Effects of inverse ratio ventilation on cardiorespiratory parameters in severe respiratory failure

Inverse ratio ventilation, with prolonged inspiratory times, appears to improve gas exchange and arterial oxygenation in patients with severe respiratory failure; however, in previous studies, pressure-controlled inverse ratio ventilation (PC-IRV), which uses a rapidly decelerating inspiratory flow pattern, was compared to conventional volume-controlled ventilation, which uses a constant inspiratory flow rate. Pressure-controlled ventilation (PCV), with a decelerating inspiratory flow pattern and conventional inspiratory-to-expiratory (I/E) ratios, also has been shown to produce improvement in PaO2 when compared to volume-controlled ventilation. It therefore is unknown if the potentially beneficial effects of PC-IRV are due to the reversal of I/E ratios or to the use of the rapidly decelerating inspiratory flow pattern. In order to investigate this issue, cardiorespiratory values were measured in ten patients with severe respiratory failure ventilated first with PCV, then PC-IRV, and finally with a second period of PCV. Tidal volume, respiratory rate, end-expiratory pressure, and fraction of oxygen in inspired gas (FIO2) were maintained at the same value for both ventilatory modalities. The PC-IRV was associated with significant increases in PaO2, arterial pH, and mean airway pressure. Significant decreases in pulmonary shunt fraction, PaCO2, and cardiac index were found with PC-IRV. No significant changes in tissue oxygen delivery or consumption occurred with either PCV or PC-IRV. These results demonstrate that inversion of conventional I/E ratios produces no significant improvement in the overall cardiorespiratory profile of critically ill patients. This study also suggests that the clinical utility of PC-IRV is limited except in the setting of the adult respiratory distress syndrome with hypoxemia or hypercapnia refractory to other therapeutic options.