Effect of low inspired oxygen fraction on respiratory indices in mechanically ventilated horses anaesthetised with isoflurane and medetomidine constant rate infusion.

This research examines the impact of different levels of inspired oxygen fraction (FiO2) on horses under anesthesia and on a standard mechanical ventilator. No significant improvements in respiratory indices were found when using a low FiO2 of 0.65 compared to a higher FiO2 of 0.90.

Study Background and Objectives

• The study sought to investigate the effects of varying inspired oxygen fraction (FiO2) levels on the respiratory status of anaesthetised, mechanically ventilated horses. FiO2 is the concentration of oxygen in the mix of gases that a patient breathes. The focus was on a frequently used FiO2 of 0.65 versus 0.90. This investigation was prompted by the observation that horses sometimes become hypoxaemic (having low oxygen levels in their blood) during anaesthesia, even when a high FiO2 is employed.

Methodology

• The researchers conducted a non-blinded, randomised clinical study with 18 healthy Thoroughbred horses, anaesthetised for experimental laryngeal surgery. Each horse was randomly assigned to one of the two FiO2 groups, either low (0.65) or high (0.90).
• Every 30 minutes during anaesthesia, arterial blood gas (ABG) analysis was performed to provide data for statistical evaluation of respiratory indices. This analysis helps to gauge how effectively oxygen is being delivered to the body and how well carbon dioxide is being removed.

Results

• The findings revealed that as expected, the partial pressure of oxygen in arterial blood (PaO2) was notably lower in horses anaesthetised with a low FiO2. However, the amount of oxygen was still sufficient to fully saturate haemoglobin, the protein responsible for carrying oxygen in the blood.
• No significant enhancements in the other respiratory indices were observed. This implies that regardless of the level of FiO2 applied, no improvement or deterioration in the horses’ ability to breathe came about.

Conclusion

• The research suggests that the practice of using a FiO2 of 0.65 gives no apparent advantage over a higher FiO2 of 0.90 for mechanically ventilated Thoroughbred horses undergoing anaesthesia in lateral recumbency (lying on their side) with isoflurane and a continual medetomidine infusion. This contributes to a better understanding of anaesthesia management in horses and could influence clinical practices.