Risk factors associated with hypoxaemia in horses undergoing general anaesthesia: A retrospective study

Background: Hypoxaemia is a common concern during equine general anaesthesia. The prevalence and predictors of hypoxaemia in horses undergoing elective anaesthesia in particular are poorly documented.

Objectives: To determine and compare the prevalence of hypoxaemia (defined as PaO₂ ≤ 80 mm Hg, on the lowest value recorded) in horses undergoing anaesthesia for exploratory laparotomy and elective procedures, and to identify risk factors in both subpopulations.

Study design: Retrospective cohort study.

Methods: Data were collected from 774 records of 708 horses undergoing general anaesthesia between April 2017 and August 2020. Potential predictors of hypoxaemia in horses undergoing anaesthesia for exploratory laparotomy or elective procedures were investigated separately by univariable penalised maximum likelihood logistic regression, followed by multivariable analysis. The lowest recorded PaO₂ was used as a single data point for the determination of hypoxaemia and arterial oxygen partial pressure to fractional inspired oxygen ratio analysis.

Results: Hypoxaemia was recorded in 23% horses undergoing exploratory laparotomy compared with 3.8% horses undergoing elective anaesthesia (P < .001). Multivariable analysis showed that weight above 550 kg, large intestinal lesions, and peak inspiratory pressure (PIP) above 30 cmH₂ O were significantly associated with hypoxaemia in horses undergoing exploratory laparotomy. Soft tissue procedures and PIP >30 cmH₂ O were significantly associated with hypoxaemia during elective anaesthesia.

Main limitations: This study only considered the minimum PaO₂ recorded during anaesthesia. Duration and treatment of hypoxaemia were not considered.

Conclusions: Based on the lowest PaO₂ value during anaesthesia, in our population, horses undergoing anaesthesia for exploratory laparotomy were over six times more likely to become hypoxaemic than horses undergoing elective procedures. Bodyweight, type of procedure, and high PIP were predictors of hypoxaemia.