近赤外分光法を用いる無侵襲生体計測—特に新しい定量的測定法について

New quantitative method for non-invasive monitoring of tissue blood oxygenation by near infrared spectrophotometry

　組織透過性の高い近赤外領域の光を用い生体組織中の血液ヘモグロビン（Hb）の酸素化，ミトコンドリア（Mt）のチトクローム・オキシダーゼ（cyt．oxidase）の酸化還元状態などを無侵襲にモニターする試みはJobsisi^1）に始まり注目を集めるようになった。著者らは数年来ラット脳を対象に脳内Hb酸素濃度，脳血液量およびcyt．oxidase （aa₃）の酸化還元レベルの測定を試みてきた^2〜4）。このHb測定法としては通常組織反射または透過光の二波長分光（二波長吸収差の記録）によるもので従来酸素濃度により吸収の大きく変化する波長を測定波長（著者らは700，760nm使用）に変化しない波長を対照波長としてこの二波長の吸収差を測定してきた^5）。対照波長として通常Hbの等吸収点波長が用いられラット脳の場合805nmを選択してきた^3〜5）。しかもこの等吸収点波長における吸光変化は全Hb量すなわち血液量の変動を表すことになる。しかし後述するが脳cyt．oxidaseは近赤外領域に幅広い吸収帯を持つためにoxidaseの酸化還元による吸光変化が重なり酸化Hb量，全Hb量の定量測定に誤差を生じる危倶がある。今回はcyt．oxidaseの吸収に影響されない波長を用いてのより精度の高い測定法について基礎的検討を行った結果を報告する。この新しい測定法はすでに1987年より用いておりその一部は同年に発表した^6）。

The near infrared absorption spectra was measured with the transmitted light through rat brain under the various condition. The absorbance changes below 780 nm were attributable to hemoglobin (Hb) in the brain tissue, whereas those above 780 nm were asso-ciated with both Hb and cytochrome oxidase.

To eliminate possible interference from cyt. oxi-dase, two wavelengths, 750 nm and 780 nm, were used to measure Hb oxygenation in the tissue.

The absorbance changes in human blood cell suspensions were measured with changes in hema-tocrit values in the optical cuvette. At two wave-lengths 750 nm and 780 nm, there was a linear re-lationship between absorbance changes and hema-tocrit values.

Through these in vitro studies, the following equation (1) and (2) were obtained to monitor quantitatively the changes of oxy-Hb content (ΔHb O2) and total-Hb content (ΔHb Vol.) in the living tissue. These are (1) ΔHbO2= -1.15 ΔA 750 +1. 39 ΔA780, (2) ΔHb Vol.= =0.29 ΔA 750+ 0.59 ΔA 780. The studies using these equations showed that the oxy-Hb content in the brain was decreased as the O² concentration in inspired gas was lowered with a half of Hb deoxygenated at 7% O². The reliabi-lity of these equations was examined under the various conditions in situ such as CO² inhalation, intravenous injection of Ca²⁺-blocker nicardipine, hemorrhage and retransfusion.

These results confirmed that these equations derived from in vitro studies, were successfully applied to the in situ measurements of the oxygenation state of Hb in the living tissues.