INTERSTITIAL LUNG DISEASE

Exercise peripheral oxygen saturation (Spo₂) accurately reflects arterial oxygen saturation (Sao₂) and predicts mortality in systemic sclerosis

J J Swigris¹, X Zhou¹, F S Wamboldt², R du Bois¹, R Keith¹, A Fischer¹, G P Cosgrove¹, S K Frankel¹, D Curran-Everett³, K K Brown¹

¹ Interstitial Lung Disease Program and Autoimmune Lung Center, National Jewich Health, Denver, Colorado, USA
² Division of Psychosocial Medicine, National Jewish Health, Denver, Colorado, USA
³ Division of Biostatistics and Bioinformatics, National Jewish Health, Denver, Colorado, USA

Dr J J Swigris, Interstitial Lung Disease Program and Autoimmune Lung Center, National Jewish Medical and Research Center, 1400 Jackson Street, Denver, CO 80206, USA; swigrisj{at}njc.org

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.

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