The Effects of Aging

In 1937, Hallock and Benson demonstrated a progressive loss of aortic distensibility with aging (1). Their observations on the pressure-volume relationship in segments of human aortas from five age groups are shown in Figure 1. The curves which represent the less distensible older aortas have a greater slope (d P/d V). The slope of each curve is not fixed, however, and it changes by bending toward the pressure axis as pressure increases. The rate of this change in slope (or the curvature) also is greater in the curves of older aortas. The present report deals with an indirect clinical index of the curvature of arterial pressurevolume curves, using the ratio of the change in pulse pressure to the change in diastolic pressure following the inhalation of amyl nitrite. This index is referred to, arbitrarily, as one of "arterial rigidity." A study of Hallock and Benson's curves made it apparent to us that the change in the slope of each curve is associated with a corresponding change in pulse pressure at different diastolic pressures. If the pressure-volume curve is relatively straight, as it is in the young, the curvature and the index will be near zero (Figure 2). If the line curves greatly toward the pressure axis, the index will be positive and large (Figure 3). If it curves the other way, the index will be negative. In vitro indices were derived from each of the five aortic distensibility curves and were found to correlate with age (Figure 4). This observation suggested that the values of this index in "normal" subjects also should increase with chronological age. The present investigation was undertaken to test this hypothesis.