Epidural test dose: isoproterenol is a reliable marker for intravascular injection in anesthetized adults

Epidural test doses containing more than 15 micrograms epinephrine are reliable for the detection of intravascular injection based on the conventional systolic blood pressure (SBP) criterion (positive if > or = 15 mm Hg increase) but not on the heart rate (HR) criterion (positive if > or = 20 bpm increase) in adult patients anesthetized with isoflurane. The present study was designed to test whether isoproterenol could be used as a reliable marker. Thirty adult patients were randomly assigned to one of two groups, each of which was anesthetized with 1% end-tidal isoflurane and nitrous oxide after endotracheal intubation. Isoproterenol group (n = 15) was given 3 mL of 1.5% lidocaine with 3 micrograms isoproterenol intravenously (i.v.) to simulate an i.v. administered test dose. The saline group (n = 15) was identical to the isoproterenol group but received 3 mL of normal saline. HR and arterial blood pressure were measured at 20-s intervals for 4 min after i.v. injection. Mean HR in the isoproterenol group was significantly higher than in the saline group from 40 to 240 s after i.v. test dose with a mean maximum HR increase of 32 +/- 7 bpm (mean +/- SD) occurring at 68 +/- 19 s. All 15 patients in the isoproterenol group developed HR increases > or = 20 bpm (sensitivity 100%). Since HR was essentially unchanged in the saline group, specificity and positive and negative predictive values were all 100%. On the other hand, 12 of 15 patients in the isoproterenol group and none in the saline group exhibited SBP increases > or = 15 mm Hg, resulting in 80% sensitivity and 83% negative predictive value. In the isoproterenol group, however, transient systolic hypotension ( < 80% of the preinjection value) occurred in five patients without untoward clinical sequelae. These results indicate that, based on the peak HR response, the epidural test dose containing 3 micrograms isoproterenol is a reliable marker for intravascular injection in adult patients during isoflurane anesthesia.