Quantitative Assessment of Extradural Bupivacaine Analgesia

Abstract

Bupivacaine (0.5%) 20 ml was administered extradurally to six healthy volunteers. It was found that simultaneous application of 10 needles to the skin could evoke pain when analgesia was obtained to one needle stimulation. In addition, a laser beam was used as a quantitative technique to activate simultaneously many cutaneous nociceptors. For 7 h, thresholds (sensory and pain) and pain-evoked brain potentials (amplitude and latency) to laser stimulation were monitored and used for quantitative assessment of onset, efficacy and duration of analgesia at various dermatomes (C7, T8, T10, T12, L1, L3, S1). The onset time of analgesia was shortest and conduction delay longest at the dermatome related to the site of injection (L3). Full analgesia was obtained at L1, L3 and S1, although the peak efficacy at S1 was delayed for 120-180 min after injection. A minor effect was found at dermatome C7 approximately 60 min after injection.

Concentração analgésica mínima da bupivacaína durante infusão peridural contínua após bloqueio subaracnóideo no período pós-operatório de cirurgias ortopédicas da perna, tornozelo e pé

“…Among patients receiving ineffective bupivacaine concentrations, upper sensory block level measured by pinprick remained between T 10 and L 4 , showing that the lack of sensitivity measured by such method does not prevent patients from referring pain in the territory innervated by L 5 and S 1 . This is in line with other studies where only high local anesthetic concentrations produced a complete sensory block of L 5 and S 1 roots [13][14][15][16] . This is because L 5 and S 1 roots are thicker, thus prolonging sensory and motor block onset and decreasing their intensity 17 .…”

Spatial summation of heat induced pain within and between dermatomes

“…epidural and spinal analgesia block pain applied to a small area effectively whereas a stimulus applied to a large area still evokes pain (Arendt-Nielsen et al, 1990;Brennum et al, 1992). Spatial summation may also play a role in relation to hyperalgesia where dorsal horn neurones expand their receptive ® elds and hence more spatial summation of the incoming activity can occur (Cook et al, 1987).…”

Evoked responses in anaesthesia

“…5 79 The amplitude of the vertex potential elicited by argon laser stimulation was reduced by 42% after administration of the local anaesthetic bupivacaine into the epidural space at L2-3. 8 A differential effect was seen at the outer dermatomal limits of the epidural. In the caudal segments, different effects on warmth (sensory) and pain (pinprick) thresholds were seen.…”