Efficacy of an Epidural Test Dose in Adult Patients Anesthetized with Isoflurane: Lidocaine Containing 15 micro gram Epinephrine Reliably Increases Arterial Blood Pressure, but Not Heart Rate

When continuous epidural anesthesia is combined with general anesthesia, the only objective sign of intravascular migration of the epidural catheter are the increments of heart rate (HR) or arterial blood pressure after a local anesthetic test dose containing epinephrine.However, the efficacy of a simulated intravenous (IV) test dose in adult patients under general anesthesia has not been determined. 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. The epinephrine group (n = 15) was given 3 mL of 1.5% lidocaine with epinephrine (1:200,000) IV to simulate an IV administered epidural test dose. The saline group (n = 15) was identical to epinephrine group, but received 3 mL of normal saline IV. HR and arterial blood pressure were measured at 20-s intervals for 4 min after IV injection. In the epinephrine group, significant increases in HR compared with the baseline value were observed from 40 to 80 s after the IV test dose with a mean maximum HR increase of 24 +/- 2 bpm (mean +/- SEM) occurring at 48 +/- 3 s. However, 5 of 15 patients in the epinephrine group developed HR increments smaller than 20 bpm (sensitivity 67%). Since HRs were essentially unchanged in the saline group, specificity, positive predictive value (+PV), and negative predictive value (-PV) were 100%, 100%, and 75%, respectively. On the other hand, all patients in the epinephrine group and none in the saline group exhibited systolic blood pressure (SBP) increases more than 15 mm Hg, giving sensitivity, specificity, +PV, and -PV all 100%. We conclude that (a) the low -PV based upon HR criterion does not provide support for the safety of the procedure, and (b) clinical judgment of suspected intravascular migration of the epidural catheter should be made, possibly using SBP criterion in adult patients under isoflurane anesthesia. (Anesth Analg 1995;80:310-4)