Restoration of arterial oxygen tension in horses recovering from general anaesthesia

Background Arterial hypoxaemia is common in anaesthetised horses, but little information exists regarding restoration of arterial oxygen tension (PaO2) during recovery from anaesthesia, or if intra-operative management factors exert any longer-term effect. Objectives To evaluate PaO2 in horses recovering from general anaesthesia up to 1 h after resuming standing. Study design Prospective observational clinical cohort study. Methods Systemically healthy adult horses undergoing inhalational general anaesthesia for elective surgical procedures were studied. Arterial blood samples were obtained anaerobically prior to pre-anaesthetic medication, at end of anaesthesia, immediately following positioning in the recovery box, then at 10-min intervals until standing. Additionally, samples were taken when horses achieved sternal recumbency, at standing (STAND) and 1 h after standing (STAND+1). Data were analysed using ANOVA and mixed-effects linear regression, with significance set at P Data from one hundred and two horses were analysed. Forty horses received controlled mandatory ventilation (CMV) throughout anaesthesia, 47 breathed spontaneously (SV) and 15 initially breathed spontaneously before CMV was imposed (S-CMV). Overall, PaO2, P(A-a)O-2 and PaCO2 remained significantly lower than baseline at STAND+1 (P<0.01). CMV resulted in higher PaO2 at the end of anaesthesia (P = 0.03) and during early recovery (P<0.01) than SV. Only in group S-CMV did PaO2, P(A-a)O-2 and PaCO2 return to baseline values at STAND+1. Highest PaO2 values associated with CMV were also associated with early recovery apnoea. Main limitations Non-standardised anaesthetic management, temporal and quantitative variation in oxygen delivery during early recovery and lack of control group where oxygen was electively withheld during recovery. Conclusions Controlled mandatory ventilation results in better pulmonary function in horses as assessed by PaO2, P(A-a)O-2 and PaCO2, an effect enhanced by an initial period of SV and still evident 1 h after standing. High PaO2 values may contribute to early recovery apnoea but this does not adversely affect outcome. The is available in Portuguese - see Supporting Information