Morbidity associated with inhalation anesthesia in horses: The role of opioid use in a university teaching hospital.

Despite concerns among equine practitioners, the effects of opioid administration on intraoperative complications during inhalation anesthesia remain incompletely characterized. This retrospective study evaluated intraoperative complications and perianesthetic mortality in horses undergoing inhalation anesthesia according to opioid use. Medical records from a university veterinary hospital were reviewed (2014-2025) and grouped based on opioid inclusion in anesthetic protocols and surgery type (colic vs. non-colic). Intraoperative hypotension (MAP < 70 mmHg), hypothermia (temperature < 35 °C), hypercapnia (EtCO2 or PaCO2 > 65 mmHg), and hypoxemia (PaO2 < 60 mmHg or SpO2 < 90%) were recorded. Perianesthetic mortality included deaths from premedication to seven days post-operatively. Proportions were compared using chi-squared tests (P < 0.05). Hypotension was the most frequent complication (338/426; 79.3%), followed by hypothermia (94/426; 22.0%), hypoxemia (43/426; 10.0%), and hypercapnia (27/426; 6.3%). Opioid use was only associated with hypothermia (P = 0.001); however, opioids were also more commonly administered during procedures lasting > 3 hours, and hypothermia was more prevalent in these procedures (P < 0.0001). Overall perianesthetic mortality was 7.9% (34/426) and was higher in colic cases (27/219 [12.3%]) than in non-colic cases (7/207 [3.3%]) (P < 0.001), as well as in horses with elevated ASA classifications (2/173 [1.1%] in ASA I-II vs. 32/253 [12.6%] in ASA III-V; P < 0.001). Opioid administration during equine inhalation anesthesia does not appear to increase intraoperative complications or perianesthetic mortality, and the association with hypothermia was likely influenced by anesthesia duration.