The interaction between applied peep and auto-peep during one-lung ventilation

Objective: To investigate the relationship between applied external positive end-expiratory pressure (PEEP) and auto-PEEP and the resultant total PEEP experienced by the patient during one-lung ventilation (OLV). Design: A prospective clinical study. Setting: A university hospital. Participants: Ten adult patients undergoing elective thoracotomies. Interventions: End-expiratory airway occlusion and measurement of plateau pressure during two-lung ventilation (TLV) and OLV with and without the application of 5 cm H2O of external PEEP via the anesthetic ventilator. The effect of variation of the inspiratory-expiratory ratio on total PEEP with and without applied external PEEP was also studied. Main Results: The mean level (+/-SD) of auto-PEEP changing from two-lung to one-lung ventilation rose from 0.9 (+/-0.8) cm H2O to 6.0 (+/-3.0) cm H2O at an inspiratory-expiratory ratio of 1:2. The application of 5 cm H2O external PEEP did not increase the total PEEP (7.3 +/- 2.0 cm H2O) significantly. The total PEEP increased significantly when the duration of expiration was decreased, and decreased when expiratory time increased. The change in total PEEP caused by the application of external PEEP during OLV correlated inversely with the preexisting level of auto-PEEP (r = -0.84). Conclusion: The change in end-expiratory pressure experienced by the ventilated lung during OLV when external PEEP is applied depends on the preexisting level of auto-PEEP. This may explain some of the inconsistencies in the clinical results of application of external PEEP during OLV. The total PEEP delivered to the patient should be measured whenever external PEEP is applied during OLV. Copyright (C) 1998 by W.B. Saunders Company.