Estimation of the Conduction Velocity Distribution of Nerve Fibers

摘要

末梢神経線経の伝導速度の分布（DCV）を推定するための新しい方法を開発した．木方法は同時に単一線経活動電位（SFAP）を推定することができる．またその結果は生理学の知識と合致した．神経束を経皮的に2箇所で電気刺激し，誘発された2つの複合的活動（CAPl，CAP2）を測定した．任意の適当に仮定したSFΛPとCAPlから最初の推定DCVを非負拘束最小二乗法求めた．次にSFAPをCAP2とさきに推定したDCVから計算した．このとき，高速な収束を得るためにローバスフィルタを用いた．（CAPcalculated　－CAPmeasured）~2と定義されたCAP誤差が十分に小さくなるまで，上記の手続きを繰返した．刺激電極と記録電極の距離の測定に誤差が特に遠位であり，推定結果に誤差を生じたので，フィボナッチ捜索を用いて，距離も推定するようにした．正確さはシミュレーションによって確かめた．すなわち，正規白色雑音を加えた適当な2つのCAPsとDCVからSFAPを推定することができた．この方法を健常成人で行い，従来報告された結果と矛盾の無いことを確認した．また，糖尿病患者でも測定した結果，伝導速度は低下していることが分かった．%A new method for estimating the distribution of conduction velocities (DC V) of peripheral nerve fibers has been developed. It also enables estimation of single fiber action potential (SFAP), which agrees with the physiological knowledge. Two compound nerve action potentials (CAPs) elicited by electrical stimulation of a nerve bundle were recorded at different conduction distances. The distances between the stimulation and recording electrodes were measured on the skin surface along the nerve bundle. Starting with an arbitrary SFAP, the first estimated DCV was calculated from a CAP by the regularized non-negative least squares method. The next SFAP was then calculated by deconvolution of the other CAP and the estimated DCV. A lowpass filter with an appropriate cutoff frequency was used to obtain better conversion. The process was iterated until the CAP error defined as (CAPcalculated - CAPmeasured)~2 was small enough. The conduction distances contained errors in measurement, especially in the distal segment, that distorted the estimated results. The Fibonacci search, therefore, was adopted to optimize the distance according to the CAP error. The accuracy of this method was demonstrated by a simulation study performed with two CAPs calculated from an arbitrary bimodal DCV and a biphasic SFAP to which a Gaussian white noise was added. The reliability of this method was checked in normal subjects by recording a pair of CAPs elicited by stimulation of the median nerve at the wrist and the elbow.