Epidural test dose and intravascular injection in obstetrics: Sensitivity, specificity, and lowest effective dose
The authors studied the sensitivity and specificity of several epidural test doses as markers of intravascular injection in laboring patients in a prospective double-blind, randomized study. Fifty-nine parturients were assigned randomly to receive an intravenous injection of either normal saline solution (3 mL, NS group) or 1.5% lidocaine with epinephrine 1:200,000 (1 mL, EPI-5 group; 2 mL, EPI-10 group; or 3 mL, EPI-15 group). The EPI-5 and EPI-10 doses were diluted to 3 mL volume with normal saline solution. All injections were given during uterine diastole. Maternal heart rate was monitored with a pulse oximeter. An observer who was unaware of the study treatment recorded the baseline and the peak maternal heart rate within the first minute after the injection and questioned the patient about tinnitus, dizziness, metallic taste, and palpitations. He then recorded his opinion as to whether the patient had received the saline or the test solution. Analysis of the maternal heart rate showed an average increase (baseline-to peak criterion) of 8 ± 10 beats/min (mean ± SD) in the NS group. In the other groups, the increase was 21 ± 8 (EPI-5 group), 31.5 ± 13 (EPI-10 group), and 29 ± 9 beats/min (EPI-15 group). A baseline-to-peak criterion of >10 beats/min identified all intravascular injections in the EPI-15 (by design) and EPI-10 groups (15 of 15 and 14 of 14, respectively) with a sensitivity of 100%. Specificity was 73% (11 of 15 true negatives). The calculated areas under the receiving operating characteristic curves for the EPI-5, EPI-10, and EPI-15 groups were, respectively, 0.83 ± 0.8 (SE), 0.91 ± 0.5, and 0.93 ± 0.5. For the EPI-10 and EPI-15 groups, positive predictive value (+PV) ranged from 24% to 41% (assuming a prevalence between 8% and 16%). The negative predictive value (-PV) was 100%. The observer also correctly guessed the treatment of all patients in the EPI-10 and EPI-15 groups. Thus, the sensitivity was 100%, whereas the specificity reached 93% (14 of 15 true negatives). For the observer, the positive predictive value was 55%-73% and the negative predictive value 100%. Analysis of the symptoms (alone and in combination) exhibited low sensitivity (<67%). We conclude that (a) a test dose containing either 10 or 15 μg of epinephrine is 100% sensitive as a marker of intravascular injection (area under the receiving operating characteristic curves 0.91-0.93 and negative predictive value 100%); (b) the positive predictive value of this technique may range between 55% and 73%; and (c) 27%-45% of epidural catheters may be unnecessarily removed in the presence of a positive test.