Effects of transfusion-induced polycythemia on O₂ transport during exercise in the dog

An increased hematocrit could enhance peripheral O₂ transport during exercise by improving arterial O₂ content. Conversely, it could reduce maximal delivery of O₂ by limiting cardiac output during exercise or by limiting the distribution of blood flow to peripheral capillaries with high O₂ extractions. We studied O₂ transport at rest and during graded treadmill exercise in splenectomized tracheostomized dogs at normal hematocrit (38 ± 3%), and 48 h after transfusion of type-matched donor cells. This procedure increased hematocrit (60 ± 3%) but also increased blood volume (P < 0.05). Following transfusion, resting cardiac output (Q̇T) and heart rate were not different. During exercise, Q̇T was significantly lower at each level of O₂ consumption (V̇O₂) at high hematocrit (P < 0.01). A reduction in Q̇T was also seen during polycythemic exercise with hypoxemia produced by breathing 12 or 10% O₂ in N₂. Despite the reduction in Q̇T, mixed venous PO₂ was not lower at high hematocrit, and the increase in base deficit with VO₂ was not different from control measurements. O₂ delivery (Q̇T x arterial content) was similar at each level of V̇O₂ at both levels of hematocrit, during both normoxic and hypoxic studies. Both systemic and pulmonary arterial pressures were increased at rest after transfusion (P < 0.05). However, pulmonary and systemic pressures were not higher than control during exercise at high hematocrit. We conclude that a hematocrit of 60% with increased blood volume is not associated with a cardiac limitation of O₂ delivery, nor does it interfere with peripheral O₂ extraction during exercise in the dog.