Dose response study of lidocaine 1% for spinal anaesthesia for lower limb and perineal surgery

To compare the sensory and motor block produced by three different volumes of intrathecal lidocaine 1% and thereby determine the appropriate volume to administer for surgery of the lower limbs and perineum.

Forty-eight patients scheduled for perineal or lower limb surgery were randomly assigned to receive 4, 6 or 8 ml lidocaine 1% intrathecally. The onset, spread, duration and regression of analgesia and motor block and side effects were evaluated (by a blinded observer whenever possible).

The maximum cephalad spread in the 6 ml (T8 +/- 3) and 8 ml (T4 +/- 1.7) groups were higher than the 4 ml group (T12 +/- 2.2, P < 0.01). In the 4 ml group, six patients (33%) did not achieve analgesia to T12 and four (22%) did not have complete motor blockade. Patients given 8 ml had longer duration of block (duration at T12: 104 +/- 23 vs 60 +/- 24, 67 +/- 14 min. P < 0.01: 8 ml vs 4, 6 ml) and slower recovery times (sensory recovery: 188 +/- 27 vs 142 +/- 27, 157 +/- 28 min. P < 0.01; 8 ml vs 4, 6 ml). Two patients (18%) from the 8 ml group and one (5%) from the 6 ml group had transient hypotension.

Four millilitres intrathecal lidocaine 1% is adequate for perineal surgery but for lower limb procedures, 6 ml is more appropriate as it consistently provides sensory analgesia above L1 dermatome and complete motor block. Eight ml gives an unnecessarily high block with higher incidence of hypotension.