EPHEDRINE
Ephedrine remains one of the most extensively studied vasopressors used to treat hypotension in the obstetric population. It has both direct and indirect mechanisms of action, stimulating both alpha- and beta-receptors to increase cardiac output, heart rate and systolic and diastolic blood pressure. Coronary, cerebral and muscle blood flow are increased, while renal and splanchnic blood flow are decreased (1). Studies have indicated that ephedrine returns uterine blood flow towards normal and reverses the fetal acidosis, hypoxia and hypercarbia associated with hypotension (2, 3). The prophylactic intramuscular administration of ephedrine before central neural blockade is no longer advocated due to its variable absorption and potential for causing hypertension (4, 5). Ephedrine may be used prophylactically or therapeutically as intravenous boluses of 5-10 mg or as an infusion (0.01%) which can be titrated to maintain blood pressure in the appropriate range. It appears that when ephedrine is used as an infusion, the incidence of nausea is less than when it is given as a bolus injection (6). The total dose of ephedrine required does not seem to differ significantly whether given as a bolus or as an infusion (6). If the administration of a total dose of 50mgs of ephedrine administered in 5-10mg intravenous increments does not correct the hypotension, phenylephrine should be substituted (7). Etilefrine, another combined alpha- and beta-agonist, compares favourably with ephedrine. Metaraminol restores maternal blood pressure but does not improve the fetal acidosis (2, 7, 8).
PHENYLEPHRINE
Early studies suggested that phenylephrine and methoxamine, both alpha-agonists, would cause uterine vasoconstriction and were, therefore, best avoided in obstetrics (9, 10). Recent work has suggested that phenylephrine can be just as effective as ephedrine in the treatment of hypotension associated with either spinal or epidural anaesthesia and produces no adverse fetal effects (11, 12). If tachycardia is undesirable, phenylephrine may be better than ephedrine. Studies to date involving phenylephrine have been on healthy women and caution has been suggested regarding its concomitant use with ritodrine or magnesium sulphate (13, 14). Phenylephrine may be given intravenously in 20-100 mcg increments (13, 14).
Other vasopressors have been used to treat hypotension secondary to obstetric anaesthesia. Dopamine has been shown to decrease uterine blood flow in animals, and has not been shown to have any benefit over ephedrine in humans (15, 16). Angiotensin II has also been investigated and been found to have no clear advantages (17).
Generally speaking, ephedrine appears to remain the vasopressor of choice to treat hypotension in the obstetric population (Chapter 6). Phenylephrine may be used if a total dose of 50 mg of ephedrine has failed to improve the blood pressure or if tachycardia is deemed detrimental to the maternal situation. In healthy parturients, phenylephrine 20-100 mcg may be an alternative mode of therapy for the hypotension associated with regional anaesthesia.
References:
1. Weiner, N. Norepinephrine, epinephrine, and the sympathomimetic amines. In: Gilman AG, Goodman LS, Rall TW, Murad F (Eds.).
The Pharmacological Basis of therapeutics. New York: Macmillan, 1985: 145-80.
2. James FM III, Greiss FC, Kemp RA. An evaluation of vasopressor therapy for maternal hypotension during spinal anesthesia. Anesthesiology 1970: 33; 25-34.
3. Shnider SM, de Lorimier AA, Holl JW, Chapler FA, Morishima HO. Vasopressors in obstetrics. I. correction of fetal acidosis with ephedrine during spinal hypotension. Am J Obstet Gynecol 1968; 102:911-919.
4. McCrae AF, Wildsmith JAW. Prevention and treatment of hypotension during central neural block. Br J Anaesth 1993; 70: 672-680.
5. Rolbin SH, Cole AFD, Hew EM, Pollard A, Virgint S. Prophylactic intramuscular ephedrine before epidural anaesthesia for Caesarean section: efficacy and actions on the foetus and newborn. Can Anaesth Soc J 1982; 29: 148-153.
6. Kang YG, Abouleish E, Caritis. Prophylactic intravenous ephedrine infusion during spinal anesthesia for Cesarean section. Anesth Analg 1982; 61: 839-842.
7. Rout CC, Rocke DA. Prevention of hypotension following spinal anesthesia for Cesarean section. Int Anesthesiol Clin 1994; 32: 117-135.
8. Shnider SM, deLorimier AA, Steffenson JL. Vasopressors in obstetrics. III. fetal effects of metaraminol infusion during obstetric spinal hypotension. Am J Obstet Gynecol 1970; 108: 1017-1022.
9. Ralston DH, Shnider SM, deLorimier AA. Effects of equipotent ephedrine, metaraminol,mephenteramine and methoxamine on uterine blood flow in the pregnant ewe. Anesthesiology 1974; 40: 354-370.
10. Greiss FC, Pick JR. The uterine vascular bed: adrenergic receptors. Obstetrics and Gynecology 1964; 23:209-213.
11. Ramanthan S, Grant GJ. Vasopressor therapy for hypotension due to epidural anesthesia for Cesarean section. Acta Anaesthesiol Scand 1988; 32: 559-565.
12. Moran DH, Perillo M, LaPorta RF, Bader AM, Datta S. Phenylephrine in the prevention of hypotension following spinal anesthesia for Cesarean delivery. J Clin Anesth 1991; 3: 301-305.
.13. Sipes SL, Chestnut DH, Vincent RD, DeBruyn CS, Bleuer SA, Chatterjee P. Which vasopressor should be used to treat hypotension during magnesium sulfate infusion and epidural anesthesia? Anesthesiology 1992; 77: 101-108.
14. McGrath JM, Chestnut DH, Vincent RD, et al. Ephedrine remains the vasopressor of choice for treatment of hypotension during ritodrine infusion and epidural anesthesia. Anesthesiology 1994; 80: 1073-1081.
15. Rolbin SH, Levinson G, Shnider SM, Biehl DR, Wright RG. Dopamine treatment of spinal hypotension decreases uterine blood flow in the pregnant ewe. Anesthesiology 1979; 51: 36-40.
16. Clark RB, Brunner JA. Dopamine for the treatment of spinal hypotension during Cesarean section. Anesthesiology 1980; 53: 514-517.
17. Ramin SM, Ramin KD, Cox K, Magness RR, Shearer VE, Gant NF. Comparison of prophylactic angiotensin II versus ephedrine infusion for prevention of maternal hypotension during spinal anesthesia. Am J Obstet Gynecol 1994; 171: 734-739.