Isoproterenol is an effective marker of intravenous injection in laboring women

The purpose of this randomized, double-blind study was to determine if isoproterenol 5 micrograms iv produces a consistent, noticeable tachycardia in healthy, laboring women. Maternal heart rate, fetal heart rate, and uterine contractions were continuously recorded and maternal blood pressure was measured every minute for 10 min before and after each patient received either normal saline (NS group; n = 10) or isoproterenol 5 micrograms (ISO group; n = 10) iv. The data-collecting investigator and a nurse palpating the patient's radial artery determined which solution they thought had been administered. The authors analyzed the maternal heart rate tracings using baseline-to-peak (a greater than or equal to 25 beat/min maternal heart rate increase occurring within 120 s of drug injection and lasting greater than or equal to 15 s) and peak-to-peak (a greater than or equal to 10 beat/min increase in the maximum maternal heart rate during the 2-min postinjection over the maximum maternal heart rate during the 2 min preinjection) criteria for detection of an intravascular marker. Mean maternal heart rate in the ISO group was significantly higher than in the NS group 20, 30, 40, 50, and 60 s following the injection (P less than 0.01). The peak-to-peak criterion and the data-collecting investigator correctly classified all patients. Five ISO group patients were not identified by the baseline-to-peak criterion. The nurse palpating the mother's radial artery misidentified two patients. Systolic blood pressure was significantly higher in ISO group than in NS group patients 1 min (P less than 0.05) and 2 min (P less than 0.01) following drug injection. Diastolic and mean blood pressures did not change. No fetal distress occurred. Isoproterenol 5 micrograms is an effective marker of intravascular injection in laboring women; however, the safety and efficacy of epidural isoproterenol must be demonstrated in animals before isoproterenol can be incorporated in an epidural anesthesia test dose.