Background
A reduction in compliance of the large-sized cardiothoracic (central) arteries is an independent risk factor for the development of cardiovascular disease with advancing age.
Methods and Results
We determined the role of habitual exercise on the age-related decrease in central arterial compliance by using both cross-sectional and interventional approaches. First, we studied 151 healthy men aged 18 to 77 years: 54 were sedentary, 45 were recreationally active, and 53 were endurance exercise–trained. Central arterial compliance (simultaneous B-mode ultrasound and arterial applanation tonometry on the common carotid artery) was lower (P,0.05) in middle-aged and older men than in young men in all 3 groups. There were no significant differences between sedentary and recreationally active men at any age. However, arterial compliance in the endurance-trained
middle-aged and older men was 20% to 35% higher than in the 2 less active groups (P,0.01). As such, age-related differences in central arterial compliance were smaller in the endurance-trained men than in the sedentary and recreationally active men. Second, we studied 20 middle-aged and older (5362 years) sedentary healthy men before and after a 3-month aerobic exercise intervention (primarily walking). Regular exercise increased central arterial compliance (P,0.01) to levels similar to those of the middle-aged and older endurance-trained men. These effects were independent of changes in body mass, adiposity, arterial blood pressure, or maximal oxygen consumption.
Conclusions
Regular aerobic-endurance exercise attenuates age-related reductions in central arterial compliance and restores levels in previously sedentary healthy middle-aged and older men. This may be one mechanism by which habitual exercise lowers the risk of cardiovascular disease in this population.
Discussion
The main new findings of the present study were as follows. First, central arterial compliance decreases with age even in healthy physically active men, suggesting an effect of primary (intrinsic physiological) aging. Second, the magnitude of the age-related reduction in central arterial compliance is attenuated in men who regularly perform vigorous endurance exercise but not significantly so in men performing less strenuous physical activity. Third, a relatively brief (13- to 14-week) period of regular aerobic exercise can restore some of the loss of central arterial compliance in previously sedentary middle-aged and older men. Fourth, the ability of regular aerobic exercise to increase central arterial compliance in this population does not depend on changes in body weight/composition, arterial blood pressure, metabolic risk factors for atherosclerosis, or maximal aerobic capacity. To our knowledge, these findings are the first to demonstrate that the age-associated reduction in central arterial compliance
can be favorably modified by regular aerobic-endurance exercise.
We2 and others23 have reported that the medium- and large-sized arteries primarily in the cardiothoracic region of middle-aged and older endurance-trained adults appear to be less stiff than those of their sedentary peers on the basis of measurements of aortic pulse wave velocity and carotid augmentation index. However, these are only indirect measures that may or may not accurately reflect population differences in the dynamic compliance of arteries in the central circulation. Moreover, although important, the cross sectional nature of these observations precluded us from assigning the group differences to the effects of regular exercise, per se, as opposed to other lifestyle or constitutional (including genetic) influences.
The results of the present study extend considerably these prior findings. First, our cross-sectional study (protocol I) allowed us to determine the general age (young adult,
middle-aged, or older adult) at which reductions in central arterial compliance are observed in sedentary healthy men as well as any possible interaction between age and habitual exercise. We found that reductions in central arterial compliance occur largely between young adulthood and middle age in both sedentary and physically active men.
Second, using direct measurements of the dynamic compliance of the carotid artery, we show that regular aerobic exercise modulates age-associated reductions in central arterial compliance. Specifically, we found that central arterial compliance was '40% higher in endurance-trained older men than in their sedentary peers. Because there were no significant differences among the young adult groups, this suggests that the decrease in central arterial compliance with age may be attenuated in men who habitually engage in vigorous endurance exercise.
Third, the 3-group design of our cross-sectional study also allowed us to determine that vigorous aerobic-endurance exercise appears to modulate age-related decreases in central arterial compliance, whereas less strenuous exercise may not. These findings suggest that a relatively robust endurance exercise stimulus may be necessary to clearly attenuate the age-related decrease in central arterial compliance. However, we should emphasize that although not statistically significant, mean levels of arterial compliance in our middle-aged and older recreationally active men were 10% to 17% greater than in their sedentary peers. Thus, this level of habitual physical activity may have a small, but physiologically important, effect on arterial compliance.
