Comparison of SpO ₂ /FiO ₂ and PaO ₂ /FiO ₂ Ratios in Mechanically Ventilated Dogs

Objective To determine whether the SpO 2 /FiO 2 (SF) ratio correlates with the PaO 2 /FiO 2 (PF) ratio in mechanically ventilated dogs. Design Prospective, multicenter, longitudinal, observational study. Setting Private veterinary referral center and a veterinary medical teaching institution. Animals Thirty‐seven client‐owned dogs requiring mechanical ventilation between April 2021 and April 2023. Interventions None. Measurements and Main Results SpO 2 and FiO 2 values in mechanically ventilated dogs were recorded at the time of arterial blood gas sampling. SpO 2 values between 80% and 97%, PaO 2 , and FiO 2 were used to calculate and determine the relationship between SF and PF ratios. The weighted Spearman correlation coefficient between the ratios was 0.70 ( p < 0.0001), and the relationship between them was described by the following linear mixed model: SF = 0.50 × (PF) + 103. The linear model predicted that SF ratios of 153, 203, and 253 correspond with PF ratios of 100, 200, and 300, respectively. There was an adequate to outstanding discriminatory ability for SF to predict PF <100 (area under the curve [AUC] = 0.94, 95% CI: 0.91–0.97), PF <200 (AUC = 0.92, 95% CI: 0.88–0.96), and PF <300 (AUC = 0.77, 95% CI: 0.69–0.85). Conclusions In this population, SF and PF ratios were strongly correlated, suggesting that SF ratios may serve as a reliable, noninvasive substitute for PF ratios when assessing oxygenation in mechanically ventilated dogs. The linear mixed model identified SF values corresponding to PF ratios of 100, 200, and 300. However, given the wide level of agreement between SaO 2 and SpO 2 measurements in this study, arterial blood gas analysis remains the gold standard for assessing oxygenation. Further studies are warranted to investigate the ability of SF and PF ratios to predict clinical outcomes.