Causes of error of respiratory pressure-volume curves in paralyzed subjects

Respiratory pressure-volume (PV) curves are commonly obtained in paralyzed patients by relating airway pressure to volume changes of a syringe (Vsyr). This is based on the implicit assumption that changes in thoracic volume (Vtho) and Vsyr are equal. We undertook to verify this assumption through simultaneous measurements of Vtho by respiratory inductive plethysmography and Vsyr in six comatose, paralyzed, intubated patients. At any constant Vsyr, Vtho fell and was smaller on deflation than on inflation during inflation-deflation (ID) cycle. The rate of fall was 110 +/- 64 (SD) ml/min. During ID cycles lasting 76 +/- 7 s, thoracic PV curves showed less hysteresis and a larger compliance on deflation than PVsyr curves (12 +/- 2 vs. 18 +/- 6% and 73 +/- 13 vs. 67 +/- 12 ml/cmH2O, P less than 0.05). With PVsyr curves, hysteresis increased and compliance on deflation decreased with increasing rate of fall of Vtho. We submit that the difference between changes in Vsyr and Vtho is best explained by gas exchange and should be taken into account when performing PV curves with a syringe in paralyzed patients.