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 Spo₂ 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 Spo₂max 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.