BACKGROUND: The present study was done to elucidate the effects of acute normovolemic hemodilution (ANH) on intrapulmonary shunt (Qs/Qt) and systemic oxygen delivery balance during one lung ventilation (OLV). METHODS: To induce one lung ventilation, an atelectasis of the right lung was produced in anesthetized mongrel dogs. In 6 dogs with OLV, ANH was produced by sequential hemodilution with hydroxyethyl starch. ANH was divided into 3 stages (ANH0: no hemodilition, ANH1: first hemodilution, ANH2: second hemodilution). Qs/Qt was measured by using blood gas analysis. Various hemodynamic parameters, oxygen delivery, and consumption were measured or calculated indirectly. RESULTS: After hemodilution, hemoglobin levels at each stage were 9.9 +/- 1.3 g/dl (ANH0), 7.0 +/- 1.0 g/dl (ANH1), and 5.2 +/- 0.7 g/dl (ANH2). The Qs/Qt of ANH2 stage increased from 25.0 11.4% of ANH0 to 35.4 9.2% (P < 0.05). Cardiac output of ANH2 increased from 2.4 +/- 0.8 ml/min of ANH0 to 3.2 +/- 0.8 ml/min (P < 0.05). Pulmonary and systemic vascular resistance measurements in ANH2 were lower than those of ANH0 (P < 0.05). The changes in pH and carbon dioxide tension and mixed venous oxygen tension by ANH were not significant in comparison with ANH0 (P > 0.05). Global oxygen delivery was markedly decreased by hemodilution in OLV (P < 0.05), whereas global oxygen consumption was maintained. CONCLUSIONS: We conclude that global oxygen delivery balance is preserved by ANH in this study. However, extreme ANH has a deleterious effect on pulmonary gas exchange, possibly through the attenuation of hypoxic pulmonary vasoconstriction during one-lung ventilation. On the basis of this study, increased cardiac output generated by ANH might be the cause of inhibition or blunting of hypoxic pulmonary vasoconstriction.