Effects of intraoperative positive end-expiratory pressure and fraction of inspired oxygen on postoperative oxygenation in dogs undergoing stifle surgery

Abstract

Objective: To compare the effects of fraction of inspired oxygen (FiO₂) with the addition of positive end-expiratory pressure (PEEP) during anesthesia on arterial oxygenation in the first 4 postoperative hours in dogs. We hypothesized that compared with dogs breathing FiO₂ ≥ 0.95 and no PEEP (ZEEP), the use of intraoperative PEEP would improve postoperative oxygenation, and that the use of PEEP combined with an FiO₂ of 0.4 would further improve it.

Study design: Prospective, randomized study.

Animals: A total of 30 dogs undergoing unilateral stifle surgery.

Methods: Using a standardized anesthetic protocol, dogs were assigned to either FiO₂ ≥ 0.95 and ZEEP, FiO₂ ≥ 0.95 and 5 cmH₂O PEEP or FiO₂ 0.4 and 5 cmH₂O PEEP. All dogs were mechanically ventilated with a tidal volume of 12 mL kg^-1. Dogs breathed room air after recovery from anesthesia. Arterial blood gases were measured during surgical closure and 10, 120 and 240 minutes after extubation. Demographic characteristics were compared with Kruskal-Wallis tests. The effects of treatment and time on the PaO₂, PaCO₂, PaO₂:FiO₂ and shunt fraction (F-shunt) were assessed with mixed-effect models.

Results: The PaO₂ and F-shunt were lower during anesthesia for dogs breathing FiO₂ 0.4. No differences among groups were measured after extubation for any variable.

Conclusions and clinical relevance: Compared with dogs ventilated with FiO₂ ≥ 0.95 and ZEEP, application of 5 cmH₂O PEEP did not improve intraoperative gas exchange. The combination of 5 cmH₂O PEEP and FiO₂ 0.4 resulted in lower intraoperative F-shunt values. However, no benefits from those maneuvers on postoperative PaO₂ and F-shunt were recorded after extubation, suggesting that alterations in pulmonary function imposed by anesthesia were reversed soon after extubation.

Keywords: hypoxemia; postoperative pulmonary complications.