Effects of Ephedrine, Phenylephrine, Norepinephrine and Vasopressin on Contractility of Human Myometrium and Umbilical Vessels: An In-vitro Study

Hypotension is one of the most common adverse effects of spinal anesthesia for cesarean deliveries, affecting as many as 55-90% of mothers. Hypotension during cesarean deliveries can have detrimental effects on the mother and neonate. Various vasopressors, such as ephedrine, phenylephrine and more recently norepinephrine, have been used for the prevention and treatment of hypotension at cesarean deliveries. Ephedrine was historically considered as the gold standard vasopressor for the management of hypotension during cesarean deliveries. This was based on studies in animal models that showed preserved uteroplacental circulation with ephedrine and not with phenylephrine. However, multiple studies in the past several decades have shown that phenylephrine compared with ephedrine results in a more favorable fetal acid-base status. Consequently, the use of phenylephrine for blood pressure management during cesarean deliveries increased. Recently, norepinephrine was introduced in the obstetrical practice for the management of hypotension at cesarean deliveries, due to its ability to maintain maternal cardiac output better than phenylephrine. Studies have also investigated the use of vasopressin to limit hypotension during CD. There have been case reports of successful vasopressin usage to treat post-spinal hypotension after CD in patients with advanced idiopathic pulmonary arterial hypertension as well as severe mitral stenosis with pulmonary hypertension. Its effect was associated with hemodynamic stability without evidence of harm to the mother or child. However, much controversy still exists surrounding the choice of vasopressor in the obstetric population, in large part due to their varying efficacies, and maternal and fetal effects. Vasopressors used for the treatment of hypotension during cesarean deliveries can have significant direct or indirect effects on the perfusion of uteroplacental and umbilical vessels. Reduction of uteroplacental perfusion and constriction of umbilical vessels can result in fetal acidosis, however, the mechanisms for these effects are unclear. The investigators hypothesize that ephedrine, phenylephrine and norepinephrine and vasopressin have variable effects on the contractility of pregnant myometrium and umbilical arteries due to their variable actions on adrenergic alpha (α) and beta (β) receptors, as well as vasopressin1 and vasopressin2 receptors located in these tissues.