Finally, our follow-up intervention study (protocol II) allowed us to confirm these cross-sectional observations by demonstrating that regular aerobic-endurance exercise can
increase central arterial compliance in previously sedentary middle-aged and older men. Moreover, this allowed us to examine the direct effect of exercise by determining whether the improvement in central arterial compliance depended on concomitant changes in other factors, such as arterial blood pressure or maximal oxygen consumption (ie, the capacity to transport oxygen to active skeletal muscle).
In our intervention study, we observed a 25% increase in arterial compliance and a 20% reduction in the b stiffness index after only 3 months of regular aerobic exercise in
previously sedentary middle-aged and older men. This improvement was not associated with changes in body mass, adiposity, arterial blood pressure, plasma cholesterol, or maximal aerobic capacity, indicating a direct (primary) effect of exercise on central arterial compliance. Importantly, after exercise training, central arterial compliance was not different from that observed in the middle-aged and older endurance athletes studied in protocol I. These results suggest that relatively short-term aerobic exercise can restore some of the loss of dynamic arterial compliance in healthy middle-aged and older men. Importantly, this can be accomplished with an intensity (moderate) and type (primarily walking) of physical activity that can be performed by most, if not all, healthy men of this age.
We can only speculate on the mechanism by which regular aerobic-endurance exercise improved arterial compliance in the present study. Arterial compliance primarily is determined by the intrinsic elastic properties of the artery.24 The elements of the arterial wall that determine its compliance are the composition of elastin and collagen (structural determinants) and the vasoconstrictor tone exerted by its smooth muscle cells (functional determinant).24 Because biochemical changes in the elastin-collagen composition of the arterial wall are believed to occur over years, it is unlikely that short-term regular aerobic exercise increased arterial compliance by this mechanism. However, it is possible that the increased pulse pressures and mechanical distension during the exercise sessions “stretched” collagen fibers and modified their cross-linking, thereby increasing arterial compliance.25 Arterial compliance also can be altered over a short time period, even acutely, via modulation of the sympathetic-adrenergic tone of smooth muscle cells in the arterial wall.26,27 In this context, it is possible that regular exercise increased arterial compliance by reducing the chronic suppressive influence exerted by sympathetic-adrenergic tone either directly or by enhancing the sympathoinhibitory effect of NO.28 Future studies will be needed to determine the physiological mechanism(s) underlying the influence of regular aerobic exercise on central arterial compliance.
Our findings have a number of potentially important clinical implications. Reductions in arterial compliance are believed to contribute significantly to the pathophysiology of
age-related increases in cardiovascular diseases.1,4 For example, reduced compliance in the central arteries has been hypothesized to be the predominant mechanism for the reduction in cardiovagal baroreflex sensitivity with aging.4 As such, it may contribute to the increased prevalence of ventricular tachyarrhythmias and cardiac sudden death with advancing age.29 It is possible that increased compliance in the carotid artery (and perhaps the ascending aorta) is associated with greater baroreflex sensitivity. Consistent with this, both preliminary30 and published31 results from our laboratory indicate that cardiovagal baroreflex sensitivity is enhanced in middle-aged and older endurance-trained adults compared with their sedentary peers. Moreover, age-related reductions in the compliance of central arteries can affect cardiac function by increasing the aortic impedance to ejection of left ventricular stroke volume.24 This, in turn, may contribute to reductions in left ventricular performance and exercise capacity with age.32 In this regard, greater central arterial compliance could contribute to the augmented left ventricular function and aerobic exercise capacity demonstrated by older endurance-trained men.33 In summary, the present results support the idea that regular aerobic-endurance exercise can attenuate reductions
in and partially restore the loss of central arterial compliance in middle-aged and older healthy men. As such, habitual aerobic exercise may be an effective lifestyle intervention for minimizing the loss in central arterial compliance with advancing age. Greater central arterial compliance may contribute to the lower incidence of cardiovascular disease observed in middle-aged and older men who exercise regularly.
Acknowledgments
This study was supported by National Institutes of Health grants AG-00847 (Dr Tanaka), AG-16071, AG-06537, and AG-13038 (Dr Seals), and HL-03840 (Dr DeSouza); by the General Clinical Research Center (No. 5-01-RR00051); and by American Heart Association grant 9960234Z (Dr Tanaka). We thank Yoli Casas, Linda Shapiro, Jayne Semmler, and Teresa Wilson for their technical assistance in the present study.