The effects on arterial haemoglobin oxygen saturation and on shunt of increasing cardiac output with dopamine or dobutamine during one-lung ventilation

Theoretically, if the cardiac output were increased in the presence of a given intrapulmonary shunt, the arterial saturation should improve as the venous oxygen extraction per ml of blood decreases if the total oxygen consumption remains constant. Previous work demonstrated that this was not achieved with adrenaline or isoprenaline as increased shunting negated any benefit from improved cardiac output and mixed venous oxygen content. However, pharmacological stimulation of cardiac output and venous oxygen without any increase in shunt should achieve the goal of improved arterial oxygenation. To test this hypothesis, seven pigs were subjected to one-lung ventilation and infused on separate occasions, with dopamine and with dobutamine in random order to increase the cardiac output. The mixed venous oxygen content, shunt fraction, oxygen consumption and arterial oxygen saturation were measured. With both dopamine and dobutamine there was a consistent rise in venous oxygen content. However, with dopamine, the mean shunt rose from 28% to 42% and with dobutamine, the mean shunt rose from 45% to 59% (both changes P<0.01). With dopamine, the mean arterial oxygen saturation fell by 4.7%, and with dobutamine by 2.9%, but neither fall was statistically significant. It is concluded that any benefit to arterial saturation which might occur from a dopamine- or dobutamine-induced increase in mixed venous oxygen content during one-lung ventilation is offset by increased shunting. During one-lung anaesthesia, there would appear to be no benefit to arterial saturation in increasing cardiac output with an infusion of either dopamine or dobutamine.