Detection of tidal recruitment/overdistension in lung-healthy mechanically ventilated patients under general anesthesia.

Background: The volume-dependent single compartment model (VDSCM) has been applied for identification of overdistension in mechanically ventilated patients with acute lung injury. In this observational study we evaluated the use of the VDSCM to identify tidal recruitment/overdistension induced by tidal volume (Vt) and positive end-expiratory pressure (PEEP) in lung-healthy anesthetized subjects.

Methods: Fifteen patients (ASA physical status I-II) undergoing general anesthesia for elective plastic breast reconstruction surgery were mechanically ventilated in volume-controlled ventilation (VCV), with Vt of 8 mL•kg(-1) and PEEP of 0 cm H(2)O. With these settings, ventilatory mode was randomly adjusted in VCV or pressure-controlled ventilation (PCV) and PEEP was sequentially increased from 0 to 5 and 10 cm H(2)O, 5 min per step. Thereafter, PEEP was decreased to 0 cm H(2)O, Vt increased to 10 mL•kg(-1) and, keeping minute ventilation constant, PEEP was similarly increased to 5 and 10 cm H(2)O. Airway pressure and flow were continuously recorded and fitted to the VDSCM with or without considering flow-dependencies. A "distension index" (%E(2)) derived from the VDSCM was used to assess Vt and PEEP-induced recruitment/overdistension. Positive and negative values of %E(2) suggest tidal overdistension or tidal recruitment, respectively. In addition, the linear respiratory system elastance was calculated. Comparisons among variables at each PEEP value, Vt setting, ventilatory mode, and regression model considering or not considering flow-dependencies were performed with the Wilcoxon-sign rank test for paired samples (P < 0.05). Multiple comparisons were corrected with the Bonferroni method. The relative change in the estimated noisy variance was used as an index of the goodness of fit of the models.

Results: VDSCM including the flow-dependent parameter significantly improved estimated noisy variance in almost all experimental conditions (11.2 to 71.4, smallest of the lower and highest of the upper 95% confidence intervals). No differences in %E(2) were observed between VCV and PCV, at comparable Vt and PEEP levels, when flow-dependencies were included in the regression model. The negligence of the flow-dependent parameter systematically led to an underestimation of %E(2) in PCV compared to VCV mode (all P < 0.02). At a given Vt, %E(2) was negative at a PEEP of 0 cm H(2)O and significantly increased with PEEP, being almost 0 at a PEEP of 5 cm H(2)O. At a given level of PEEP, %E(2) significantly increased with Vt.

Conclusions: The distension index %E(2), derived from the VDSCM considering flow-dependencies, seems able to identify tidal recruitment/overdistension induced by Vt and PEEP independent of flow waveform in healthy lung-anesthetized patients.