Flow-controlled ventilation (FCV) improves regional ventilation in obese patients - a randomized controlled crossover trial

Background In obese patients, high closing capacity and low functional residual capacity increase the risk for expiratory alveolar collapse. Constant expiratory flow, as provided by the new flow-controlled ventilation (FCV) mode, was shown to improve lung recruitment. We hypothesized that lung aeration and respiratory mechanics improve in obese patients during FCV. Methods We compared FCV and volume-controlled (VCV) ventilation in 23 obese patients in a randomized crossover setting. Starting with baseline measurements, ventilation settings were kept identical except for the ventilation mode related differences (VCV: inspiration to expiration ratio 1:2 with passive expiration, FCV: inspiration to expiration ratio 1:1 with active, linearized expiration). Primary endpoint of the study was the change of end-expiratory lung volume compared to baseline ventilation. Secondary endpoints were the change of mean lung volume, respiratory mechanics and hemodynamic variables. Results The loss of end-expiratory lung volume and mean lung volume compared to baseline was lower during FCV compared to VCV (end-expiratory lung volume: FCV, - 126 +/- 207 ml; VCV, - 316 +/- 254 ml; p < 0.001, mean lung volume: FCV, - 108.2 +/- 198.6 ml; VCV, - 315.8 +/- 252.1 ml; p < 0.001) and at comparable plateau pressure (baseline, 19.6 +/- 3.7; VCV, 20.2 +/- 3.4; FCV, 20.2 +/- 3.8 cmH(2)O; p = 0.441), mean tracheal pressure was higher (baseline, 13.1 +/- 1.1; VCV, 12.9 +/- 1.2; FCV, 14.8 +/- 2.2 cmH(2)O; p < 0.001). All other respiratory and hemodynamic variables were comparable between the ventilation modes. Conclusions This study demonstrates that, compared to VCV, FCV improves regional ventilation distribution of the lung at comparable PEEP, tidal volume, P-Plat and ventilation frequency. The increase in end-expiratory lung volume during FCV was probably caused by the increased mean tracheal pressure which can be attributed to the linearized expiratory pressure decline.