不是减少--减少吗？

In the previous section, it has been explained how a reduction in capillary transit time may reduce the diffusing capacity. The mean transit time is reduced when cardiac output is raised, and this may increase diffusing capacity substantially, for example during exercise (see next).

在前一节中，已经解释了毛细管传输时间的减少如何降低弥散量。心输出量增加（如运动时），毛细血管平均通过时间减少，这可能会显著增加弥散量（见下一页）。

已经说了“毛细管传输时间的减少是降低弥散量的”，为何“心输出量增加（如运动时），毛细血管平均通过时间减少，反而显著增加弥散量”？

我们看一下 previous section

Capillary Transit Time

Capillary transit time is a most important factor determining both the pulmonary end-capillary Po2 and the diffusing capacity. It is seen from Figure 8.2, A, that, if the capillary transit time is reduced to below 0.25 seconds there will be an appreciable gradient between the alveolar and end-capillary Po2. Because the diffusion gradient from alveolar gas to mean pulmonary capillary blood will be increased, the oxygen-diffusing capacity must be decreased.

The mean pulmonary capillary transit time equals the pulmonary capillary blood volume divided by the pulmonary blood flow (approximately equal to cardiac output). This gives a normal time of the order of 0.8 seconds with a subject at rest, though estimates vary from 0.1 to 3 seconds. It is therefore likely that, in a similar fashion to ventilation and perfusion, there is a wide range of normal capillary transit times affected by many factors such as posture, lung volume, cardiac output, and so on. Blood from capillaries with the shortest time will yield desaturated blood, and this will not be compensated by blood from capillaries with longer than average transit times, for the reason shown in Figure 7.15.

毛细管通过时间

毛细血管通过时间是决定肺末梢毛细血管PO₂和弥散量的最重要因素。从图8.2，A可以看出，如果毛细血管通过时间减少到0.25s以下，肺泡和末端毛细血管PO₂之间会有明显的梯度。由于肺泡气体到平均肺毛细血管的弥散梯度将增大，因此O₂的弥散量必会降低。

平均肺毛细血管通过时间等于肺毛细血管血容量除以肺血流量（约等于心输出量）。受试者休息时的正常时间约为0.8s，但估计值0.1s～3s。因此，与通气和灌注类似，正常毛细血管通过时间的范围很广，受体位、肺容积、心输出量等诸多因素的影响。毛细血管通过时间最短的的血液氧合程度降低，而这不会被来自传输时间超过平均时间的毛细血管的血液所代偿，原因如图7.15所示。

我们看一下 next section

Exercise. During exercise diffusing capacity may be double the value obtained at rest, as a result of increased cardiac output causing a reduction in capillary transit time and pulmonary capillary recruitment in nondependent lung zones (page 76). Because of this large effect of cardiac output on the measurement of diffusing capacity, some groups advocate using simultaneous noninvasive measures of cardiac output to aid interpretation of the diffusing capacity result.

运动。运动期间，由于心输出量增加导致毛细血管通过时间缩短和非重力依赖性肺毛细血管重新开放，弥散量可能是静息时的两倍（第76页）。由于心输出量对弥散量测量的巨大影响，某些研究团队主张使用同步无创心输出量测量来帮助解释弥散量的结果。