How do epidurals cause fetal decelerations or fetal distress?
The epidural placement leads to a decrease in maternal blood pressure. We try to limit the blood pressure decrease by giving a liter of IV fluids prior to the epidural being placed. But, because the epidural does cause vasodilation of the blood vessels, it most definitely leads to blood pressure decreases. Most commonly, it happens right about 20 minutes after the epidural is placed (sometimes sooner). If the BP drops enough, the blood flow to the placenta is decreased, and the baby compensates by dropping his/her heart rate to conserve oxygen consumption. That's the heart rate drops/decelerations. If the BP drops super low, you're going to have severe decels.
Fix the BP - you usually fix the decels.
How do you fix the low blood pressure? The nurse will give another bolus of IV fluids and can give a dose (or several doses) of Ephedrine. The IV fluids help "bulk up" the level of fluid in the blood vessels, with increases pressure on the vessel walls, which leads to a higher blood pressure.
Ephedrine sulfate is a potent sympathomimetic that stimulates both α and β receptors and has clinical uses related to both actions. Itsperipheralactions, which it owes in part to the release of norepinephrine, simulate responses that are obtained when adrenergic nerves are stimulated.
In other words, this is how it works in the body to increase blood pressure and to increase cardiac output, which also positively effects how much blood flow the placenta gets from the mother's body.