Estimated Pao 2: A Continuous and Noninvasive Method to Estimate Pao 2 and Oxygenation Index

Abstract

BACKGROUND: Pa o 2 is the gold standard to assess acute hypoxic respiratory failure, but it is only routinely available by intermittent spot checks, precluding any automatic continuous analysis for bedside tools. OBJECTIVE: To validate a continuous and noninvasive method to estimate hypoxemia severity for all Sp o 2 values. DERIVATION COHORT: All patients who had an arterial blood gas and simultaneous continuous noninvasive monitoring from 2011 to 2019 at Boston Children’s Hospital (Boston, MA) PICU. VALIDATION COHORT: External cohort at Sainte-Justine Hospital PICU (Montreal, QC, Canada) from 2017 to 2020. PREDICTION MODEL: We estimated the Pa o 2 using three kinds of neural networks and an empirically optimized mathematical model derived from known physiologic equations. RESULTS: We included 52,879 Pa o 2 (3,252 patients) in the derivation dataset and 12,047 Pa o 2 (926 patients) in the validation dataset. The mean function on the last minute before the arterial blood gas had the lowest bias (bias –0.1% validation cohort). A difference greater than or equal to 3% between pulse rate and electrical heart rate decreased the intraclass correlation coefficients (0.75 vs 0.44; p < 0.001) implying measurement noise. Our estimated Pa o 2 equation had the highest intraclass correlation coefficient (0.38; 95% CI, 0.36–0.39; validation cohort) and outperformed neural networks and existing equations. Using the estimated Pa o 2 to estimate the oxygenation index showed a significantly better hypoxemia classification (kappa) than oxygenation saturation index for both Sp o 2 less than or equal to 97% (0.79 vs 0.60; p < 0.001) and Sp o 2 greater than 97% (0.58 vs 0.52; p < 0.001). CONCLUSION: The estimated Pa o 2 using pulse rate and electrical heart rate Sp o 2 validation allows a continuous and noninvasive estimation of the oxygenation index that is valid for Sp o 2 less than or equal to 97% and for Sp o 2 greater than 97%. Display of continuous analysis of estimated Pa o 2 and estimated oxygenation index may provide decision support to assist with hypoxemia diagnosis and oxygen titration in critically ill patients.