Influence of changing lateral recumbency and mode of ventilation on the alveolar-arterial oxygen tension gradient and selected laboratory analytes in adult isoflurane anesthetized horses

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This study investigated the influence of changing recumbency and mode of ventilation over repeated anesthesias on the alveolar to arterial oxygen tension gradient (P_A-aO₂) and laboratory analytes in eight horses during a year-long imaging study. Anesthesia was induced with xylazine, diazepam or guaifenesin, and ketamine and maintained with isoflurane. Horses were positioned in right or left lateral recumbency for computed tomography. Ventilation was controlled during 47% of the anesthetics. Blood was sampled from an arterial catheter prior to (30 ± 5 min from connection to anesthetic circuit), within 5 min of changing lateral recumbency, and prior to circuit disconnection (24 ± 6 min after second sample) for measurement of pH, partial pressure of arterial oxygen (PaO₂) and partial pressure of arterial carbon dioxide, blood glucose and electrolytes. P_A-aO₂ was calculated. Data from five anesthetic episodes for each horse were summarized as mean ± standard error and analyzed using a mixed-model ANOVA. t tests were used for pairwise comparisons. P<0.05. PaO₂ decreased after turning (198 vs. 347 mmHg), then increased to 291 mmHg prior to disconnection. Correspondingly, P_A-aO₂ was wider (252 vs.120 mmHg), and improved before disconnection (190 mmHg). Body temperature, ionized-Ca²⁺ and blood glucose were lower, and Na⁺ was higher at the last time point. In conclusion, turning anesthetized horses decreases PaO₂ andresults in a widening P_A-aO₂ suggesting a cautious approach in animals with pre-existing hypoxemia.