Comparison of SpO₂/FiO₂ and PaO₂/FiO₂ Ratios in Mechanically Ventilated Dogs

To determine whether the SpO₂/FiO₂ (SF) ratio correlates with the PaO₂/FiO₂ (PF) ratio in mechanically ventilated dogs.

Prospective, multicenter, longitudinal, observational study.

Private veterinary referral center and a veterinary medical teaching institution.

Thirty-seven client-owned dogs requiring mechanical ventilation between April 2021 and April 2023.

None.

SpO₂ and FiO₂ values in mechanically ventilated dogs were recorded at the time of arterial blood gas sampling. SpO₂ values between 80% and 97%, PaO₂, and FiO₂ 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).

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 SaO₂ and SpO₂ 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.