Simulation of an epidural test dose with intravenous isoproterenol in isoflurane-anesthetized adults

Although a simulated intravenous (I.V.) test dose containing 3 microg isoproterenol results in a reliable heart rate (HR) increase in anesthetized patients, hypotension may limit its clinical utility. The present study was designed to determine the incidence of hypotension and the reliability of smaller doses of isoproterenol. Forty-five healthy adult patients were anesthetized with 1% end-tidal isoflurane and nitrous oxide after endotracheal intubation and were randomized to one of three groups according to the dose of isoproterenol. Isoproterenol 1-, 2-, and 3-microg groups (n = 15 each) received 3 mL of 1.5% lidocaine with 1, 2, and 3 microg isoproterenol I.V., respectively, to simulate an intravascularly administered test dose. HR and systolic blood pressure were measured at 20-s intervals for 4 min after injection. Mean maximal HR increases were 15 +/- 6, 23 +/- 10, and 32 +/- 7 bpm (mean +/- SD) in the isoproterenol 1-, 2-, and 3-microg groups, respectively. However, the incidence and degree of systolic hypotension were similar among groups. Isoproterenol 3 microg produced 100% sensitivity in both the conventional (> or = 20 bpm increase) and the modified (> or = 10 bpm increase) HR criteria, but 2 microg resulted in 100% sensitivity on the modified criterion alone. Isoproterenol 1 microg did not elicit reliable HR changes. Significant correlation was demonstrated between the isoproterenol dose (microg/kg) and the maximal HR increase. Ninety-five percent confidence intervals to increase HR by 10 and 20 bpm were 0.015-0.02 microg/kg and 0.03-0.04 microg/kg, respectively. The application of isoproterenol as a test dose component seems promising, pending detailed studies of neural toxicity. The appropriate dose needs to be tailored according to the patient's weight.

To determine whether an epidural catheter may be in a blood vessel, various vasoactive drugs are often administered. The author found that isoproterenol might be a useful drug in place of epinephrine.