Use of the SpO₂/FiO₂ Diagram to Assess Gas Exchange in Horses Under General Anaesthesia

The aim of this study was to evaluate the validity of the SpO₂/FiO₂ diagram in estimating gas exchange in horses under general anaesthesia. In this prospective, controlled clinical study were included 10 horses under general anaesthesia. FiO₂ was progressively reduced with the following steps: 0.6, 0.4, 0.3 and 0.21; SpO₂ was recorded at each step. An arterial blood sample was collected at the steps of 1.0 and 0.21, to calculate intrapulmonary shunt with the Fshunt formula. The Fshunt value calculated at 0.21 FiO₂ was defined as "Fshunt 0.21", the one calculated at 1.0 FiO₂ as "Fshunt 1.0". The FiO₂ vs SpO₂ data points were analyzed using a computer algorithm which uses the haemoglobin and a fixed value for arterial-venous oxygen difference of 3.5 mL/dL. The algorithm estimates a shunt value fitting the obtained data with an ideal SpO₂/FiO₂ curve. The value of shunt (Sshunt) was considered for the study. Correlation between "Fshunt 1.0", "Fshunt0.21" and SShunt was determined using the Spearman Rank Correlation Coefficient test, the analysis of the regression curve and the coefficient of determination (r²). Values of P < .05 were considered statistically significant. A significant and strong correlation (P = .0069; r = 0.839; r²=0.6194) and a significant and moderate correlation (P = .0443; r = 0.644; r²=0.2336) was found between Sshunt and "Fshunt 1.0" and between Sshunt and "Fshunt 0.21", respectively. The SpO₂/FiO₂ diagram proved to be a useful and non-invasive tool to characterize gas exchange in horses under general anaesthesia and mechanical ventilation.