The effect of the inspired oxygen fraction on arterial blood oxygenation in spontaneously breathing, isoflurane anaesthetized horses: a retrospective study.

Objectives: To investigate the influence of two inspired oxygen fractions (FiO(2)) on the arterial oxygenation in horses anaesthetized with isoflurane.

Study Design: Retrospective, case-control clinical study.

Animals: Two hundred equine patients undergoing non-abdominal surgery (ASA class 1-2), using a standardized anaesthetic protocol and selected from anaesthetic records of a period of three years, based on pre-defined inclusion criteria.

Methods: In group O (n = 100), medical oxygen acted as carrier gas, while in group M (n = 100), a medical mixture of oxygen and air (FiO(2) 0.60) was used. Demographic data, FiO(2), arterial oxygen tension (PaO(2)) and routinely monitored physiologic data were recorded. The alveolar-arterial oxygen tension difference [P(A-a)O(2)] and PaO(2)/FiO(2) ratio were calculated. The area under the curve, standardized to the anaesthetic duration, was calculated and statistically compared between groups using t-tests or Mann-Whitney tests as appropriate. Categorical data were compared using Chi-square tests.

Results: No significant differences in age, body weight, sex, breed, surgical procedure, position, anaesthetic duration or arterial carbon dioxide tension were found. Mean FiO(2) was 0.78 in group O and 0.60 in group M. Compared to group O, significantly lower values for PaO(2) and for P(A-a)O(2) were found in group M. In contrast, the PaO(2)/FiO(2) ratio and the percentage of horses with a PaO(2) <100 mmHg (13.33 kPa) were comparable in both groups.

Conclusions: Although a reduction of the inspired oxygen fraction resulted in a lower PaO(2), the P(A-a)O(2) was also lower and the number of horses with PaO(2) values <100 mmHg was comparable.

Clinical Relevance: In healthy isoflurane anaesthetized horses, the use of a mixture of oxygen and air as carrier gas seems acceptable, but further, prospective studies are needed to confirm whether it results in a lower degree of ventilation/perfusion mismatching.