Low oxygen saturation and mortality in an adult cohort: the Tromsø study

Oxygen saturation has been shown in risk score models to predict mortality in emergency medicine. The aim of this study was to determine whether low oxygen saturation measured by a single-point measurement by pulse oximetry (SpO₂) is associated with increased mortality in the general adult population.

Pulse oximetry was performed in 5,152 participants in a cross-sectional survey in Tromsø, Norway, in 2001–2002 (“Tromsø 5”). Ten-year follow-up data for all-cause mortality and cause of death were obtained from the National Population and the Cause of Death Registries, respectively. Cause of death was grouped into four categories: cardiovascular disease, cancer except lung cancer, pulmonary disease, and others. SpO₂ categories were assessed as predictors for all-cause mortality and death using Cox proportional-hazards regression models after correcting for age, sex, smoking history, body mass index (BMI), C-reactive protein level, self-reported diseases, respiratory symptoms, and spirometry results.

The mean age was 65.8 years, and 56% were women. During the follow-up, 1,046 (20.3%) participants died. The age- and sex-adjusted hazard ratios (HRs) (95% confidence intervals) for all-cause mortality were 1.99 (1.33–2.96) for SpO₂ ≤ 92% and 1.36 (1.15–1.60) for SpO₂ 93–95%, compared with SpO₂ ≥ 96%. In the multivariable Cox proportional-hazards regression models that included self-reported diseases, respiratory symptoms, smoking history, BMI, and CRP levels as the explanatory variables, SpO₂ remained a significant predictor of all-cause mortality. However, after including forced expiratory volume in 1 s percent predicted (FEV₁% predicted), this association was no longer significant. Mortality caused by pulmonary diseases was significantly associated with SpO₂ even when FEV₁% predicted was included in the model.

Low oxygen saturation was independently associated with increased all-cause mortality and mortality caused by pulmonary diseases. When FEV₁% predicted was included in the analysis, the strength of the association weakened but was still statistically significant for mortality caused by pulmonary diseases.