Introduction: We recently observed elevated cell free hemoglobin (CFH), a pulmonary vasoconstrictor, in humans with pulmonary arterial hypertension (PAH). The mechanism of CFH elevation in PAH is unknown. We hypothesized that CFH elevation would be multifactorial, mediated by dysfunction of hemoglobin processing proteins and hemolysis across the pulmonary vasculature.
Methods: We measured haptoglobin, hemopexin, CD163, and hemo oxygenase-1 concentration in peripheral blood from subjects with idiopathic or heritable PAH, unaffected bone morphogenetic receptor protein type II mutation carriers (UMC), and subjects without PH. We also measured the transpulmonary CFH gradient (wedge minus pulmonary artery value) in patients with PAH or pulmonary venous hypertension (PVH) at the time of right heart catheterization. Non-parametric tests were used; p<0.05 was considered significant.
Results: We observed 33% higher soluble CD163 and 72% higher hemopexin in PAH subjects compared with noPH controls (p=0.001 and 0.001, respectively; Table). Haptoglobin was highest among UMCs and over two-fold higher in UMCs compared with HPAH (1781±860μg/mL vs. 821±775μg/mL, p<0.001). CFH increased across the lungs by 22% in patients with PAH (n=12) versus a 4% decrease in PVH (n=9) (+7.0±8.8mg/dL vs -1.8±8.5mg/dL, p=0.03). Transpulmonary CFH gradient correlated with pulmonary vascular resistance (rs= 0.63, p = 0.027) in PAH subjects.
Conclusion: Elevation of CD163 in PAH suggests dysfunctional macrophage clearance of CFH. Markedly elevated haptoglobin in UMCs versus HPAH suggests haptoglobin production play a role in PAH penetrance. CFH is generated across the lungs in PAH subjects indicating low level hemolysis that appears to contribute directly to vascular load. Further studies are needed to characterize the clinical implications of these findings.