Respiratory function in systemic lupus erythematosus: relation with activity and severity

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Abstract
The objective of this study was to examine the relation between respiratory function tests, disease activity and disease severity in ambulatory patients with systemic lupus erythematosus (SLE) who did not present with overt respiratory problems. Lung volumes, maximal expiratory flows at 50% and 25% of vital capacity (MEF50 and MEF25), bronchial threshold to methacholine (PD15FEV 1), transfer factor CO (KCO) were measured in 24 consecutive SLE outpatients (22 women, age 41 ± 14.8 years) and in 24 healthy controls matched for age and sex. In SLE patients alveolar-arterial oxygen gradient (AaO2) was also measured. Disease activity was assessed by European Consensus Lupus Activity Measurement (ECLAM) scoring system and disease severity by Lupus Severity of Disease Index. In comparison to controls SLE patients showed a significant decrease of total lung capacity (TLC) (91.7 ± 16.5 vs 102.7 ± 12.9% predicted, P < 0.01), MEF25 (58.4 ± 25.2 vs 73.5 ± 19.5% predicted, P < 0.005), PD15FEV1 (2164 ± 1122 vs 4230 ± 1014 μg methacholine, P < 0.0001) and KCO (77.1 ± 20.5 vs 96.3 ± 12.4% predicted, P < 0.001). AaO2 (mean value 13.2 ± 8.4) was abnormally high (>20 mm Hg) in 12 patients. The ECLAM score of activity was inversely related with KCO (r = 0.48, P < 0.02). The severity index was significantly related with FEV1/VC ratio (r = 0.43, P < 0.05), MEF50 (r = 0.51, P < 0.01), MEF25 (r = 0.40, P < 0.05) and PD15FEV1 (r = 0.51, P < 0.01). In eight patients, evaluated also after treatment intensification, there was a significant increase in KCO (from 71.8 ± 24.7 to 84.9 ± 22.3% predicted, P < 0.01) along with a decrease in ECLAM score (from 3.0 ± 1.34 to 0.69 ± 0.75, P < 0.01). The relation between disease activity and KCO suggests a relation between systemic and alveolar inflammation whereas the relation between severity index, airway patency and reactivity indices suggests a cumulative damage to the airways in SLE patients, even in the absence of overt respiratory manifestations.