Postoperative oxygenation in healthy dogs following mechanical ventilation with fractions of inspired oxygen of 0.4 or >0.9.

Objective: To evaluate arterial oxygenation during the first 4 postoperative hours in dogs administered different fractions of inspired oxygen (FiO ₂ ) during general anesthesia with mechanical ventilation.
Study Design: Prospective, randomized clinical trial.
Animals: A total of 20 healthy female dogs, weighing >15 kg and body condition scores 3-7/9, admitted for ovariohysterectomy.
Methods: Dogs were randomized to breathe an FiO ₂ >0.9 or 0.4 during isoflurane anesthesia with intermittent positive pressure ventilation. The intraoperative PaO ₂ :FiO ₂ ratio was recorded during closure of the linea alba. Arterial blood was obtained 5, 60 and 240 minutes after extubation for measurement of PaO ₂ and PaCO ₂ (FiO ₂  = 0.21). Demographic characteristics, duration of anesthesia, PaO ₂ :FiO ₂ ratio and anesthetic agents were compared between groups with Wilcoxon tests. The postoperative PaO ₂ , PaCO ₂ , rectal temperature, a visual sedation score and events of hypoxemia (PaO ₂ < 80 mmHg) were compared between groups with mixed-effects models or generalized linear mixed models.
Results: Groups were indistinguishable by demographic characteristics, duration of anesthesia, anesthetic agents administered and intraoperative PaO ₂ :FiO ₂ ratio (all p > 0.08). Postoperative PaO ₂ , PaCO ₂ , rectal temperature or sedation score were not different between groups (all p > 0.07). During the first 4 postoperative hours, hypoxemia occurred in three and seven dogs that breathed FiO ₂ >0.9 or 0.4 during anesthesia, respectively (p = 0.04).
Conclusions and Clinical Relevance: The results identified no advantage to decreasing FiO ₂ to 0.4 during anesthesia with mechanical ventilation with respect to postoperative oxygenation. Moreover, the incidence of hypoxemia in the first 4 hours after anesthesia was higher in these dogs than in dogs breathing FiO ₂ >0.9.
(Copyright © 2020 Association of Veterinary Anaesthetists and American College of Veterinary Anesthesia and Analgesia. Published by Elsevier Ltd. All rights reserved.)