Pulmonary Hypertension and the Hypoxic Response in SCD (PUSH)

National Heart, Lung, And Blood Institute

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Abstract

Our published preliminary studies indicate that pulmonary hypertension occurs in nearly one-third of adults with sickle cell disease, that it is associated with increased mortality although pulmonary artery pressures are lower than in patients with primary pulmonary hypertension, and that chronic hemolysis with nitric oxide scavenging may be a part of the pathogenesis. The present proposal is based on three postulates. First, the problem of sickle cell-associated pulmonary hypertension may begin during childhood and adolescence. Second, the pathogenesis of sickle cell-associated pulmonary hypertension may not only include the effects of chronic hemolysis, but also the consequences of chronic hypoxia related to severe anemia and repeated vaso-occlusive episodes. Pulmonary hypertension is a recognized complication of conditions marked by chronic hypoxia, and we have recently found evidence that pulmonary hypertension complicates Chuvash polycythemia, a congenital disorder of oxygen sensing in which the hypoxic response is constitutively up regulated in the absence of hypoxia and in which high hemoglobin concentrations would promote nitric oxide scavenging. Third, the pathophysiology of sickle cell-associated pulmonary hypertension may be elucidated by comparing components of the hypoxic response in patients with sickle cell disease and Chuvash polycythemia according to the presence or absence of pulmonary hypertenson. Both sickle cell disease and Chuvash polycythemia may be characterized by nitric oxide scavenging and upregulated hypoxia inducible factor, leading to stimulation of pulmonary vascular proliferative pathways that eventuate in pulmonary hypertension. Comparing specific responses in both conditions may identify shared pathways that have a central role in sickle cell-related pulmonary hypertenison. New Findings to date: 1. The genetic bases of the highly variable degrees of anaemia and haemolysis in persons with Hb SS are not fully known, but several studies have indicated that G6PD deficiency is not a factor. The G6PD(202A) and G6PD(376G) alleles and alpha-thalassaemia were determined by molecular genetic testing in 261 children and adolescents with Hb SS in a multicentre study. G6PD(202A,376G) (G6PD A-) was defined as hemizygosity for both alleles in males and homozygosity in females. Among the participants 41% were receiving hydroxycarbamide. The prevalence of G6PD(202A,376G) was 13.6% in males and 3.3% in females with an overall prevalence of 8.7%. G6PD(202A,376G) was associated with a 10 g/l decrease in haemoglobin concentration (P = 0.008) but not with increased haemolysis as measured by lactate dehydrogenase, bilirubin, aspartate-aminotransferase, reticulocyte count or a haemolytic component derived from these markers (P >0.09). Similar results were found within a sub-group of children who were not receiving hydroxycarbamide. By comparison, single and double alpha-globin deletions were associated with progressively higher haemoglobin concentrations (P = 0.005 for trend), progressively lower values for haemolytic component (P = 0.007), and increased severe pain episodes (P <0.001). In conclusion, G6PD(202A,376G) may be associated with lower haemoglobin concentration in sickle cell anaemia by a mechanism other than increased haemolysis. 2. Plasma concentrations of interleukin-8, interleukin-10 and VEGF were elevated in the patients with sickle cell disease compared to controls (P <or =0.003). By logistic regression, greater values for PDGF-BB (P = 0.009), interleukin-6 (P = 0.019) and the hemolytic component (P = 0.026) were independently associated with increased odds of elevated tricuspid regurgitation velocity while higher VEGF concentrations were associated with decreased odds (P = 0.005) among the patients with sickle cell disease. These findings, which are consistent with reports that PDGF-BB stimulates and VEGF inhibits vascular smooth muscle cell proliferation, did not apply to E/Etdi. Circulating concentrations of angiogenic and pro-Inflammatory markers are altered in sickle cell disease children and adolescents with elevated tricuspid regurgitation velocity, a subgroup that may be at risk for developing worsening pulmonary hypertension. Further studies to understand the molecular changes in these children are indicated. 3. Hydroxyurea and higher hemoglobin F improve the clinical course and survival in sickle cell disease, but their roles in protecting from pulmonary hypertension are not clear. We studied 399 children and adolescents with sickle cell disease at steady state;38% were being treated with hydroxyurea. Patients on hydroxyurea had higher hemoglobin concentration and lower values for a hemolytic component derived from 4 markers of hemolysis (P <or = .002) but no difference in tricuspid regurgitation velocity compared with those not receiving hydroxyurea;they also had higher hemoglobin F (P <.001) and erythropoietin (P = .012) levels. Hemoglobin F correlated positively with erythropoietin even after adjustment for hemoglobin concentration (P <.001). Greater hemoglobin F and erythropoietin each independently predicted higher regurgitation velocity in addition to the hemolytic component (P <or = .023). In conclusion, increase in hemoglobin F in sickle cell disease may be associated with relatively lower tissue oxygen delivery as reflected in higher erythropoietin concentration. Greater levels of erythropoietin or hemoglobin F were independently associated with higher tricuspid regurgitation velocity after adjustment for degree of hemolysis, suggesting an independent relationship of hypoxia with higher systolic pulmonary artery pressure. The hemolysis-lowering and hemoglobin F-augmenting effects of hydroxyurea may exert countervailing influences on pulmonary blood pressure in sickle cell disease. 4. Low steady state haemoglobin oxygen saturation in patients with sickle cell anaemia has been associated with the degree of anaemia and haemolysis. How much pulmonary dysfunction contributes to low saturation is not clear. In a prospective study of children and adolescents with sickle cell disease aged 3-20 years at steady state and matched controls, 52% of 391 patients versus 24% of 63 controls had steady state oxygen saturation <99% (P <0.0001), 9% of patients versus no controls had saturation <95% (P = 0.008) and 8% of patients versus no controls had exercise-induced reduction in saturation >or =3%. Decreasing haemoglobin concentration (P <or = 0.001) and increasing haemolysis (P <or = 0.003) but not pulmonary function tests were independent predictors of both lower steady-state saturation and exercise-induced reduction in saturation. Neither history of stroke nor history of acute chest syndrome was significantly associated with lower steady-state oxygen saturation or exercise-induced reduction in saturation. Tricuspid regurgitation velocity was higher in patients with lower steady state haemoglobin oxygen saturation (P = 0.003) and with greater decline in oxygen saturation during the six-minute walk (P = 0.022). In conclusion, lower haemoglobin oxygen saturation is independently associated with increasing degrees of anaemia and haemolysis but not pulmonary function abnormalities among children and adolescents with sickle cell disease.