Tidal Volume Variability During Airway Pressure Release Ventilation: Case Summary and Theoretical Analysis

Airway pressure-release ventilation (APRV) is used in the management of patients with severe or refractory respiratory failure. In addition to reversal of inspiratory-expiratory ratios, this pressure control mode also allows unrestricted spontaneous breathing. The spontaneous tidal volume (V_T), as well as the V_T resulting from transition between the high and low airway pressures, is uncontrolled. There are limited data on the within-patient variation of actual V_T and the safety of these modes. The authors present a patient with severe ARDS who was managed with biphasic modes (APRV and bi-level positive airway pressure). Serial V_T measurements showed that V_T ranged from 4 to 12 mL/kg predicted body weight. Computed tomography scan images and chest radiographs obtained before and following APRV showed lung parenchyma changes that may be related to ventilator-induced lung injury. We also present a mathematical model that is useful for simulating APRV and demonstrating the issues related to volume delivery for mandatory breaths during the transition between the 2 pressure levels. A key finding of this analysis is the interdependence of release volume, autoPEEP, and the T_low time setting. Furthermore, it is virtually impossible to target a specific P_aCO2 with a desired level V_T and autoPEEP in a passive model, emphasizing the importance of spontaneous breathing with this mode. This case report suggests caution when using these modes, and that end-inspiratory lung volumes and V_T should be limited to avoid lung injury. The important point of this case study and model analysis is that the application of APRV is more complex than it appears to be. It requires a lot more knowledge and skill than may be apparent from descriptions in the literature.