Exercise SpO2 Accurately Reflects SaO2 and Predicts Mortality in Systemic Sclerosis

Abstract

Background: Measures of oxygenation have not been assessed for prognostic significance in systemic sclerosis-related interstitial lung disease (SSc-ILD).

Methods: We identified 83 subjects with SSc-ILD who performed a maximal cardiopulmonary exercise test (CPET) with arterial line at our center. We examined agreement between peripheral (SpO2) and arterial oxygen saturation (SaO2). Next, we analyzed survival differences between subgroups of subjects stratified on SpO2. Finally, we used Cox proportional hazards analyses to examine the prognostic capabilities of SpO2.

Results: At maximal exercise, the SpO2-SaO2 difference was 2.98 ± 2.98 and only 15 subjects had a SpO2-SaO2 difference > four points. The survival of SSc-ILD subjects whose maximum exercise SpO2 (SpO2MAX) fell below 89% or whose SpO2MAX fell > 4 points from baseline was worse than subjects in comparator groups (log-rank p = 0.01 and 0.01 respectively). The hazard of death during the median 7.1 years of follow-up was 2.4 times greater for subjects whose SpO2MAX fell below 89% (hazard ratio [HR] = 2.4, 95% confidence interval [CI] 1.1-4.9, p=0.02) or whose SpO2MAX fell > 4 points from baseline (HR = 2.4, CI 1.1-5.0, p=0.02).

Conclusion: Among patients with SSc-ILD, SpO2 is an adequate reflection of SaO2, and radial arterial lines need not be inserted during CPET in these patients. Given the ease of measurement and its prognostic value, SpO2 should be considered as a meaningful clinical and research outcome in patients with SSc-ILD.