Exercise peripheral oxygen saturation (SpO2) accurately reflects arterial oxygen saturation (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: 83 subjects with SSc-ILD performed a maximal cardiopulmonary exercise test with an arterial line. The agreement between peripheral oxygen saturation (SpO2) and arterial oxygen saturation (SaO2) was examined and survival differences between subgroups of subjects stratified on SpO 2 were analysed. Cox proportional hazards analyses were used to examine the prognostic capabilities of SpO2. Results: At maximal exercise the mean (SD) difference between SpO2 and SaO2 was 2.98 (2.98) and only 15 subjects had a difference of >4 points. The survival of subjects with SSc-ILD 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 2.4, 95% CI 1.1 to 4.9, p = 0.02) or whose SpO 2max fell >4 points from baseline (hazard ratio 2.4, 95% CI 1.1 to 5.0, p = 0.02). Conclusion: In patients with SSc-ILD, SpO2 is an adequate reflection of SaO2 and radial arterial lines need not be inserted during cardiopulmonary exercise tests 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.