頭蓋内圧亢進時における脳血管のCO2反応性に関する実験的研究

CEREBROVASCULAR CO2 REACTIVITY UNDER INTRACRANIAL HYPERTENSION IN DOGS

I．はじめに

　急性頭蓋内圧亢進は古典的脳浮腫・脳腫脹の概念でまとめて考えられていたが，最近，脳血管床の増大と神経組織の含水量増加とに分けて理解されるようになつた。前者がEvansら^3）が提唱したvasomotor paralysisの概念であり，Langfittら^13）はこの概念を急性脳腫脹の重要な要素として取り上げている。急性脳腫脹はFig．1に示すようなvicious cycle^16,18）によつて発生する。すなわち，①頭蓋内圧亢進は脳血流の減少を招き，その結果生ずる脳hypoxiaは脳組織内のCO₂および〔H^＋〕濃度の増加をもたらし，cerebrovascular toneを低下させ，この脳血管床の増大とvenous stasisによつて頭蓋内圧はさらに上昇する。一方cerebrovascular toneの低下は，②脳・血液関門とは無関係な水分代謝異常によるhydrostatic edemaを生ずる。また脳hypoxiaは，③脳血管透過性を亢進させ，脳浮腫を生ずる。現在，急性脳腫脹は脳内血液量増加によるものとする説^13）が有力であり，cerebrovascular toneの低下を基盤として発生してくると考えられている。そこで著者は急性脳腫脹のmechanism究明の鍵と考えられるcerebrovasculartoneが頭蓋内圧亢進時にどのように変化しているものかをCO₂に対する反応性を指標として実験的に検討した。

Cerebrovascular tone and CO₂ reactivity wereobserved under intracranial hypertension caused byextradural balloon compression method in dogs.Parameters which were recorded continuously ona polygraph were ICP, local CBF, systemic bloodpressure, EEG and ECG. CO₂ reactivity underintracranial hypertension could be classified intothe following 4 stages.

Stage 1: (ICP less than 50mmHg) Both ICP andCBF were increased upon CO₂ inhalation.

Stage 2: (ICP 50~70mmHg) ICP was increased,but CBF was decreased upon CO₂, when CO₂ wascut off, ICP was furthermore elevated, and reboundincrease of CBF, so-called biphasic response, wasseen.

Stage 3: (ICP 70~100mmHg) Vasopressor re-sponse was seen upon CO₂, but CBF responsepattern was similar to stage 2.

Stage 4: (ICP more than 100mmHg) Both ICPand CBF were decreased upon CO₂, i. e. so-callednegative CO₂ reactivity was seen.

It was very important that the dissociation ofCBF and ICP upon CO₂ inhalation was observed instage 2. Because it is presumed that ICP increaseupon CO₂ shows cerebrovascular dilatation, and de-creased CBF upon CO₂ means decrease of cerebro-vascular tone. Both decreased ICP and decreasedCBF upon CO₂ were observed in stage 4, so-calledCO₂ paralysis. CO₂ paralysis in stage 4 returnedto normal cerebrovascular tone and CO₂ reactivityby the administration of THAM, but the responsepattern upon CO₂ was unchanged by the adminis-tration of mannitol. It was concluded that hydrogenion was concerned with cerebrovascular tone andCO₂ reactivity.

On the basis of the experimental results, cerebro-vascular CO₂ reactivity should be discussed in aview of ICP as well as CBF.