Effect of extradural compliance and resistance on spread of extradural analgesia

Abstract

We have studied the effect of extradural compliance and extradural resistance on the spread of extradural analgesia. In 111 patients aged 21-75 yr, compliance and resistance of the extradural space were calculated by a mathematical analysis (using the Windkessel theory) of the extradural pressure-response curve to injection of a given volume of local anaesthetic. The calculated mean extradural compliance was 0.39 (SD 0.13) ml mm Hg-1 and this increased with advancing age (P < 0.01). The total number of analgesic segments blocked was related to extradural compliance (P < 0.01). Segmental dose requirement was related inversely to extradural compliance (P < 0.01). Calculated extradural resistance was 26.8 (14.5) mm Hg s ml-1 and this decreased with advancing age (P < 0.05). The total number of analgesic segments blocked was related inversely to extradural resistance (P < 0.05). Segmental dose requirement was related to extradural resistance (P < 0.05). © 1990 Copyright: 1990 British Journal of Anaesthesia.