Objective: To determine the frequency of unexplained reversible hypoxemia in patients with systemic lupus erythematosus and to assess the relation between hypoxemia and elevated plasma levels of complement split products.
Design: Cohort study.
Setting: Inpatient and outpatient facilities of the New York University Medical Center/Bellevue Hospital and the Hospital for Joint Diseases.
Patients: Case patients were 22 patients hospitalized with disease exacerbation and no evidence of parenchymal lung disease on chest roentgenogram. Four patients with stable disease were followed in the outpatient clinic, and five healthy normal volunteers served as controls.
Measurements: Plasma levels of complement split products (C3a, factor Bb fragment), alveolar-arterial (A-a) Po2 gradients, and pulmonary function were measured.
Main results: Nine episodes of hypoxemia or hypocapnia (mean A-a gradient, 30.4 +/- 4.8 mm Hg) or both (despite normal chest roentgenogram results) were noted in six hospitalized patients (group 1). Gas exchange improved within 72 hours of steroid therapy (mean A-a gradient, 11.6 +/- 4.3 mm Hg; P less than 0.01). These patients had an elevated initial mean C3a level (938.4 +/- 246.8 ng/mL) that decreased within 72 hours (407.8 +/- 80.9 ng/mL; P less than 0.01), concomitant with improved oxygenation. Ventilation-perfusion scans, obtained for four of six group 1 patients, excluded pulmonary emboli. Four hospitalized patients (group 2) had a normal A-a gradient (mean, 7.5 +/- 2.7 mm Hg). The mean C3a level of this group (358.3 +/- 39.2 ng/mL) was lower than that of group 1 (P less than 0.05). Four patients with stable disease (group 3) had a mean A-a gradient and a mean C3a level of 3.3 +/- 2.7 mm Hg and 237.8 +/- 105.7 ng/mL, respectively, similar to values found in five normal volunteers, in whom the mean A-a gradient was 3.7 +/- 1.7 mm Hg and the mean C3a level was 124.8 +/- 9.2 ng/mL.
Conclusion: A syndrome of reversible hypoxemia, unassociated with parenchymal lung disease, is unexpectedly common in acutely ill, hospitalized patients with systemic lupus erythematosus. The pathogenesis of this syndrome is unclear, although the data are compatible with the hypothesis that hypoxemia may be related to pulmonary leukoaggregation.