Polycythemia

Abstract

Dr. Richard J. Cross: Most cases of polycythemia are secondary to a systemic hypoxemia. Perhaps the most clear-cut example of this is found in those who reside at high altitudes. Extensive studies of these subjects have provided considerable information about how they compensate for their environment. The increase in hemoglobin mass provides higher oxygen tensions in the tissues, although it tends to diminish oxygen uptake in the lungs. Recent studies have shown that in residents at high altitudes there is a decrease in the affinity of hemoglobin for oxygen, which serves to raise still higher the oxygen tension in the tissues. Mild degrees of oxyhemoglobin unsaturation are often encountered in polycythemia vera; their cause is uncertain. Marked unsaturation is more rarely encountered and may indicate that the polycythemia is secondary to damage to the respiratory center or other interference with the normal ventilatory drive. Other causes of polycythemia in the absence of heart or lung disease include methemoglobin, which when present in large amounts can reduce the oxygen supply to the tissues and stimulate erythropoiesis. Polycythemia vera has long been classified as a myeloproliferative disorder, but great areas of ignorance remain concerning its pathogenesis and etiology. Recent research in this disease has uncovered certain facts of considerable interest. (1) The metabolic activity of the leukocytes of polycythemia vera with a leukemoid reaction differ from the morphologically indistinguishable leukocytes of chronic myeloid leukemia. (2) Polycythemia vera results from an overproduction of erythrocytes of normal life span. (3) The accelerated disappearance of iron from the plasma of patients with polycythemia vera is probably related to erythroid activity, increased iron demand and possibly to iron deficiency; whether other factors are involved is not known. (4) The observation of Chromatographic differences in the whole blood of patients with polycythemia may provide some interesting leads in the study of the pathogenesis of this disorder. Considerable evidence has recently been uncovered which suggests that the formation of red blood cells may be under hormonal control, but no clear-cut proof of this hypothesis has yet been adduced. Current research in this area seems to indicate that erythropoiesis is an enormously complex process of which we have little understanding at present.