Effect of posture and some c.s.f. characteristics on spinal anaesthesia with isobaric 0.5 % bupivacaine

Abstract

The effect of the sitting position (0,2.5, 5.0 or 7.5 min), during and after the injection of 3 ml of 0.5 % isobanc bupivacaine-HQ, on the segmental spread of spinal analgesia was studied in 40 patients. Ten patients injected in the lateral horizontal position acted as controls The spread of analgesia was significantly greater in those who sat for 2.5min or more compared with those who were immediately put in the supine position. Prolongation of the sitting time did not produce a higher analgesic block. The motor block was complete in all patients; its duration was significantly longer in the horizontal group than in all the sitting groups. There was no significant correlation between the different c.s.f. indices (pressure, protein and chloride ion concentra-tion, pH, and specific gravity) and the spinal block. In a previous study we found that when patients were in a sitting position the spread of isobaric 0.75 % bupivacaine analgesia was significantly grea-ter compared with patients who were supine (Tuominen, Kalso and Rosenberg, 1982). A similar trend was seen with 0.5 % bupivacaine. The sitting time was set at 2.5 min. The present study compares the influence of different sitting times on the spread and duration of spinal block. The spread of spinal analgesia with isobaric bupivacaine may be influenced by a number of factors: bupivacaine has a high affinity for proteins; the pH of the cerebrospinal fluid (c.s.f.) affects the ionization of bupivacaine; the c.s.f. chloride con-centration influences the osmotic equilibrium; the concentration of local anaesthetics in the c.s.f. is inversely proportional to the distance from the injec-tion site; and it has been suggested that the c.s.f. pressure is inversely proportional to the height of the spinal block (Helrich et al., 1950; Lund, 1971