Lessons learned from airway pressure release ventilation

Objective

To review a single institution's experience with APRV with respect to safety, complication detection, and efficacy at correcting hypercarbia and hypoxemia.

Methods

Consecutive patients transitioned from either volume or pressure targeted ventilation to APRV in a University hospital ICU were retrospectively reviewed. Patients initially ventilated with APRV were excluded. Initial APRV settings for correction of hypoxemia (pO₂ ≤ 60 on FIO₂ ≥ 0.9) were a Phigh at the prior plateau pressure, a Thigh of 6.0 s and a Tlow of 0.8 s. Hypercarbic (pCO₂ ≥ 55 and pH ≤ 7.3) patients were set at a Thigh of 5.0 s with a Tlow of 1.0 s. IRB approved data included principal diagnoses, ventilation parameters, laboratory values, and ventilator associated complications. Data before and after APRV were compared using an unpaired two-tailed t-test; significance at P < 0.05 (*).

Results

Patient mix was 43% trauma, 32% sepsis, 8% cardiac surgery, 12% vascular surgery and 5% other. Transitioning to APRV was most frequent for hypoxemia (88%) and hypercarbia less often (12%). The mean time to correct hypoxemia (SaO₂ ≥ 92%) was 7 ± 4 min while the mean time to correct pCO₂ (pCO₂ ≤ 40 Torr) was 42 ± 7 min. Maximal CO2 clearance was achieved by 76 ± 12 min. The mean minute ventilation on APRV decreased by 3.3 ± 0.9 l/min (*) but achieved superior CO₂ clearance and oxygenation. The mean time to achieve FIO₂ ≤ 0.6 was 5.2 ± 0.9 hours. Four of 38 patients developed a pneumothorax although none developed hypotension; one had bilateral pneumothoraces. All four patients evidenced decreased CO₂ clearance and decreased release phase volumes as their only manifestation of a pneumothorax. 97% of patients on APRV with a Phigh ≥ 20 cmH₂O pressure who were transported out of the ICU using bag-valve ventilation developed hypoxemia within 5 min. 100% of patients with a Phigh ≤ 20 cmH₂O pressure were safely hand ventilated during transport without developing hypoxemia.

Conclusion

APRV is a safe rescue mode of ventilation for hypoxemic or hypercarbic respiratory failure and requires a lower minute ventilation than does conventional modes. Decreasing release phase volumes and a rising pCO₂ are excellent clues of a pneumothorax in a patient on APRV. Thus, routine end-tidal CO₂ monitoring is recommended for patients on APRV. Preparations for safe intra-hospital transport may be keyed to the Phigh required for adequate ventilation and oxygenation.