A total of 86.1% of patients had at least 1 PaOdos/FiO2 ratio during the first 24 hours of ventilation (Supplemental Table 2 a ). 81, 95% CI = 0.81-0.82) than with the worst PaO2/FiO2 ratio (AUROC = 0.78, 95% CI = 0.78-0.79, p < 0.001). Results were similar when SF-TAR was compared with the single worst SpO2/FiO2 ratio in the first 24 hours (AUROC = 0.79; 95% CI = 0.79-0.80, p < 0.01). Even when SF-TAR data were limited to either the first 6 or 12 hours of mechanical ventilation, the AUCs for these models were both 0.80 (95% CI = 0.79-0.80 and 0.80-0.81, respectively). Each 10% increase in SF-TAR during the first 24 hours of ventilation was associated with a 24% increase in the odds of hospital mortality (adjusted odds ratio = 1.24, 95% CI = 1.23-1.26, p < 0.001).
Validation Investigation
The MIMIC cohort included 13,755 hospitalizations with mechanical ventilation. The patients’ mean age (SD) was 63.1 (16.0) years, and the median duration of ventilation was 22 hours (IQR = 9-75; Supplemental Table 1 a ). Correlation between SpO2/FiO2 and PaO2/FiO2 ratios was moderate at 0.49 when both values were available concurrently. The hospital mortality rate was 9.7% among patients with SF-TAR of 0% and was 53.0% among those with SF-TAR of 91% to 100% (Figure 2 and Supplemental Table 4 a ). Each 10% increase in SF-TAR was associated with an adjusted odds ratio of 1.26 for hospital mortality (95% CI = 1.23-1.30, p < 0.001).
The UCD cohort included 1088 hospital encounters involving mechanical ventilation where SpO2/FiO2 and PaO2/FiO2 ratios were available concurrently. The mean age (SD) was 54.9 (17.0) years; the median duration of mechanical ventilation was 35 hours (IQR = 12 to 126 hours; see Supplemental Table 1 a ). Correlation between SpO2/FiO2 and PaO2/FiO2 values was 0.50 for all values and 0.56 when comparisons were restricted to SpO2/FiO2 values with saturation less than or equal to 96%. The smooch hospital mortality rate was 9.3% in patients with an SF-TAR of 0% and was 47.4% with an SF-TAR of 91% to 100% (Figure 2 and Supplemental Tables 4 and 5 a ). Each 10% increase in SF-TAR was associated with an adjusted odds ratio for hospital mortality of 1.21 (95% CI = 1.12-1.31, p < 0.001) when a SOFA score lacking the respiratory subscore was included in the model, and 1.16 (95% CI = 1.08-1.28, p < 0.001) using the full SOFA score.
Discussion
In this study, we examined the value of using SpO2/FiO2 ratios to predict mortality among a mixed population of mechanically ventilated patients. Although individual SpO2/FiO2 ratios are ubiquitously available and frequently assessed, they were not consistently correlated with PaO2/FiO2 ratios, which were substantially less available or even unavailable in a sizable proportion of ventilated patients. We thus developed a longitudinal measure of oxygenation (SF-TAR) that is based on the percentage of time that patients exhibited severe hypoxemia (SpO2/FiO2 ratio < 150), and we tested its utility for identifying patients at increased risk of death. In 3 independent datasets, we found that the SF-TAR during the first 24 hours of ventilation was significantly associated with hospital mortality and that the SF-TAR could discriminate between survivors and nonsurvivors at least as well as the current ABG-based gold standard for quantifying severity and prognostication in AHRF.
This study has several implications for diagnosis and prognostication in patients with AHRF, including those with and without ARDS. Recent studies have shown that clinician recognition of ARDS remains poor, with several barriers that hamper both diagnosis and classification of severity. 6,31,32 Lack of standardized timing and frequency of ABG sampling may be a contributory factor. 33,34 Additionally, initiatives to reduce unnecessary testing in the ICU may further decrease the availability of ABG-derived PaO2/FiO2 values. 35-38 In contrast to ABGs, the SpO2 values are ubiquitously available, can be frequently or continuously measured, and are free of risk. Thus, our data suggest that use of SpO2/FiO2 ratios, either as single worst values or when used in a longitudinal SF-TAR approach, should play a central role in improving the recognition of disease severity and prognostication of mortality in mechanically ventilated patients, with and without ARDS, particularly when SpO2 is less than or equal to 96%, when estimates of hypoxemia by SpO2/FiO2 are more accurate. 39