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_rs) is of high clinical interest. Spontaneous breathing activity during pressure support ventilation (PSV) can impede the correct assessment of C_rs and also alter the true C_rs by inducing lung recruitment. We describe a method for determination of C_rs during PSV and assess its accuracy in a study on 20 mechanically ventilated patients. To assess C_rs during pressure support ventilation (C_rs,PSV), we performed repeated changes in pressure support level by ± 2 cmH₂O. C_rs,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 C_rs, measured during volume controlled ventilation (C_rs,VCV). In a post-hoc analysis, we assessed C_rs during the last 20% of the volume-controlled inflation (C_rs,VCV20). Values were compared by linear regression and Bland–Altman methods comparison. Comparing C_rs,PSV to the reference value C_rs,VCV, we found a coefficient of determination (r²) of 0.90, but a relatively high bias of − 7 ml/cm H₂O (95% limits of agreement − 16.7 to + 2.7 ml/cmH₂O). Comparison with C_rs,VCV20 resulted in a negligible bias (− 1.3 ml/cmH₂O, 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 C_rs during PSV. Despite its limited accuracy, it might be useful for non-invasive monitoring of C_rs in patients undergoing pressure support ventilation.