Effect of different inspired fractions of oxygen on F-shunt and arterial partial pressure of oxygen in isoflurane-anaesthetized and mechanically ventilated Shetland ponies.

The research investigates the impact of the fraction of injected oxygen (FiO) on the intrapulmonary shunt fraction, as measured by F-shunt, in ponies. The study, conducted with Shetland ponies, concludes that using a reduced FiO does not improve oxygenation in those that are anaesthetized and mechanically ventilated.

Research Method

• The researchers conducted a prospective random study including 23 adult Shetland ponies, participating in a total of 32 anaesthetic procedures.
• Ponies were premedicated intravenously (IV) with detomidine and either morphine or butorphanol. The anaesthesia was induced using ketamine and midazolam administered through IV.
• The ponies were then assigned randomly to two groups. In one, anaesthesia was maintained with isoflurane in oxygen (group TH – FiO = 0.95). The other group was administered a mixture of oxygen and medical air (group TL – FiO = 0.65). All ponies were given a constant infusion of detomidine.
• Mechanical ventilation was used to maintain the partial pressure of carbon dioxide between 40 and 50 mmHg. Every 30 minutes, an arterial blood gas analysis was performed.
• The F-Shunt equation, which measures intrapulmonary shunt fraction, was calculated at four specific time points – T0, T30, T60 and T90.
• Results were annotated as mean ± standard deviation, and a linear mixed model analysis was employed for data analysis. Statistical significance was determined through a p-value of less than 0.05.

Results

• For the group maintained on isoflurane and oxygen (TH), the arterial partial pressure of oxygen (PaO) was higher than in the group where the ponies were given a mixture of oxygen and medical air (TL). Furthermore, in group TH, F-shunt was less than TL.
• Significant differences were found at T60 and T90 time points. At T60, the F-shunt for group TH was 13.2% ± 4.3, whereas for group TL it was 19.4% ± 8.3 (p = 0.016). At T90, the F-shunt was 11.7% ± 3.5 for group TH, and 18.6% ± 9.5 for group TL (p = 0.036).

Conclusion

• The study concluded that using a reduced fraction of inspired oxygen does not have a beneficial impact on the oxygenation of anaesthetized and mechanically ventilated Shetland ponies. This means that the proportional balance of oxygen and other gases did not increase the efficacy of oxygen being absorbed during these anaesthetic procedures.