Spread of epidural analgesia following a constant pressure injection--an investigation of relationships between locus of injection, epidural pressure and spread of analgesia.

(1) The spread of epidural analgesia following injection of 15 ml of 2% mepivacaine was 17.3 +/- 0.6, 14.3 +/- 0.4, and 13.3 +/- 0.7 spinal segments in cervical, thoracic, and lumbar epidural analgesia, respectively. The patient's age showed significant correlation with the spread of epidural analgesia in cervical (r = 0.5776, p < 0.001), thoracic (r = 0.3758, p < 0.01), and lumbar area (r = 0.8195, p < 0.001). The spread of cervical epidural analgesia was more caudad than cephalad (p < 0.05), but in lumbar epidural analgesia it was more cephalad than caudad (p < 0.05). There was no difference between the cephalad and caudad spread in thoracic epidural analgesia. (2) The epidural pressure immediately after injection of 15 ml of 2% mepivacaine into the lumbar epidural space at a constant pressure (80 mmHg) correlated to the patient's age (r = -0.5714, p < 0.001) and the spread of analgesia (r = -0.3904, p < 0.05). The lower epidural pressure associated with higher age, the wider spread of analgesia. There was no significant correlation between the residual pressure at 60 seconds and the age or the spread of analgesia.