Evaluation of the Effect of the Inspired Oxygen Fraction on Blood Oxygenation during Inhalant Anaesthesia in Horses: A Systematic Review with Meta-Analysis

SIMPLE SUMMARY: In anaesthetized horses, blood oxygenation impairment often occurs. This systematic review compared the effects of low and high inspired oxygen fractions on the arterial oxygen tension and other pulmonary gas exchange parameters in horses during inhalation anaesthesia. Five studies, four experimental and one clinical, were deemed suitable for inclusion. A meta-analysis was performed on the four experimental studies. The oxygen partial pressure was significantly lower with a lower inspired oxygen fraction. However, indices of pulmonary gas exchange were significantly worsened. It is concluded that, while only a limited number of studies are available, the use of a higher inspired oxygen fraction in horses during inhalation anaesthesia will result in higher levels of oxygen in the blood; it will also worsen the lung gas exchange status. Further studies are needed to increase the level of evidence on this subject. ABSTRACT: In anaesthetized horses, pronounced ventilation/perfusion mismatching often occurs. Several authors have investigated the effect of lower inspired oxygen fractions (FiO(2)) to reduce formation of absorption atelectasis. This systematic review compared the effects of low (<0.6) and high (>0.8) FiO(2) on the arterial oxygen tension (PaO(2)), the alveolar-to-arterial oxygen tension difference (P(A-a)O(2)), and the PaO(2)/FiO(2) ratio in horses during inhalation anaesthesia. Using the Systematic Review Protocol for Animal Intervention Studies, four experimental and one clinical investigations were deemed suitable for inclusion. A meta-analysis was performed on the four experimental studies. The PaO(2) was significantly lower (p = 0.0007, mean difference −23.54 kPa, 95% CI −37.18, −9.90) with a lower FiO(2). However, the P(A-a)O(2) was also significantly lower (p < 0.00001, mean difference −20.80 kPa, 95% CI −26.28, −15.32) when using a low FiO(2). For the PaO(2)/FiO(2) ratio, only one study fitted the inclusion criteria, so no meta-analysis was performed. It is concluded that, while only a limited number of studies are available, the use of a higher FiO(2) in horses during inhalation anaesthesia will result in higher levels of PaO(2), but also a larger P(A-a)O(2) difference. Further studies are needed to increase the level of evidence on this subject.