The effects of maternal plasma dobutamine levels on fetal oxygenation in anaesthetized sheep

To measure the effects of dobutamine infusion on fetal oxygenation during isoflurane anaesthesia in pregnant ewes.

Prospective randomized experimental study.

Seven clinically normal adult pregnant Rambouillet-Dorset cross ewes with fetuses of 117-122 days gestational age.

The ewes were anaesthetized with ketamine (2 mg kg^-1) IM, and isoflurane (F_E'ISO 2.0%) in oxygen. After instrumentation and stabilization, dobutamine was infused at 4 µg kg^-1minute^-1 for 60 minutes and 10 µg kg^-1minute^-1 for 60 minutes in random order, separated by a 20-minute washout period. Catheters were placed in the maternal and fetal carotid arteries; these were used for continuous blood pressure measurement and intermittent blood sampling.

Maternal mean systemic carotid arterial pressure was 60 mm Hg prior to dobutamine infusion. After 5 minutes of dobutamine infusion, fetal oxygen saturation increased (p < 0.05) from 0.62 (0.17-0.71, minimum-maximum) to 0.72 (0.28-0.78) at a dose of 4 µg kg^-1minute^-1 and to 0.70 (0.20-0.73) at a dose of 10 µg kg^-1minute^-1. These increases were maintained during the infusion and were not significantly different between doses. Maternal oxygen saturation remained constant at 1.0 before and during all infusions. Although maternal heart rate and blood pressure increased (p < 0.05) by 90% and 25%, respectively, with dobutamine, this stimulant effect was not evident in the corresponding fetal variables. Maternal haemoglobin concentration increased 30% (p < 0.05) with each infusion.

Dobutamine at 4 µg kg^-1minute^-1 increases fetal oxygenation that is not improved by a dose of 10 µg kg^-1minute^-1. This increase is largely due to an increase in maternal haemoglobin concentration that, in turn, increases oxygen delivery to the placenta.

The use of dobutamine to treat hypotension in pregnant sheep during isoflurane anaesthesia improves fetal oxygenation. This may be true in other species.