Aging and Cardiovascular Function

Introduction

World records in endurance sports are not accomplished at age 55. Why? Because one of the unavoidable consequences of aging is a decline in the maximal capacity of the cardiovascular system to pump blood and deliver oxygen while removing metabolic waste products. The components of cardiovascular pump performance are 1) the maximal heart rate that can be achieved. 2) The size and contractility of the heart muscle 3) The compliance (stiffness) of the arterial tree. We will look briefly at what is known about aging effects on each of these variables.

Maximal Heart Rate

Young children generally have a maximal heart rate approaching 220 beats per minute. This maximal rate falls throughout life. By age 60 maximal heart rate in a group of 100 men will average about 160 beats per minute. This fall in heart rate seems to be a linear process so that maximal heart rate can be estimated by the formula 220- AGE. This is an ESTIMATE, however. If we actually measure the maximal heart rates of those same 100 men during a maximal exercise test we would probably see a range of heart rates between 140 and 180. There is no strong evidence to suggest that training influences the decline in maximal heart rate. This reduction appears to be due to alterations in the cardiac electrical conduction system (SA node and Bundle of His), as well as down regulation of beta-1 receptors, which decreases the heart's sensitivity to catecholamine stimulation.

Maximal Stroke Volume

The research picture regarding age effects om maximal stroke volume is far less clear. This is in part due to the technical challenges involved in making these measurements. Studies showing a decline, an increase, and no change can be found in the literature. It appears that if middle-aged and older adults continue to train intensely, stroke volume is well maintained. Heart size in older athletes has been shown to be similar to that of young athletes, and bigger than their sedentary, same-aged peers. Ultimately, maximal stroke volume appears to decrease due to a 1) decrease in training volume and 2) an increase in peripheral resistance.

The Peripheral Resistance

The blood pumped out of the heart enters the systemic arterial system. In our youth, this system of arteries is quite flexible or compliant. This is important for the performance of the heart. Compliant vessel walls stretch when blood is pumped through them, lowering the resistance that the heart must overcome to eject it volume of blood each beat. As we age, these vessels lose their elasticity. Consequently, resting blood pressure and blood pressure during exercise slowly increase as we age. Continued training appears to reduce this aging effect, but does not eliminate it. Increased peripheral resistance results in a decrease in maximal blood flow to working muscles. However, at submaximal exercise intensities, the 10-15% decrease in blood flow is compensated for by increased oxygen extraction (a-v O2 difference). This compensation is probably possible due to the increased transit time of the blood through the capillary tree.

The Big Picture

In the sedentary population, cardiovascular performance declines progressively with age. However, much of this decline is due to 1)physical inactivity and 2) increased body weight (fat). Maximal oxygen consumption declines about 10% per decade after age 25. However, if body composition is maintained and physical activity levels are kept constant, the decline in VO2max due to aging is only about 5% per decade. Prior to age 50, this decline may even be less, perhaps 1-2% per decade in hard training masters athletes. Ultimately, cardiovascular capacity is reduced however, due to the unavoidable decline in maximal heart rate.