[The dose-effect-ratio of isobaric bupivacaine in spinal analgesia (author's transl)].

Isobaric bupivacaine 0.5% with adrenaline 1:200,000 was used for spinal anaesthesia. The influence of dose (mg) and volume (ml) on segmental spread and duration of spinal block was investigated. Bupivacaine 10 or 15 mg was injected in 2 or 3 ml of solution. The segmental spread and duration of analgesic block was tested at certain time intervals. The results showed that: 1. an increase in dose is not followed by a larger segmental spread or longer duration of analgesic block, 2. the increase of the volume injected is followed by an increase in segmental spread of analgesic block. This is independent of the concentration of the local anaesthetic, 3. the segmental regression of analgesic block starts in the cephalad segments. Therefore, higher spread of analgesic block provides prolonged analgesia in the lower segments. The results of this investigation are controversal in relation to the reports in the literature of the last two decades. The increase in dosage of the local anaesthetic did not increase the segmental spread and duration of analgesia. Only the volume injected is responsible for an increase in segmental spread of analgesia. Of course the concentration of the local anaesthetic injected has to be at least the minimal concentration necessary to result in a sensory nerve block.