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

Schauvliege, Stijn, Savvas, Ioannis, Gasthuys, Frank
Veterinary anaesthesia and analgesia 2015 v.42 no.3 pp. 280-285
air, blood, body weight, breathing, carbon dioxide, chi-square distribution, horses, isoflurane, oxygen, patients, prospective studies, retrospective studies, surgery, t-test
To investigate the influence of two inspired oxygen fractions (FIO2) on the arterial oxygenation in horses anaesthetized with isoflurane.Retrospective, case-control clinical study.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.In group O (n = 100), medical oxygen acted as carrier gas, while in group M (n = 100), a medical mixture of oxygen and air (FIO2 0.60) was used. Demographic data, FIO2, arterial oxygen tension (PaO2) and routinely monitored physiologic data were recorded. The alveolar-arterial oxygen tension difference [P(A-a)O2] and PaO2/FIO2 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.No significant differences in age, body weight, sex, breed, surgical procedure, position, anaesthetic duration or arterial carbon dioxide tension were found. Mean FIO2 was 0.78 in group O and 0.60 in group M. Compared to group O, significantly lower values for PaO2 and for P(A-a)O2 were found in group M. In contrast, the PaO2/FIO2 ratio and the percentage of horses with a PaO2 <100 mmHg (13.33 kPa) were comparable in both groups.Although a reduction of the inspired oxygen fraction resulted in a lower PaO2, the P(A-a)O2 was also lower and the number of horses with PaO2 values <100 mmHg was comparable.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.