Assessment of oxygenation in patients with respiratory diseases is critical for treatment, and it influences outcome beyond estimates provided by prognostic scores. Partial pressure of oxygen in arterial blood (PaO₂) and arterial saturation measured via pulse oximetry (SpO₂) are the two important parameters for monitoring blood oxygenation. PaO₂ is the gold standard that requires invasive arterial blood gas tests for measurement. Pulse oximetry is non-invasive, easily and routinely available, and can be used for monitoring continuously as well. Normal ranges for PaO₂ in healthy volunteers at sea level are between 80 and 100 mm Hg and SpO₂ of 95 and 97%. The amount of dissolved oxygen will readily increase at partial pressures of arterial oxygen exceeding 100 mm Hg and will almost completely saturate hemoglobin.

The ratio of arterial oxygen partial pressure (PaO₂) to fraction of inspired oxygen (FiO²) [P/F ratio] is used as a clinical indicator of disease severity in patients receiving varying levels of FiO₂. Berlin definition of acute respiratory distress syndrome (ARDS) includes arterial PaO₂, setting 3 categories of ARDS based on the degree of hypoxemia: mild (200 mm Hg<P/F ratio ≤300 mm Hg), moderate (100 mm Hg<P/F ratio≤200 mm Hg), and severe (P/F ratio≤100 mm Hg). Monitoring trends in arterial blood gas data provide more clinically relevant information than single measurements. However, repeated arterial puncture in small animals is often impractical and placement of indwelling arterial catheters to facilitate serial sample acquisition may be challenging, especially in distressed patients. These challenges have led clinicians to investigate SpO₂/FiO₂ (S/F) ratio as a non-invasive and alternative marker of P/F ratio.

Many studies investigated the correlation of SpO₂/FiO₂ (SF) with PaO₂/FiO₂ (PF) ratios. In human, S/F ratio has been demonstrated to correlate well with the P/F ratio in both adult and pediatric studies. Rice and colleagues looked at the correlation of S/F ratios in mechanically ventilated patients with ARDS and reported that the relationship between S/F ratio and P/F ratio was described by the following equation: SF=64+0.84´(PF)(p<0.0001; r=0.89). They concluded that an S/F ratio of 235 corresponded with a PF ratio of 200 with 85% sensitivity with 85% specificity, while an S/F ratio of 315 corresponded with a PF ratio of 300 with 91% sensitivity with 56% specificity, respectively. Also, Lobete and colleagues measured oxygen saturation by pulse oximetry and concluded that S/F ratio values for P/F ratio criteria of 100, 200, and 300 were 146 (95% CI: 142–150), 236 (95% CI: 228–244), and 296 (95% CI: 285–308), respectively. Areas under receiver operating characteristic curves for diagnosis of P/F ratio less than 100, 200, and 300 with the S/F ratio were 0.98, 0.95, and 0.95, respectively, and therefore concluded that oxygen saturation as measured by SpO₂/FiO₂ ratio is an adequate noninvasive surrogate marker for PF ratio. Based on these findings, the SF ratio has been shown to be an independent, valid diagnostic indicator for acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) in humans.

In veterinary medicine, the correlation between P/F ratio and S/F ratio was investigated in dogs with variable levels of intervention including breathing room air, nasal cannula, high-flow nasal cannula oxygen, and mechanical ventilation. In a pilot study, arterial blood gas analysis of 38 dogs requiring oxygen assessment were evaluated while dogs were spontaneously breathing room air. Results showed a moderate correlation (r=0.618) as seen, and it was concluded that S/F ratio may be a useful surrogate for P/F ratio in dogs. Farrell and colleagues further demonstrated a stronger correlation (r=0.76) in dogs with respiratory disease that are anesthetized for mechanical ventilation and breathing variable levels of known FiO₂. Carver and colleagues also provide compelling evidence of excellent correlation (r=0.90 to 0.94) between P/F ratio and S/F ratio in healthy dogs recovering post-operatively on either room air or nasal oxygen insufflation. Recently, a retrospective study demonstrated a strong correlation (r=0.89) in dogs with high-flow nasal cannula oxygen therapy, suggesting that SF is a useful surrogative for PF in this patient population.

It is important to note that all these studies evaluated SpO₂<98%, as the correlation between SF and PF ratios would be expected to be lost as the oxygen hemoglobin dissociation curve flattens in that above that value, thus losing the linear correlation between SpO₂ and PaO₂. Additionally, the estimation of the P/F ratio from SpO₂ ignores the effects of body temperature, pH and hemoglobin characteristics on the relationship between saturation and PaO₂. SpO₂ also will not accurately reflect PaO₂ when there are elevated levels of dyshemoglobins such as carboxyhemoglobin and methemoglobin. In conclusion, studies suggest that estimation of S/F ratio is a non-invasive, precise, and reliable marker of P/F ratio for assessing disease severity. S/F ratio is incorporated into severity stratification and prognostication in human patients with acute lung injury and ARDS.

References

1.  Haskins SC. Chapter 15—Hypoxemia. In: Silverstein DC, Hopper KBT. Small Animal Critical Care Medicine. 2nd ed. St. Louis, MO: W.B. Saunders; 2015:81–86.

2.  Lobete C, Medina A, Rey C, Mayordomo-Colunga J, Concha A, Menéndez S. Correlation of oxygen saturation as measured by pulse oximetry/fraction of inspired oxygen ratio with PaO₂/fraction of inspired oxygen ratio in a heterogeneous sample of critically ill children. J Crit Care. 2013;28(4):538.e1-538.e7.6.

3.  Rice TW, Wheeler AP, Bernard GR, Hayden DL, Schoenfeld DA, Ware LB. Comparison of the SpO₂/FIO₂ ratio and the PaO₂/FIO₂ ratio in patients with acute lung injury or ARDS. Chest. 2007;132(2):410–417.

4.  Brown SM, Grissom CK, Moss M, Rice TW, Schoenfeld D, Hou PC, et al. Nonlinear imputation of PaO₂/FIO₂ from SpO2/FIO2 among patients with acute respiratory distress syndrome. Chest. 2016;150(2):307–313.

5.  Calabro JM, Prittie JE, Palma DAD. Preliminary evaluation of the utility of comparing SpO₂/FiO₂ and PaO₂/FiO₂ ratios in dogs. J Vet Emerg Crit Care. 2013;23(3):280–285.

6.  Farrell KS, Hopper K, Cagle LA, Epstein SE. Evaluation of pulse oximetry as a surrogate for PaO₂ in awake dogs breathing room air and anesthetized dogs on mechanical ventilation. J Vet Emerg Crit Care. 2019;29(6):622–629.

7.  Carver A, Bragg R, Sullivan L. Evaluation of PaO₂/FiO₂ and SaO₂/FiO₂ ratios in postoperative dogs recovering on room air or nasal oxygen insufflation. J Vet Emerg Crit Care. 2016;26(3):437–45.