Time-cycled inverse ratio ventilation does not improve gas exchange during anaesthesia.

Inverse ratio ventilation (IRV) has been reported to improve oxygenation at lower peak airway pressures in patients with respiratory failure. Therefore we hypothesised that IRV might also improve oxygen exchange during anaesthesia. Conventional ratio ventilation (CRV) and IRV were compared in 24 low-risk surgical patients who were paralysed and whose lungs were ventilated with air/O2 by a non-rebreathing circuit and a Siemens 900-C servo ventilator. Two levels of time-cycled IRV (I:E ratios of 60/40 and 77/23) were bracketed by control periods with CRV (I:E ratio of 35/65). Inspired O2 fraction, O2 uptake and CO2 elimination, arterial blood gases, pulmonary ventilation and mechanics, heart rate and blood pressure were measured. From these data alveolar and dead space ventilation and four oxygen tension-based indices of gas exchange were calculated. During IRV, mean airway pressure (mean AWP) was increased but there were no changes in oxygen exchange indices, pulmonary mechanics, HR or BP. A sub-set of the sample with moderately impaired oxygen exchange, defined as the upper quartile for (A-a)DO2, was examined separately with identical results. Multivariate models were tested to identify variables which predicted O2 exchange during CRV. Patient age was the only predictor consistently significant in all models. We conclude that age is an important determinant of impaired pulmonary oxygen exchange during anaesthesia, and that increasing mean AWP by TC-IRV has no beneficial effects on pulmonary mechanics or gas exchange.