROME/

Background: It is not known whether inverse I:E ratio ventilation (IRV ) offers any real benefit over conventional mechanical ventilation wit h positive end-expiratory pressure (CMV-PEEP) at similar levels of end -expiratory pressure. Methods: The effects of volume-controlled and pr essure-controlled IRV (VC-IRV and PC-IRV, respectively) on (V) over do t(A)/(Q) over dot inequality were compared with those of CMV-PEEP at a similar level of end-expiratory pressure and with CMV without PEEP (C MV) in eight patients in the early stages of acute respiratory distres s syndrome (ARDS). Respiratory blood gases, inert gases, lung mechanic s, and hemodynamics were measured 30 min after the onset of each venti latory mode. Results: Recruitment of nonventilated, poorly ventilated (or both) but well-perfused alveoli increased the partial pressure of oxygen (Pa-O2) during CMV-PEEP (+13 mmHg) and IRV-VC (+10 mmHg; P < 0. 05) compared with CMV. In contrast, PC-IRV did not affect Pa-O2 but ca used a decrease in P-CO2 (-7 mmHg; P < 0.05). The latter was due to a concomitant decrease in dead space (P < 0.01) and shift to the right o f (V) over dot(A)/(Q) over dot distributions. During PC-IRV, the incre ase in the mean of blood flow distribution (mean Q;P < 0.01) without a change in the disper sion (log SD Q) did not result in an increase in Pa-O2, probably because it reflected redistribution of blood flow wit hin well-ventilated areas. Conclusions: Short-term PC-IRV improved car bon dioxide clearance, but the lung became less efficient as an oxygen exchanger. Furthermore, based on mean airway and plateau pressures, t he risk of barotrauma was not reduced with this type of ventilation.