[The effect of the injection speed on the blockade characteristics of hyperbaric bupivacaine and tetracaine in spinal anesthesia].

A lack of uniform methodology used in the assessment of different injection speeds in spinal anesthesia by different authors formed the basis of the current study, which compared under randomized conditions the effects of various injection speeds during intrathecal administration of 4 ml 0.5% hyperbaric bupivacaine or 0.5% hyperbaric tetracaine. MATERIALS AND METHODS. Eighty male ASA Physical Status II and III patients scheduled for transurethral resection of the prostate under spinal anesthesia were selected. They were randomly divided into four groups of 20 each. Patients were given 4 ml of either 0.5% hyperbaric bupivacaine or 0.5% hyperbaric tetracaine according to the following scheme: I: 4 ml tetracaine/0.25 ml.s-1; II: 4 ml tetracaine/0.5 ml.s-1: III: 4 ml bupivacaine/0.25 ml.s-1; IV: 4 ml bupivacaine/0.5 ml.s-1. The study was carried out in a double-blind fashion and puncture was performed at the L3-4 interspace using a 25-gauge needle with the patient in the sitting position. Following injection of the anesthetic solution (0.25 ml.s-1 or 0.5 ml.s-1 without barbotage), the patient was immediately placed in the lithotomy position with the table remaining horizontal. The level of anesthesia using pin prick and the degree of motor blockade, using a 0 to 3+ scale where 0 represented no motor weakness and 3+ complete motor block, were assessed at specific intervals. Statistical analysis was performed using the Mann-Whitney rank sum test: P value of less than 0.05 was considered statistically significant. RESULTS. Bupivacaine injected at 0.25 ml.s-1 was associated with a higher dermatome level than at 0.5 ml.s-1 (P less than 0.05). Time to highest dermatome, however, was shorter with bupivacaine 0.5 ml.s-1 compared to 0.25 ml.s-1 (P less than 0.05) (Table 3, Fig. 1). Time to highest level of analgesia was shorter with tetracaine 0.25 ml.s-1 compared to 0.5 ml.s-1 (P less than 0.05) (Fig. 2). At 0.25 ml.s-1, tetracaine achieved the highest dermatome faster than bupivacaine (P less than 0.05). Tetracaine injected at 0.5 ml.s-1, however, was associated with a higher segmental level than bupivacaine (P less than 0.05). At 0.25 ml.s-1 bupivacaine and tetracaine achieved a 3+ motor block faster than at 0.5 ml.s-1 (P less than 0.05). Complete motor blockade, however, was significantly longer with tetracaine at both injection speeds (0.25 and 0.5 ml.s-1) compared to bupivacaine (P less than 0.05). CONCLUSIONS. The results suggest that 4 ml 0.5% hyperbaric bupivacaine or tetracaine injected at 0.25 or 0.5 ml.s-1 provides a rapid and reproducible spread of analgesia for transurethral surgery. However, our findings suggest that speed of injection is of little i