Raymond S A, Steffensen S C, Gugino L D, Strichartz G R
Department of Anesthesia, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts 02115.
Anesth Analg. 1989 May;68(5):563-70.
The quantitative relation between the concentration of local anesthetic (LA), the length of nerve exposed, and severity of conduction blockade was studied with use of a chamber where exposure length was varied as the concentration of lidocaine was held constant. Recordings of the compound action potential and of single axons established that small variations in the length of nerve exposed to LA strongly modulate conduction block even at exposure lengths in excess of 2 cm. Therefore, exposure length is a significant factor in determining blocking potency, and only at very high concentrations of LA, where voltage-dependent Na conductance is almost completely blocked, is the critical exposure length less than three nodes of Ranvier. The concentration required for 50% block of impulses in single fibers (that is, where 50% of the impulses would fail to propagate through the exposed region of the nerve) diminished as the exposed length of nerve increased, approximately halving as exposure length was changed from 6 mm to 15-25 mm. Conduction latency increased with the exposure length becoming sharply more variable as the critical exposure length for conduction block was approached. The results are consistent with the hypothesis of decremental conduction, where a partial active response in nodes exposed to marginal blocking concentrations extends the decay of the action potential along the axon, and do not support the interpretation that lengths of several centimeters affect blocking concentration because such distances increase the probability that three nodes will be blocked in succession. This study contradicts the broader common assumption that beyond three nodes, the length of nerve exposed is not a factor in nerve block with local anesthetics.
利用一个腔室研究了局部麻醉药（LA）浓度、神经暴露长度与传导阻滞严重程度之间的定量关系，在该腔室中，当利多卡因浓度保持恒定时，改变暴露长度。复合动作电位和单根轴突的记录表明，即使在超过2厘米的暴露长度下，暴露于LA的神经长度的微小变化也会强烈调节传导阻滞。因此，暴露长度是决定阻滞效能的一个重要因素，只有在非常高的LA浓度下，电压依赖性钠电导几乎完全被阻断时，关键暴露长度才小于三个郎飞结。单纤维中50%冲动阻滞所需的浓度（即50%的冲动无法通过神经暴露区域传导）随着神经暴露长度的增加而降低，当暴露长度从6毫米变为15 - 25毫米时，大约减半。传导潜伏期随着暴露长度的增加而增加，随着接近传导阻滞的关键暴露长度，其变化变得更加剧烈。这些结果与递减传导的假设一致，即在暴露于边缘阻滞浓度的节点中部分主动反应会使动作电位沿轴突的衰减延长，并且不支持这样的解释，即几厘米的长度会影响阻滞浓度是因为这样的距离增加了三个节点连续被阻滞的概率。这项研究与更广泛的普遍假设相矛盾，即超过三个节点后，暴露的神经长度不是局部麻醉药神经阻滞的一个因素。
