Pulmonary gas exchange and exercise capacity in patients with systemic lupus erythematosus

Objective. Exercise tolerance is often reduced in patients with systemic lupus erythematosus (SLE). Mechanisms have been proposed but the underlying causes have not yet been elucidated. The study of pulmonary gas exchange during exercise may be helpful in revealing circulatory, ventilatory, and metabolic abnormalities. We hypothesized that in SLE, exercise aerobic capacity would be reduced due to chronic inactivity and poor muscle energetics. Methods. Thirteen women with SLE and low disease activity were studied; 5 age matched subjects served as controls. Clinical examination, chest radiography, electrocardiogram, and pulmonary function test were all normal. Subjects underwent 1 min incremental cycle ergometer exercise to exhaustion. Oxygen uptake (VO₂), CO₂ output (VCO₂), minute ventilation (V(E)), heart rate (HR), and arterial O₂ saturation were monitored. Anaerobic threshold (AT), VO₂/HR, ΔVO₂/ΔWatt, respiratory rate (RR), T(i)/T(tot), V(E)/VCO₂, and breathing reserve (BR) were computed. Results. At rest, patients exhibited high V(E), respiratory alkalosis, and a wide alveolar-arterial O₂ gradient [(A - a)O₂] during 50% O₂ breathing. Other indexes of respiratory function were within the normal range. In the 6 patients with SLE where pulmonary artery systolic pressure at Doppler echocardiography was measurable, mean level was in the upper limits of normal. During exercise, maximal aerobic capacity was reduced in all patients (VO₂ peak, 1098 ± 74 vs 2150 ± 160 ml/min, p <0.01; AT, 36 ± 3 vs 48 ± 3% predicted VO₂ max, p <0.05). Ventilation adjusted for the metabolic demand (V(E)/VCO₂ at AT) was increased (31 ± 1 vs 24 ± 1; p <0.05). A normal breathing pattern was observed during all tests. No patient stopped exercising because of ventilatory limitation (i.e., they had normal breathing reserve). Conclusion. Reduced muscle aerobic capacity is common in SLE and is most likely because of peripheral muscle deconditioning. Increased ventilatory demand, secondary to diffuse interstitial lung disease, is not a significant contributor to the reduction in exercise tolerance.