抄録
心筋の粘弾性は, 心臓の拡張期特性を評価する上で重要であるにも関わらず, 従来, 非侵襲的に計測することができなかった.我々は, 超音波を用いて, 拍動によって大きく動いている心臓壁上の振幅数十ミクロン以下の微小運動速度波形を百Hzまでの帯域にわたり高精度に計測する手法を開発した.さらに超音波ビームを約16方向に送信することにより, 左心室の数百点における微小運動速度波形を同時に計測した.その速度波形にフーリエ変換施し得られた位相値を数百点で求めカラーコードによってカラー画像を生成した.このカラー画像を約2ms間隔で表示することで, パルス状の振動(振幅は心室中隔壁に垂直方向)が心室中隔壁に沿って伝搬する様子を画像化した.本報告では, さらにこのパルス波の瞬時の位相速度の周波数分散性を決定し, 血液に浸った粘弾性板内のラム波の波動伝搬モデルを用いて, 心室中隔壁のずり粘弾性定数を決定した.60Hz成分に関して, 大動脈弁閉鎖時刻の前後20msにおいて, 位相速度は, 5m/sから3m/sへ減少し, 得られたずり弾性定数は, 24-30kPaでほぼ一定であったが, ずり粘性定数は, 400Pa・sから70Pa・sに減少していた.これらの特性は, 健常者5名に関してほぼ同様な傾向があった.また同様な周波数における文献値とも対応している.
Though myocardial viscoelasticity is essential in the evaluation of heart diastolic properties, it has never been noninvasively measured in vivo. By the ultrasonic measurement of the myocardial motion, we have already found that some pulsive waves are spontaneously excited by aortic-valve closure (AVC) at end-systole (T_0). In this study, using a sparse sector scan, in which the beam directions are restricted to about 16, the pulsive waves were measured almost simultaneously at about 160 points set along the heart wall at a sufficiently high frame rate. The consecutive spatial phase distributions clearly revealed wave propagation along the heart wall for the first time. The propagation time of the wave along the heart wall is very small, namely, several milliseconds, and cannot be measured by conventional equipment. Based on this phenomenon, we developed a means to measure the myocardial viscoelasticity in vivo. The phase velocity of the wave is determined for each frequency component. By comparing the dispersion of the phase velocity with the theoretical one of the Lamb wave, which propagates along the viscoelastic plate (heart wall) immersed in blood, the instantaneous viscoelasticity is determined noninvasively. This is the first report of such noninvasive determination. In in vivo experiments applied to five healthy subjects, propagation of the pulsive wave was clearly visible in all subjects. For the 60-Hz component, the typical propagation speed rapidly decreased from 5m/s to 3m/s around the time of AVC. The typical value of elasticity was about 24-30kPa and did not change around the time of AVC. The typical transient values of viscosity decreased rapidly from 400Pa・s at t=T_0-8ms to 70Pa・s at t=T_0+10ms. These values are comparable to those obtained using audio frequency in in vitro experiments reported in the literature.
