Effect of low inspired oxygen fraction on respiratory indices in mechanically ventilated horses anaesthetised with isoflurane and medetomidine constant rate infusion.
Authors: Taylor A H, Seymour C J
Journal: Veterinary journal (London, England : 1997)
Summary
# Editorial Summary: Low Inspired Oxygen in Equine Anaesthesia Although hypoxaemia remains a clinical concern during equine anaesthesia despite high oxygen supplementation, the potential benefits of deliberately reducing inspired oxygen fraction (FiO2) to minimise atelectasis and improve pulmonary function—well established in human anaesthesia—remain unclear in horses under clinically realistic conditions. Taylor and Seymour recruited 18 healthy Thoroughbreds undergoing experimental laryngeal surgery, randomly assigning them to receive either 0.65 or 0.90 FiO2 during isoflurane–medetomidine anaesthesia with mechanical ventilation, then performed serial arterial blood gas analysis every 30 minutes to measure respiratory indices including oxygenation, ventilation and acid–base status. Predictably, arterial partial pressure of oxygen (PaO2) decreased significantly in the low FiO2 group; however, oxygen saturation remained adequate for full haemoglobin binding, and critically, no statistically significant improvements were demonstrated in any other respiratory parameter—including CO2 elimination, alveolar–arterial oxygen gradient or acid–base variables—between the two groups. For practitioners managing mechanically ventilated horses in lateral recumbency under these specific anaesthetic conditions, increasing FiO2 to 0.90 offers no measurable respiratory disadvantage over the commonly used 0.65, suggesting that routine practice patterns are unlikely to impair pulmonary function in healthy animals, though this finding may not generalise to other anaesthetic protocols, patient positions or compromised individuals.
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Practical Takeaways
- •Despite common practice, using FiO2 0.65 does not improve respiratory outcomes compared to FiO2 0.90 in mechanically ventilated horses under isoflurane/medetomidine anaesthesia
- •Higher FiO2 (0.90) is preferable as it maintains adequate oxygenation without sacrificing other respiratory parameters
- •If anaesthetising healthy Thoroughbreds for lateral recumbency procedures, set FiO2 to 0.90 rather than 0.65 unless specific contraindications exist
Key Findings
- •PaO2 was significantly lower with FiO2 0.65 compared to 0.90, but remained sufficient for haemoglobin saturation
- •No significant improvements in other respiratory indices (PaCO2, pH, bicarbonate, base excess) were found with low FiO2
- •FiO2 0.65 provided no clinical benefit over FiO2 0.90 in mechanically ventilated horses anaesthetised in lateral recumbency
- •Low FiO2 did not reduce atelectasis or improve lung function in this clinical setting