Equivalent dose of ephedrine and phenylephrine in the prevention of post-spinal hypotension in Caesarean section.
BACKGROUND
Comparative studies of ephedrine and phenylephrine in prevention of hypotension after spinal anaesthesia for Caesarean section have lacked a consensus on dose equivalence. The aim of this study was to determine the minimum vasopressor dose for each of these drugs to calculate the dose ratio for clinical equivalence in the prevention of hypotension.


METHODS
Patients with a normal singleton pregnancy beyond 36 weeks gestation undergoing elective Caesarean section under spinal anaesthesia were randomized into two groups. The first patient in Group A received 50 mg of ephedrine in saline 0.9% w/v, 500 ml, at 999 ml h(-1), the maximum rate possible on the pump and the first patient in Group B received 500 microg of phenylephrine in saline 0.9% w/v, 500 ml, at the same rate. The initial dose for dilution was an arbitrary choice. The dose of vasopressor in the saline bag for every subsequent patient was established by the efficacy of the dose in preventing hypotension in the previous patient according to the technique of up-down sequential allocation. Minimum vasopressor dose for each drug was determined according to the Dixon-Massey formula.


RESULTS
The minimum vasopressor dose in saline 500 ml was 532.9 microg (95% CI 506.0-559.8) for phenylephrine and 43.3 mg (95% CI 39.2-47.3) for ephedrine. The concentration needed for equivalence at an infusion rate of 999 ml h(-1) was 1.07 microg ml(-1) for phenylephrine and 86.66 microg ml(-1) for ephedrine. Mean (sd) dose used for phenylephrine was 496.45 (78.3) microg and for ephedrine 39.64 (6.33) mg.


CONCLUSIONS
This study demonstrates a potency ratio of 81.2 (95% CI 73.0-89.7) for equivalence between phenylephrine and ephedrine in prevention of hypotension after spinal anaesthesia for Caesarean section.
Saravanan
S
S
Department of Anaesthesia, Hull Royal Infirmary, Anlaby Road, Hull, UK.
Kocarev
M
M
Wilson
R C
RC
Watkins
E
E
Columb
M O
MO
Lyons
G
G
D000293:Adolescent; D000328:Adult; D000773:Anesthesia, Obstetrical / Q000009:adverse effects*; D000775:Anesthesia, Spinal / Q000009:adverse effects*; D002585:Cesarean Section; D004305:Dose-Response Relationship, Drug; D004311:Double-Blind Method; D004334:Drug Administration Schedule; D004809:Ephedrine / Q000008:administration & dosage* / Q000627:therapeutic use; D005260:Female; D006801:Humans; D007022:Hypotension / Q000209:etiology / Q000517:prevention & control*; D007430:Intraoperative Care / Q000379:methods; D007431:Intraoperative Complications / Q000517:prevention & control; D010656:Phenylephrine / Q000008:administration & dosage* / Q000627:therapeutic use; D011183:Postoperative Complications / Q000517:prevention & control*; D011247:Pregnancy; D011446:Prospective Studies; D014662:Vasoconstrictor Agents / Q000008:administration & dosage / Q000627:therapeutic use
D014662:Vasoconstrictor Agents; D010656:Phenylephrine; D004809:Ephedrine