頭蓋内圧亢進時の頭蓋内圧脈波の波形変化—スペクトル分析による研究

CHANGES OF THE INTRACRANIAL PRESSURE PULSE WAVEFORM CAUSED BY RAISED INTRACRANIAL PRESSURE:APPLICATION OF SPECTRAL ANALYSIS FOR THE STUDY OF WAVEFORM

　抄録　硬膜外バルーンの膨張により頭蓋内圧を上昇させ，これに伴う頭蓋内圧脈波のスペクトルおよび動脈圧から髄液への脈動の伝達効率（振幅伝達関数）の変化を検討した。頭蓋内圧脈波および動脈圧波は基本的に同じスペクトルパターンを示し，基礎波（FW）と3つの高調波（HW2, HW3, HW 4）を含んでいた。頭蓋内圧の上昇に伴って，頭蓋内圧脈波の全てのスペクトル成分において振幅および振幅伝達関数は増大した。FWおよびHW2成分では，約35mmHgの頭蓋内圧でこれらの変化に明らかなbreak pointが認められ，高い頭蓋内圧の範囲では振幅および振幅伝達関数の増大がより顕著となった。振幅および振幅伝達関数におけるbreak pointの出現は，脳動脈tonusの著明な低下，すなわち脳血流のautoregulationの消耗を示すと考えられた。頭蓋内圧脈波が正弦波と比較してどれ程歪んでいるかを示す指標として歪率を用いた。頭蓋内圧の上昇に伴って脈波の歪率は低下するが，35mmHgの頭蓋内圧でbreak pointを示し，低い頭蓋内圧の範囲では歪率の低下はより顕著であった。頭蓋内圧脈波のスペクトル分析は臨床上，有用な指標になりうると思われた。

Changes of spectrum of the intracranial pressure (ICP) pulse wave were studied while ICP was raised by the epidural balloon inflation. ICP and systemic blood pressure were estimated in the lateral ventricle and thoracic aorta, respectively. Transmission of pulsation from arteraial pressure to ICP was estimated by the amplitude transfer function (TFa). Waveform of ICP pulse was also evaluated by distortion factor to represent how ICP pulse is defferent form a pure sine wave.

The spectra of ICP and arterial pressure showed basically the same pattern ; they consisted of funda-mental wave (FW) and three harmonic waves (HW 2, HW 3, HW 4). Amplitude and values of TFa of each spectral component were increased as ICP was raised and revealed significant corre-lation. FW and HW 2 components showed break points when ICP was raised up to 35 mmHg, thereafter increments of amplitude and TFa became more remarkable in the range of higher ICP.

TFa and amplitude of each spectral component of ICP pulse revealed significant correlation. These findings proved that changes of ICP pulse were the results of changed efficiency of pulse transmis-sion from arterial pressure to CSF.

It was already reported that the cerebrovascular tonus had decisive influences on the transmission of pulsation from arterial pressure to ICP because lowered vacular tonus would diminish the damping effect by arterial wall and facilitate the transmis-sion of arterial pressure pulse to ICP. The break point of amplitude and TFa were considered to indicate that tonus of cerebral artery was remark-ably lowered and the autoregulation of cerebral blood flow was exhausted.

The value of distortion factor was lowered as ICP was raised and also revealed a break point at ICP of 35 mmHg. Lowered value of distortion factor was the result of the changing proportion of spectral components of the ICP pulse, that was, FW component became more dominant as ICP was raised. The distortion factor was considered to be a useful index to present the shape of the ICP pulse waveform.