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Adults with polycythaemia secondary to cyanotic congenital heart disease may be at greater risk from injudicious venesection than from their polycythaemia.1 Despite an established literature on the subject, patients are frequently put at risk from acute volume depletion and chronic iron deficiency. This article aims to clarify some of the issues surrounding venesection and to set out guidelines for when and how to venesect these patients.
The polycythaemia of chronic hypoxaemia may be more precisely termed erythrocytosis as, in contrast to polycythaemia rubra vera,2 it is the red blood cell mass alone that is increased. This secondary polycythaemia, or erythrocytosis, is a physiological response to tissue hypoxia. Hypoxia increases erythropoetin, which in turn stimulates the bone marrow to produce increased numbers of circulating red cells, enhancing oxygen carrying capacity as well as producing an increase in the erythrocyte mass, haematocrit, and whole blood viscosity. The improved tissue oxygenation that results from this adaptation may be sufficient to reach a new equilibrium at a higher haematocrit.3 However, adaptive failure can occur if the increased whole blood viscosity impairs oxygen delivery and negates the beneficial effects of erythrocytosis.1 4
Polycythaemic cyanotic patients experience symptoms caused by the detrimental effects of hyperviscosity on tissue oxygen delivery rather than by a high haematocrit itself (table …
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