Increasing the haemoglobin concentration results in increased oxygen transport at the cost of increased blood viscosity. This suggested the concept of an optimum packed cell volume for maximising oxygen transport and a study was therefore conducted seeking supportive evidence. Linear cardiac output was measured as minute distance by Doppler ultrasound in 40 patients with haemopoietic disorders who had stable haemoglobin concentrations ranging from 30 to 200 g/l. The correlation between

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... in concentration and minute distance (r= -0-45; p<0-01) was negative, and correlations between haemoglobin concentration and mean blood pressure (r=0-66; p<0-001) and haemoglobin concentration and peripheralresistance (r=0*64; p<0-001)were positive. Calculated oxygen transport increased across the whole range of haemoglobin values. These results suggest (a) that adjustment of peripheral resistance in response to oxygen availability overrides the influence of blood viscosity on cardiac output and (b) that the optimum packed cell volume for oxygen transport is the highest that can be achieved. encountered (0 60 (60%) or 20 g/dl) the relation between haemo-