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Szegedi et al.(1) studied the effects of normovol-emic acute hemodilution on arterial oxygenation in supine subjects during one-lung anesthesia (OLA). After hemodilution, patients who had chronic obstructive airways disease (COAD) exhibited decreases in Pao2, whereas there were no changes in subjects with normal lungs, nor in a control group with COAD who did not undergo hemodilution. The effects of hemodilution during OLA have not been previously described, and the reasons for their findings are not obvious. Intuitively, one may suspect that hemodilution increased pulmonary shunting in the COAD group. There are however, a number of factors influencing arterial oxygenation during OLA that need to be considered (2). These factors may be introduced by considering the shunt equation: