[Motor block induced by spinal injection of pethidine. Quantification and comparison with lidocaine]

Intrathecal pethidine (or meperidine) at a dose of 1 mg.kg-1 has proved to be effective and useful for surgical procedures by producing a spinal nerve block due to its local anaesthetic properties. Motor blockade was assessed by dynamometric measurement of the dorsal flexion of foot in ten ASA I patients after subarachnoid pethidine, and in seven ASA I patients after subarachnoid 5% lidocaine at the same dosage. Measurements were made every 5 min during the first 30 min, then every 15 min until complete motor block recovery was obtained. Values were expressed as a percentage of the initial muscle strength registered before lumbar puncture. The two drugs gave a dramatic and similar decrease in muscle strength (92 vs 95%; NS). However, the delay of maximal effect was significantly longer with pethidine than with lidocaine (33.5 +/- 14 min vs 19 +/- 5.6 min; p less than 0.05). The duration of the motor blockade was similar for both drugs (116 vs 106 min) and the 25%-75% time intervals were identical (32 +/- 14 min vs 33 +/- 12.6 min). Two severe respiratory depressions were observed with pethidine, which were rapidly reversible by intravenous naloxone. Numerous other minor side-effects occurred more frequently with meperidine than with lidocaine. It was concluded that dynamometry gave an accurate description of motor blockade induced by spinal anaesthesia.