Ephedrine Versus Phenylephrine Infusion for Prevention of Spinal Hypotension During Cesarean Section: Effect on Maternal Cardiodynamics and Fetal Circulation: Randomized Double-blind Study

Spinal anesthesia is widely used as the procedure of choice for cesarean delivery. In comparison to epidural anesthesia it is faster, easier to perform, patients are more comfortable, complication rates are lower, and it is more cost effective. Spinal anesthesia is an accepted technique in elective cesarean sections. However, hypotension, resulted from sympathectomy is a common problem, especially in pregnant women.

Spinal block causes peripheral vasodilation and venous pooling, which may result in maternal hypotension. Maternal hypotension after spinal anesthesia for cesarean delivery, without prophylactic measures, has a very high incidence (80%-100%). Even though highly investigated, spinal induced hypotension remains a major concern, and it has been referred to as the "Holy Grail" of obstetric anesthesia. The detrimental effects of the spinal induced hypotension are maternal and fetal. Maternal effects are nausea, vomiting and dizziness. Hypotension results in reduced uterine and intervillous blood flow with potential fetal hypoxia and acidosis.

Treatment and prevention of hypotension has been the subject of much investigation and controversy. Prophylactic measures include: 1) left lateral tilt, 2) fluid preload, 3) vasopressors,4) low dose spinal anesthesia. A 15° left lateral tilt is used routinely during cesarean section, to prevent aorto-caval compression, however it is not sufficient as a sole method. Left uterine displacement is achieved by tilting the operating table or by placing a wedge under the woman's hip. Aorto-caval compression also may increase the spread of spinal anesthesia. Among the non-pharmacological interventions studied to minimize the incidence of hypotension sitting the patient up for up to 7 min after CSE anesthesia for cesarean section reduced intraoperative ephedrine requirement without affecting the success of the spinal anesthetic. In contrast, sitting up for 9 min resulted in the need for rescue epidural anesthesia without additional benefit.

Phenylephrine Treatment of vascular failure in shock, shock-like states, drug-induced hypotension or hypersensitivity; correction of paroxysmal supraventricular tachycardia; prolongation of spinal anesthesia; vasoconstriction in regional analgesia; maintenance of adequate level of BP during spinal and inhalation anesthesia. It has a number of important attributes for treating spinal hypotension: (i) as an alpha-adrenergic agonist, its mechanism of action directly addresses the decrease in systemic vascular resistance following spinal anesthesia;(ii) phenylephrine has a faster onset of action compared with ephedrine; (iii) ephedrine is associated with a five-fold increased risk of fetal acidosis; and (iv) ephedrine is more likely to cross the placenta and increase concentrations of lactate, glucose, and catecholamines in the fetal circulation compared with phenylephrine. However, phenylephrine used alone may be accompanied by maternal bradycardia and does not benefit from widespread clinical experience, as does ephedrine do. Thus, phenylephrine has not yet become popular, particularly for prophylactic use. Clinical experience suggests that phenylephrine may be useful in addition to ephedrine when the latter fails to correct hypotension.

Ephedrine sulphate is a potent sympathomimetic that stimulates both α and β receptors and has clinical uses related to both actions. Its peripheral actions, which it owes in part to the release of norepinephrine, simulate responses that are obtained when adrenergic nerves are stimulated. These include an increase in blood pressure, stimulation of heart muscle, constriction of arterioles.