[The effect of volume and dosage of isobaric bupivacaine on the sensory spread of spinal anesthesia].

There is some controversy about the relationship of volume, concentration and total dose of bupivacaine in the sensory spread of spinal anesthesia. In this study the effects of volume and dose were investigated. MATERIAL AND METHODS. In this study 120 patients undergoing lower extremity, inguinal or transurethral surgery were randomly divided into six groups. Bupivacaine 15 mg with the addition of epinephrine 1:200,000 was administered in 2 ml (0.75%), 3 ml (0.5%), 6 ml (0.25%) and 9 ml (0.166%) solutions. In addition 3 ml isobaric bupivacaine in doses of 7.5 mg (0.25%), 15 mg (0.5%) and 22.5 mg (0.75%). The spinal puncture was performed via the midline approach at the L3-4 interspace, with the patient in the sitting position. The injection speed was 0.5 ml per second. Immediately after the injection the patients were placed in the supine position. The spread of sensory blockade was assessed by means of the pin-prick method in the midline. Motor blockade was assessed on the Bromage scale 0-3. RESULTS. There were no statistically significant differences in motor blockade or cardiovascular changes. The maximum cephalad spread of analgesia (30 min) between the 15 mg groups with different volumes and concentration was: group I (9 ml): T7.7, group II (6 ml): T7.8, group III (3 ml): T8.5 and group IV (2 ml): T10.1. The differences between group IV 2 ml and the groups receiving 3, 6 and 9 ml were statistically significant (P less than 0.05). There were no statistically significant differences in maximum cephalad spread between the 7.5 mg (3 ml), 15 mg (3 ml) and the 22.5 mg (3 ml) groups. The regression after 180 min was significantly shorter in the 7.5 mg group than in the 15 mg and 22.5 mg groups (P less than 0.05). DISCUSSION. Earlier published results indicate that the dose of isobaric bupivacaine is more important in spinal anesthesia than the concentration or the volume of the solution. The comparison between 3 ml:6 ml and 3 ml:9 ml bupivacaine showed no statistically significant differences in cephalad spread. A volume-dependent increase in segmental spread was between the 2 ml (0.75%) and 3 ml (0.5%) bupivacaine. The same statistically significant differences were between the 2 ml and 6 ml groups and the 2 ml and 9 ml groups. No statistically significant difference in cephalad spread resulted from increasing the dose of bupivacaine from 7.5 mg to 22.5 mg. Earlier studies on the effects of changes in volume, concentration and dose of bupivacaine showed similar "jumps of blockade" between 2 ml and 3 ml injected volume. Assembling the results the relation between volume and total dose does not suggest a no linear dependence. The anatomic configuration of the spinal cord at the conus medullaris may affect the distribution of the solution.