単一の色素希釈曲線から左右短絡率を求める新しい方法—短絡を混和とみなす理論

A new method for calculating shunt ratio by the single dye-dilution curve

　指示薬希釈法のうち色素または熱を用いる方法は心疾患の定性的，定量的測定に大きな役割をもっている。話釈法による心拍出最測定の前提条件として，1）測定している期間中流量は一定である，2）注入された指示薬は系の途中で失われない，3）注入は瞬間的になされる，4）混和は完全になされる，の4つがある。臨床上は3），4）ははるかにゆるい条件で行われているがFick法との相関は極めて高い^11,12）。

　左右短絡心疾患での短絡率の測定法で最も信頼されているのはFick法であるが，短絡率20％以下の症例には有用でない。色素希釈法では経験的半定量法として，時間因子，濃度因子の様々な組み合わせがされてきた。定量法として理論構成が明解なのは中村ら^2）の方法である。

The dye-dilution curves always showed exponen-tial decrease in the case with left-to-right shunt. When the total area of the first circulation and the successive all shunts which were calcucated by Lilienfield & Kovach's method are desigened as S, and the area of first circulation calculated by Hetzel's method as SI, the shunt ratio is expressed by the folumura:

r =1-S₁/S

The shunt ratio of 17 patients with ASD and 12 patients with VSD were calculated by this met-hod. The correlation coefficient of the results bet-ween Fick's direct method and this method was 0.900 (p<0.001). This method showed also a good correlation in theoritical structure with semi-quan-titative folumura presented by Yokota et al under the same consideration.

If shunts influence the ascending limb of the first circulation, the area calculated by Hetzel's method is always greater than the true value. This pheno-menum must be explicated by means of simulation. The simulation was achieved as following. Healthy adult man's dye-dilution curve was used as the pro-totype. 2 assumptions were employed. 1) Time fac-tor is proportional to the transitional time from the injection site to the ear. 2) Peak-to-peak time of first circulation is equal to that of the successive shunts. The results were, 1) When the increase of lung blood volume excessed 50% to shunt volume, simulation showed good correlation with this me-thod, 2) the increase of lung blood volume was less than 25%, this method showed far little value as the true shunt ratio was exceeding 50%.

This study presents a theoritical basis to apply this method to the thermo-dilution. But in thermo-dilution, Hetzel's factor must be altered. Because heat takes up spaces not only lung vascular blood but also extra lung water. In dog, this factor was settled, but in man, is not yet settled.