Polianski J, Valette H, Mensch-Dechène J, Allouche D, Lockhart A
Bull Eur Physiopathol Respir. 1977 Sep-Oct;13(5):645-57.
The risk of incorporating inapparent recirculation under the extrapolated downslope of dye dilution curves is greater with exponential than gamma function extrapolation since the latter makes use of ascending and early descending limbs of the curve. Extrapolation of a true gamma function is not affected by the level at which extrapolation begins. With curves obtained in eight normal subjects, cardiac output (phi) was comparable by exponential and gamma extrapolation if the latter began between 75 and 55% of the peak concentration (Cmax.). phi was underestimated or overestimated according to whether extrapolation began higher or lower than the above limits. Therefore, experimental curves were not true gamma functions. However, the level at which extrapolation began had little effect on pulmonary mean transit time and pulmonary blood volume (PBV) calculated by use of the double injection-single sampling method. Similar effects on phi of the level at which extrapolation by a gamma function begins were found with 24 chronic bronchitics. PBV calculated with a gamma function extrapolated from 0.75 X Cmax. downwards and with an exponential function averaged approximately 340 ml and did not differ from one another. The data suggest that the low values of PBV in chronic bronchitis are not an artifact due to the method of extrapolation.
在染料稀释曲线的外推下降段纳入不明显再循环的风险，指数外推法比伽马函数外推法更大，因为后者利用了曲线的上升和早期下降支。真正的伽马函数外推不受外推起始水平的影响。在八名正常受试者获得的曲线中，如果伽马函数外推从峰值浓度（Cmax）的75%至55%之间开始，心输出量（phi）通过指数外推法和伽马外推法相当。根据外推是否高于或低于上述限度，phi会被低估或高估。因此，实验曲线并非真正的伽马函数。然而，外推起始水平对使用双注射 - 单采样法计算的肺平均通过时间和肺血容量（PBV）影响很小。在24名慢性支气管炎患者中也发现了伽马函数外推起始水平对phi的类似影响。用从0.75×Cmax向下外推的伽马函数和指数函数计算的PBV平均约为340 ml，两者无差异。数据表明，慢性支气管炎中PBV的低值并非外推方法导致的假象。
