Dietz F B, Jaffe R A
Department of Anesthesia, Stanford University School of Medicine, California 94305-5115, USA.
Anesthesiology. 1997 Jan;86(1):172-80. doi: 10.1097/00000542-199701000-00021.
Clinically, bupivacaine can provide excellent sensory anesthesia with minimal impairment of motor function. However, the mechanisms by which local anesthetics produce differential sensory-motor nerve block is still unknown. The primary site of action for spinal and epidural anesthetics is thought to be the intradural segment of the spinal root. To determine the differential susceptibility of single motor and sensory nerve fibers to local anesthetic conduction block, bupivacaine effects on individual dorsal root (DR) and ventral root (VR) axons were studied.
Lumbar DRs and VRs were excised from anesthetized adult male rats. Single-fiber dissection and recording techniques were used to isolate activity in individual axons. Supramaximal constant-voltage stimuli at 0.3 Hz were delivered to the root. During in vitro perfusion, each root was exposed to increasing concentrations of bupivacaine, and the minimum blocking concentration (C(m)) and the concentration that increased conduction latency by 50% (latency EC50) were measured.
Ventral root axons were significantly more sensitive to the steady-state conduction blocking effects of bupivacaine than were either myelinated or unmyelinated DR axons (DR-C(m), 32.4 microM; VR-C(m), 13.8 microM; P < 0.0001). In addition, VR axons were more susceptible to the latency-increasing effects of bupivacaine than were DR axons (DR-EC50 = 20.7 microM; VR-EC50 = 8.5 microM; P < 0.0001). Within axon groups, differential sensitivity as a function of conduction velocity (axon diameter), or length of nerve exposed to the anesthetic could not be demonstrated.
In contrast to clinical expectations, low concentrations of bupivacaine preferentially block motor (VR) axons in the rat.
临床上，布比卡因能提供出色的感觉麻醉，对运动功能的损害最小。然而，局部麻醉药产生不同感觉 - 运动神经阻滞的机制仍不清楚。脊髓和硬膜外麻醉的主要作用部位被认为是脊髓神经根的硬膜内段。为了确定单个运动和感觉神经纤维对局部麻醉药传导阻滞的不同敏感性，研究了布比卡因对单个背根（DR）和腹根（VR）轴突的影响。
从麻醉的成年雄性大鼠身上切除腰段DR和VR。采用单纤维解剖和记录技术分离单个轴突的活动。以0.3 Hz的超最大恒压刺激施加到神经根。在体外灌注过程中，每个神经根暴露于浓度逐渐增加的布比卡因中，并测量最小阻滞浓度（C(m)）和使传导潜伏期增加50%的浓度（潜伏期EC50）。
腹根轴突对布比卡因的稳态传导阻滞作用比有髓或无髓DR轴突更敏感（DR - C(m)，32.4 μM；VR - C(m)，13.8 μM；P < 0.0001）。此外，VR轴突比DR轴突对布比卡因的潜伏期增加作用更敏感（DR - EC50 = 20.7 μM；VR - EC50 = 8.5 μM；P < 0.0001）。在轴突组内，未发现作为传导速度（轴突直径）或暴露于麻醉药的神经长度函数的差异敏感性。
与临床预期相反，低浓度的布比卡因在大鼠中优先阻滞运动（VR）轴突。
