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Abstract

Introduction: Blood gas analysis is often used to evaluate oxygenation status in critically ill patients. Although arterial blood gas (ABG) remains the gold standard, pulse oximetry (SpO2) is non-invasive and more readily available. Hypothesis: The purpose of this study was to evaluate the correlation of SpO2 and arterial pO2. We hypothesized that SpO2 would correlate with arterial pO2 as predicted by the standard oxygen-hemoglobin dissociation curve. Methods: We performed a prospective cohort study of patients in the emergency department (ED) and intensive care unit (ICU) at a single academic tertiary referral center. Patients were eligible for enrollment if the treating physician ordered an ABG. SpO2 and arterial pO2 were analyzed using paired t-test, Pearson’s chi-square and Pearson’s correlation. Results: There were 156 patients enrolled and 129 patients completed the study. Of the patients completing the study, 53 (41.1%) were in the ED, 41 (31.8%) were in the medical ICU and 35 (27.1%) were in the surgical ICU. There were 123 (95.3%) patients with SpO2 >90%. Of these patients, 116 (94.3%) had SpO2 >90% and arterial pO2 >60 mmHg, 7 (5.7%) had SpO2 >90% and arterial pO2 <60 mmHg. Of these 7 patients, 5 were alkalemic (pH=7.44, 7.44, 7.46, 7.46, 7.52), 1 patient had a neutral pH (pH=7.40) and 1 patient was acidemic (pH=7.25). Conclusions: SpO2 correlated well with arterial pO2 as predicted by the standard oxygen-hemoglobin dissociation curve in a undifferentiated critically ill patient population. In this study, a SpO2 >90% correlated with an arterial pO2 >60 mmHg more than 94% of the time. As alkalemia shifts the standard oxygen-hemoglobin dissociation curve to the left it may cause SpO2 values >90% despite arterial pO2 values <60 mmHg. This shift in the standard oxygen-hemoglobin dissociation curve may be clinically significant and should be considered when interpreting SpO2 in the setting of alkalemia.