Determination of SpO₂/FiO₂ thresholds to impute for PaO₂/FiO₂ ratios in the Sequential Organ Failure Assessment score

Introduction

The Sequential Organ Failure Assessment (SOFA) score is a well-validated and effective method by which to measure organ dysfunction in critically ill patients and is a good indicator of prognosis. However, repeated arterial blood gas data are often lacking for calculating the respiratory component of the SOFA score on a daily basis. The aim of this study was to derive SpO₂/FiO₂ (S/F) ratio correlations with the PaO₂/FiO₂ (P/F) ratio in assessing the respiratory parameters of the SOFA score, incorporating the level of positive end-expiratory pressure (PEEP).

Methods

Matched measurements of SpO₂ (limited to values ≤ 98%) and PaO₂ from patients admitted with acute respiratory distress syndrome were used to determine the relationship between S/F and P/F, using three separate linear regression models, for patients on <8 cmH₂O PEEP, 8–12 cmH₂O PEEP and >12 cmH₂O PEEP. S/F threshold values correlating with P/F ratios of 100, 200, 300 and 400 were determined, using the equation derived from the model.

Results

The linear regression coefficients of determination (R²) for the three models (PEEP < 8 cmH₂O, n = 1,107; PEEP 8–12 cmH₂O, n = 1,404 and PEEP >12 cmH₂O, n = 405) were 0.58, 0.61 and 0.59, respectively. S/F threshold values correlating with P/F ratios of 100, 200, 300 and 400, according to PEEP, are presented in Table 1.

Table 1 (abstract P499)

Conclusion

S/F ratios categorized by the level of PEEP can be used to impute for P/F ratios in the calculation of the respiratory component of the SOFA score.