The efficient sequestration of hemoglobin by the red blood cell membrane
and the presence of multiple hemoglobin clearance mechanisms suggest a critical
need to prevent the buildup of this molecule in the plasma. A growing list
of clinical manifestations attributed to hemoglobin release in a variety of
acquired and iatrogenic hemolytic disorders suggests that hemolysis and hemoglobinemia
should be considered as a novel mechanism of human disease.
Pertinent scientific literature databases and references were searched
through October 2004 using terms that encompassed various aspects of hemolysis,
hemoglobin preparations, clinical symptoms associated with plasma hemoglobin,
nitric oxide in hemolysis, anemia, pulmonary hypertension, paroxysmal nocturnal
hemoglobinuria, and sickle-cell disease.
Hemoglobin is released into the plasma from the erythrocyte during intravascular
hemolysis in hereditary, acquired, and iatrogenic hemolytic conditions. When
the capacity of protective hemoglobin-scavenging mechanisms has been saturated,
levels of cell-free hemoglobin increase in the plasma, resulting in the consumption
of nitric oxide and clinical sequelae. Nitric oxide plays a major role in
vascular homeostasis and has been shown to be a critical regulator of basal
and stress-mediated smooth muscle relaxation and vasomotor tone, endothelial
adhesion molecule expression, and platelet activation and aggregation. Thus,
clinical consequences of excessive cell-free plasma hemoglobin levels during
intravascular hemolysis or the administration of hemoglobin preparations include
dystonias involving the gastrointestinal, cardiovascular, pulmonary, and urogenital
systems, as well as clotting disorders. Many of the clinical sequelae of intravascular
hemolysis in a prototypic hemolytic disease, paroxysmal nocturnal hemoglobinuria,
are readily explained by hemoglobin-mediated nitric oxide scavenging.
A growing body of evidence supports the existence of a novel mechanism
of human disease, namely, hemolysis-associated smooth muscle dystonia, vasculopathy,
and endothelial dysfunction.