Testing an epidural catheter in obstetrics: epinephrine or isoproterenol?

Despite the widespread use of an epinephrine-containing test dose to detect intravascular catheter placement, its use in obstetric anesthesia remains controversial. Reasons for this controversy include a reduced chronotropic response to P-adrenergic agents during pregnancy, increased maternal heart rate variability during uterine contractions, and the observed reduction of uterine blood flow in animal models after the administration of epinephrin e. Additional concerns include the use of epinephrine in preeclamptic patie nts who show an increased response to vasoactive drugs with the risk of a p ossible hypertensive crisis with i.v, injection of epinephrine. In these pa tients a further reduction of the already compromised uterine blood flow ma y subject the fetus to an increased risk of asphyxia. Isoproterenol, as a p ure P-adrenergic drug, may offer several advantages. It leads to a more rel iable heart rate increase in parturients without an increase in mean or dia stolic blood pressure, and uterine blood flow is enhanced with i.v. injecti on. This would suggest a larger safety margin for mother and fetus. However , because there are insufficient data to rule out potential neurotoxicity, the use of isoproterenol has not been approved for epidural or intrathecal use and its place in obstetric anesthesia remains to be demonstrated. As no other safer substitute for epinephrine is currently available, an increasi ng number of anesthesiologists omit a standard test dose containing epineph rine in laboring patients and rely on close clinical observation, frequent aspiration through a multi-orifice catheter and fractionated injections. (C ) 2001 Harcourt Publishers Ltd.