The factors influencing the circulation time.

mT HE MEASUREMENT of the circulation time through the heart and lungs is one of the most widely used tests of circulatory efficiency in clinical medicine. Yet, surprisingly, the term is usually poorly defined, and there is certainly considerable disagreement concerning its parameters. As Wiggers1 points out, circulation time is not the time required for the blood to make a complete circulation, because any given corpuscle has such a wide variety of paths over which to travel as to make such a concept meaningless. With many authors the term "circulation time" seems to imply a measure of the mean velocity of flow of an injected substance from the point of injection to the place of detection. Blumgart2 in his informative review remarks that velocity of blood flow as such cannot be measured and defines the measurable circulation time as being "the interval of time necessary for the fastest particle of a foreign substance to traverse the shortest available path between the point of injection and the place of detection." If a given amount of indicator is injected at time To, and the concentration of indicator at the place of detection is plotted against time, then, disregarding recirculation, a curve, as shown in figure 1, will be obtained. This has been demonstrated by Hamilton and co-workers3a, b in their studies on an injection method for the determination of cardiac output. Following Blumgart's definition, the circulation time would be the time interval from the instant of injection (To) to the instant (T1) when the first particle of indicator appears. If the