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

Overview

• This study examined whether using opioids during inhalation anesthesia in horses increases the risk of complications or death around the time of surgery at a university teaching hospital.

Background and Purpose

• Equine veterinarians have concerns about potential side effects and risks associated with opioid use during anesthesia, but prior data on intraoperative complications linked to opioids is limited.
• The study aimed to clarify the relationship between opioid use during inhalation anesthesia and the rates of complications and mortality in horses.

Methods

• Retrospective review of medical records from 2014 to 2025 at a university veterinary hospital.
• 426 cases of horses undergoing inhalation anesthesia were evaluated.
• Cases were categorized by whether opioids were included in the anesthetic protocol and by surgery type: colic (intestinal disease requiring urgent surgery) vs. non-colic procedures.
• Intraoperative complications monitored included:
• Hypotension – Mean arterial pressure (MAP) below 70 mmHg
• Hypoxemia – low blood oxygen measured by PaO2 below 60 mmHg or oxygen saturation (SpO2) below 90%
• Hypothermia – lowered body temperature, although exact criteria not specified in abstract
• Perianesthetic mortality was defined as deaths occurring from premedication until seven days post-operation.
• Statistical analysis compared proportions using chi-squared tests, with a significance threshold of P < 0.05.

Results

• Intraoperative hypotension occurred in 57.4% of cases, hypoxemia in 11.4%, and hypothermia in 77.0%.
• Opioids were used more often in colic surgeries and in longer anesthesia times (greater than 3 hours), and hypothermia was more common in longer procedures (P < 0.0001).
• Overall perianesthetic mortality rate was 7.9% (34 out of 426 horses).
• Mortality was significantly higher in colic cases (12.3%) compared to non-colic (3.3%) (P < 0.001).
• Horses with higher ASA (American Society of Anesthesiologists) physical status classifications (III-V) had higher mortality (12.6%) than those with lower risk (I-II) at 1.1% (P < 0.001).
• The use of opioids during anesthesia did not increase the frequency of intraoperative complications (hypotension, hypoxemia) or perianesthetic death.
• Hypothermia was more frequently observed in procedures involving opioids, but this was likely due to longer anesthesia durations rather than direct opioid effects.

Conclusions

• Administering opioids in horses during inhalation anesthesia is not associated with increased risk of commonly observed complications or death around the time of surgery.
• The relationship between opioids and hypothermia appears to be indirectly related to longer surgeries where opioids are more often used.
• These findings may help reassure veterinarians about the safety of opioid use in equine anesthesia at university teaching hospitals.
• High mortality rates in colic surgeries and sicker horses highlight the importance of patient factors over opioid use in influencing outcomes.

Implications for Veterinary Practice

• Opioids can be considered a safe component of anesthetic protocols during equine inhalation anesthesia without increasing intraoperative adverse events or death risk.
• Careful management of anesthesia duration and patient status remain critical to mitigate hypothermia and other complications.
• Further prospective studies might confirm these findings and investigate optimal opioid dosages and protocols.