Ephedrine
Ephedrine is a vasoactive drug commonly used to treat maternal hypotension during regional anesthesia, such as epidurals or spinals. It works mainly through beta-adrenergic stimulation, increasing maternal heart rate and cardiac output and, to a lesser extent, inducing vasoconstriction through alpha-receptors and release of norepinephrine, thereby raising blood pressure. Because its effect depends on norepinephrine release it is slow in onset (several minutes) and prone to tachyphylaxis. Ephedrine easily crosses the placenta (UV/MA ratio about 1.1, compared with about 0.2 for phenylephrine), which can impact fetal circulation.
Fetal Effect: The primary concern with ephedrine is its association with a decrease in fetal umbilical arterial pH and base excess, which results from beta-adrenergic stimulation of the fetus itself: ephedrine crosses freely and raises fetal lactate, glucose and catecholamine concentrations, that is, it increases fetal metabolic rate, rather than reducing uteroplacental blood flow as was once assumed. The difference is dose-related and usually modest (umbilical arterial pH a few hundredths lower, and in meta-analyses no clear increase in true fetal acidosis with pH below 7.20 after elective cesarean), but it has led most practitioners to favor phenylephrine as the first-line agent, keeping ephedrine for hypotension accompanied by maternal bradycardia or for patients unresponsive to phenylephrine.
Phenylephrine
Phenylephrine is another vasoactive drug used to manage maternal hypotension, particularly during regional anesthesia for cesarean deliveries. It works primarily by inducing vasoconstriction through stimulation of alpha-1 receptors, which raises maternal blood pressure and, through the baroreceptor reflex, slows the heart rate and lowers cardiac output; reflex bradycardia is its characteristic maternal side effect, and when the heart rate falls below about 50 to 60 beats per minute with hypotension the infusion is paused and ephedrine or an anticholinergic is given. Phenylephrine crosses the placenta only to a small degree (UV/MA ratio about 0.2), and its effects on the fetus are generally more favorable compared to ephedrine.
Fetal Effect: Phenylephrine is less likely to cause fetal acidosis compared to ephedrine. When used in conjunction with a crystalloid co-load given at the time of the spinal, phenylephrine has been shown to effectively maintain maternal blood pressure while minimizing adverse effects on the fetus, such as reduced umbilical arterial pH or acidosis. The 2018 international consensus statement therefore recommends a prophylactic phenylephrine infusion (started at about 25 to 50 mcg/min immediately after the spinal and titrated to keep systolic pressure at or above 90% of baseline, never below 80%) as first-line management of spinal hypotension for cesarean delivery, with norepinephrine as an acceptable alternative that causes less bradycardia. This makes phenylephrine the preferred choice for managing maternal hypotension during obstetric anesthesia, as it effectively stabilizes maternal hemodynamics without compromising fetal health.
| Feature | Ephedrine | Phenylephrine |
|---|---|---|
| Mechanism | Beta-adrenergic: raises heart rate and cardiac output; indirect alpha effect through norepinephrine release (slow onset, tachyphylaxis) | Direct vasoconstriction through alpha-1 receptor stimulation (fast onset, titratable infusion) |
| Maternal heart rate | Increases; useful when hypotension comes with bradycardia | Reflex bradycardia and a fall in cardiac output; pause the infusion and give ephedrine or an anticholinergic if the rate falls below about 50 to 60 with hypotension |
| Placental transfer | Crosses easily (UV/MA about 1.1) and stimulates the fetus directly | Crosses only slightly (UV/MA about 0.2) |
| Fetal acid–base effect | Lower umbilical arterial pH and base excess from fetal beta-stimulation (higher fetal lactate, glucose and catecholamines); modest and dose-related | Less likely to cause fetal acidosis or reduced umbilical arterial pH |
| Practical use | Second line: hypotension with maternal bradycardia, or failure of phenylephrine | First line (international consensus 2018): prophylactic infusion 25 to 50 mcg/min titrated to at least 90% of baseline systolic pressure, with a crystalloid co-load; norepinephrine is an alternative |
