The influence of body mass and thoracic dimensions on arterial oxygenation in anaesthetized horses and ponies

Objective To examine the relationship between body mass and thoracic dimensions on arterial oxygen tensions (PaO(2)) in anaesthetized horses and ponies positioned in dorsal recumbency. Study design Prospective clinical study. Animals Thirty six client-owned horses and ponies, mean [+/- SD (range)] age 8.1 +/- 4.8 (1.5-20) years and mean body mass 467 +/- 115 (203-656) kg. Methods Before general anaesthesia, food and water were withheld for 12 and 1 hours respectively. Body mass (kg), height at the withers (H), thoracic circumference (C), thoracic depth (length between dorsal spinous process and sternum; D), thoracic width (between point of shoulders; W), and thoracic diagonal length (point of shoulder to last rib; L) were measured. Pre-anaesthetic medication was with intravenous (IV) romifidine (0.1 mg kg(-1)). Anaesthesia was induced with an IV ketamine (2.2 mg kg(-1)) and diazepam (0.05 mg kg(-1)) combination and maintained with halothane in 1:1 oxygen:nitrous oxide (N(2)O) mixture. Animals were positioned in dorsal recumbency and allowed to breathe spontaneously. Nitrous oxide was discontinued after 10 minutes, and arterial blood samples obtained and analysed for gas tensions at 15, 30 and 60 minutes after connection to the anaesthetic breathing circuit. Data were analysed using ANOVA and Pearson's correlation co-efficient. Results The height per unit body mass (H kg(-1)) and thoracic circumference per unit body mass (C kg(-1)) correlated strongly (r = 0.85, p < 0.001 and r = 0.82, p < 0.001 respectively) with arterial oxygen tensions (PaO(2)) at 15 minutes. Conclusions There is a strong positive correlation between H kg(-1) and C kg(-1) and PaO(2) after 15 minutes of anaesthesia in halothane-anaesthetized horses positioned in dorsal recumbency. Clinical relevance Readily obtained linear measurements (height and thoracic circumference) and body mass may be used to predict the ability of horses to oxygenate during anaesthesia.