Metabolic acidosis in healthy mules under general anaesthesia with halothane

Objective To report the severe metabolic acidosis identified in a group of 11 healthy mules anaesthetized with halothane for castration. Study design Data generated from a prospective study. Animals Eleven mules aged 2.5-8years, weighing 230-315kg and 11 horses aged 1.5-3.5years, weighing 315-480kg. Methods Animals were anaesthetized for castration as part of an electroencephalographic study. Preanaesthetic medication was acepromazine (0.03mgkg^-1) administered through a preplaced jugular venous catheter. Anaesthesia was induced 30-90minutes later with intravenous thiopental (10mgkg^-1). After orotracheal intubation, anaesthesia was maintained with halothane vaporised in oxygen. The animals' lungs were ventilated to maintain the end-tidal CO₂ concentration between 3.9 and 4.5kPa (29-34mmHg). Anaesthetic monitoring included invasive blood pressure measurement via the auricular artery (mules) and submandibular branch of the facial artery (horses). Arterial blood gas samples were drawn from these catheters at three time points during surgery and pH, PaCO₂, base excess (ecf) and were measured. Values were compared between groups using a Mann-Whitney test. p was taken as <0.05. Results are reported as median (range). Results PaCO₂ did not differ between groups but pH was significantly lower in mules [7.178 (7.00-7.29)] compared to horses [7.367 (7.24-7.43)] (p=0.0002). values were significantly lower in the mules [16.6 (13.0-22.3) mM] compared to horses [23.7 (20.9-23.7) mM] (p=0.0001), whilst base excess (ecf) was significantly more negative in the mules [-11.4 (-1.27 to -16) mM] compared to horses [-1.3 (-5.8 to +2.4) mM] (p=0.0004). Conclusion and clinical relevance This study demonstrated severe metabolic acidosis in healthy mules, which may have prompted intervention with drug therapies in a clinical arena. It is probable that the acidosis existed prior to anaesthesia and caused by diet, but other possible causes are considered.