In the first of two papers, the details and results of a systematic, carefully standardized application of dye dilution technic to the measurement of cardiac output are reporte(l. A substantially reduced output was a consistent finding in older subjects. Fa(tors responsible for this are analyzed in this article, and further interpretations are made in the succeedinig a1rticle. One result of the analysis of time-concentration curves of dye provides an interesting relationship to (linical estimates of circulation time.INVESTIGATIONS of cardiac output ill older, but presumably "normal," adults were made by Starr and his coworkers,1 by Lewis2 and by Aperia3 16 to 20 years ago, using the indirect gas methods then available. Although the insensitivity of these methods could mask a sizeable change, Starr and his associates noted that in the "period before 20 years the average cardiac index is higher than at any time later, after 50 it slowly declines. The number of cases was too small to demonstrate the significance of the difference." Their series included only nine cases over 50 years of age; the oldest was 76. Lewis, in a systematic agewise study, found a small decrease in cardiac index in 100 male subjects between 40 and 89 years of age. He considered that the observed changes represented neither statistically nor physiologically a significant decline and that they were predominantly the result of a decrease in oxygen consumption. A decrease of about the same order may be calculated from the data reported by Aperia,3 utilizing the acetylene technic of Grollman. These indirect Fick gas methods are now recognized as yielding values which are approximately 25 per cent too low, but even allowing for this, a sizeable decrease in cardiac output cannot be said to have been demonstrated by these studies, despite a consistent suggestion in this direction.Age comparisons, using the Fick principle with right heart catheterization, were sought in the studies of Stead and his colleagues,4

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This report describes a method for utilizing externally induced impacts as a means of studying the propagation of pressure waves in intact human arteries. Because it is a new method its description is prefaced by a brief presentation of several factors relevant to wave propagation in living vessels; the description is followed by a critical evaluation.