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

In mechanically ventilated patients, measurement of respiratory system compliance (Crs ) is of high clinical interest. Spontaneous breathing activity during pressure support ventilation (PSV) can impede the correct assessment of Crs and also alter the true Crs by inducing lung recruitment. We describe a method for determination of Crs during PSV and assess its accuracy in a study on 20 mechanically ventilated patients. To assess Crs during pressure support ventilation (Crs,PSV ), we performed repeated changes in pressure support level by ± 2 cmH2 O. Crs,PSV 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 Crs , measured during volume controlled ventilation (Crs,VCV ). In a post-hoc analysis, we assessed Crs during the last 20% of the volume-controlled inflation (Crs,VCV20 ). Values were compared by linear regression and Bland-Altman methods comparison. Comparing Crs,PSV to the reference value Crs,VCV , we found a coefficient of determination (r2 ) of 0.90, but a relatively high bias of - 7 ml/cm H2 O (95% limits of agreement - 16.7 to + 2.7 ml/cmH2 O). Comparison with Crs,VCV20 resulted in a negligible bias (- 1.3 ml/cmH2 O, 95% limits of agreement - 13.9 to + 11.3) and r2 of 0.81. We conclude that the novel method provides an estimate of end-inspiratory Crs during PSV. Despite its limited accuracy, it might be useful for non-invasive monitoring of Crs in patients undergoing pressure support ventilation.