Carbon Monoxide Levels and Sickle Cell Disease Severity

Sickle cell disease is an autosomal recessive disorder and the most common genetic disease affecting African-Americans. Approximately 0.15% of African-Americans are homozygous for sickle cell disease, and 8% have sickle cell trait. Hemoglobin S polymerization leads to red cell rigidity, microvascular obstruction, inflammation, and end-organ ischemic injury. Our published data indicate that up to 50% of sickle cell patients have vascular dysfunction due to impaired bioavailability of endogenous nitric oxide, due in large part to scavenging of nitric oxide by cell-free hemoglobin. In previous studies we have demonstrated that steady-state serum LDH is strongly associated with 1) other markers of intravascular hemolysis including plasma cell-free hemoglobin and arginase levels, 2) levels of soluble endothelial adhesion molecules, and 3) an impaired vasodilatory response to an NO donor. Further, significant steady-state LDH elevation identified a subset of patients in our cohort as well as the CSSCD cohort at increased risk for developing pulmonary hypertension, cutaneous leg ulceration, priapism, and early death. Previous biochemical studies have demonstrated significant transient increases in serum LDH and plasma hemoglobin levels during VOC, and this presumed hyperhemolysis has been confirmed by 51Cr labeled RBC studies that revealed further decreases in RBC survival during VOC. However, serum LDH levels are not a specific biomarker of hemolysis, and furthermore these observations on RBC survival have not been correlated with markers of intravascular hemolysis at baseline in patients with sickle cell disease in order to confirm the presence of chronic hyperhemolysis subphenotypes in sickle cell disease as posited in our previous work.

The current gold standards of random and cohort labeling of RBCs used to quantitate RBC survival suffer from many technical drawbacks that make them impractical for routine clinical use. The production rate of expired CO has previously been used to assess RBC survival, based upon the principal that virtually all CO produced in human beings results from cleavage of the ?-methene bond of heme and is completely excreted via the lungs. Because RBC destruction accounts for approximately 80% of heme turnover in the body, endogenous CO production can be used as a quantitative indicator of RBC life span. Furne et al have previously reported on the development of a simple, rapid, and noninvasive method for determining RBC life span based on measurement of exhaled alveolar CO concentration immediately upon awakening corrected for atmospheric CO, as determined with a device that simulates the body s equilibration with CO with results comparable to standard labeling techniques. We propose that this methodology could also provide a quantitative, simple, and noninvasive test to study the RBC life span and thus rate of hemolysis in those patients with sickle cell disease.

This trial will aim to 1) establish the use of end-alveolar CO concentration as a quantitative measure of RBC life span and hemolytic rate in subjects with sickle cell disease; 2) investigate the association between end-alveolar CO concentration-derived RBC life span and laboratory measures of hemolytic severity; and 3) investigate the association between end-alveolar CO concentration-derived RBC life span and the incidence of various clinical sequelae of sickle cell disease.