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.