Oxygen supplementation before induction of general anaesthesia in horses.

REASONS FOR PERFORMING STUDY: Hypoventilation or apnoea, caused by the induction of general anaesthesia, may cause hypoxaemia. Preoxygenation may lengthen the period before this happens. No scientific studies are published on preoxygenation in equine anaesthesia.

OBJECTIVES: To determine whether supplementation of oxygen at a flow rate of 15 l/min for 3 min via a nasal cannula before induction of general anaesthesia is effective in elevating the arterial partial pressure of oxygen (PaO2 ) directly after induction.

STUDY DESIGN: Randomised, prospective clinical trial.

METHODS: A total of 18 American Society of Anesthesiologists physical status 1 or 2 adult horses undergoing elective anaesthesia were randomly allocated to one of 2 groups. The first group (control) received no oxygen supplementation before induction of general anaesthesia, whereas the second group (oxygen) did. All horses were anaesthetised with intravenous detomidine, butorphanol, ketamine, midazolam and isoflurane. Directly after induction (T = 0) and 30 min later (T = 30) an arterial blood sample was taken for blood gas analysis. At T = 30 an estimate of intrapulmonary shunt fraction (Qs/Qt) was calculated.

RESULTS: At T = 0 arterial partial pressure of oxygen (PaO2 ) was significantly higher in the oxygen group compared with the control group (11.0 ± 2.6 kPa vs. 7.4 ± 1.6 kPa; mean ± s.d., P = 0.005) and at T = 30 differences were not statistically significant. Partial pressure of carbon dioxide (PaCO2 ) and Qs/Qt did not differ between groups.

CONCLUSIONS: Supplementing oxygen by a nasal cannula before induction of general anaesthesia in horses is feasible and does effectively elevate the PaO2 immediately after induction. Future research is needed to determine whether supplementation of oxygen before induction of general anaesthesia in horses will affect outcomes.