Determination of respiratory system compliance during pressure support ventilation by small variations of pressure support.

In mechanically ventilated patients, measurement of respiratory system compliance (C) is of high clinical interest. Spontaneous breathing activity during pressure support ventilation (PSV) can impede the correct assessment of C and also alter the true C by inducing lung recruitment. We describe a method for determination of C during PSV and assess its accuracy in a study on 20 mechanically ventilated patients. To assess C during pressure support ventilation (C), we performed repeated changes in pressure support level by ± 2 cmHO. C was calculated from the volume change induced by these changes in pressure support level, taking into account the inspiration time and the expiratory time constant. As reference methods, we used C, measured during volume controlled ventilation (C). In a post-hoc analysis, we assessed C during the last 20% of the volume-controlled inflation (C). Values were compared by linear regression and Bland-Altman methods comparison. Comparing C to the reference value C, we found a coefficient of determination (r) of 0.90, but a relatively high bias of - 7 ml/cm HO (95% limits of agreement - 16.7 to + 2.7 ml/cmHO). Comparison with C resulted in a negligible bias (- 1.3 ml/cmHO, 95% limits of agreement - 13.9 to + 11.3) and r of 0.81. We conclude that the novel method provides an estimate of end-inspiratory C during PSV. Despite its limited accuracy, it might be useful for non-invasive monitoring of C in patients undergoing pressure support ventilation.