q 1999, British Geriatrics Society
Age and Ageing 1999; 28: 337–338
EDITORIAL
Ageing and the baroreflex The baroreflex arc is important in enabling people to function in the upright position, as it is the principal mechanism responsible for short-term (seconds to minutes) blood pressure control. Blood pressure sensors in the carotid and aortic arch are linked through glossopharyngeal and vagal nerves to central processing centres in the brainstem, which modulate efferent sympathetic and parasympathetic nervous system activity to the vasculature and heart. Through this mechanism, falls in blood pressure result in reflex vasoconstriction and increased heart rate and stroke volume. Conditions which cause severe disruption to the baroreflex mechanism (such as invasive neck tumours) can result in profound disability, with subjects being confined to the supine position. Ageassociated changes in the baroreflex are of great physiological and clinical importance. Measuring baroreflex sensitivity Several methods have been used to measure baroreflex sensitivity. The traditional ‘Oxford’ approach has been to determine the slope of the relationship between blood pressure and heart rate changes to bolus injections of short-lasting drugs, which either increase or decrease blood pressure. Reduced baroreflex sensitivity is manifest by a shallower slope of the change in heart rate against blood pressure. Using this method, reduced baroreflex sensitivity in normotensive older subjects and hypertensive middle aged subjects was identified in the early 1970s. A disadvantage of the technique is the need to administer pressor and vasodilator drugs, which are potentially hazardous in acutely ill or frail older subjects. For this reason, other methods were developed. These include analysis of blood pressure and heart-rate changes during phase IV of a Valsalva manoeuvre. Non-invasive spectral analysis and impulse-response function analysis of continuous heart rate and blood pressure recording are a particularly attractive method for research on older subjects. These methods require only a brief period of simultaneous heart rate and blood pressure monitoring, usually obtainable from a finger cuff sensor, without the need for invasive techniques or administration of vasoactive agents. Increasing age is associated with increasing blood pressure and reduced baroreflex sensitivity, and the contribution of age per se to reduced baroreflex sensitivity has been unclear. In this issue of Age and Ageing, the Leicester group [1] describe the association
between age, blood pressure and baroreflex sensitivity in a sample of 70 ‘normotensive’ subjects aged 22–82 years using several methods. They confirm that ageing is associated with a reduction in baroreflex sensitivity up to the fourth decade. However, beyond this there is little further decline. Age is the dominant factor associated with reduced baroreflex sensitivity, although increasing blood pressure is associated with further blunting of baroreflex sensitivity in this older normotensive population. Interpretation of cross-sectional ageing studies is always confounded by possible cohort changes in lifestyle and environment, and confirmation in longitudinal observation studies is required. However, these and other data demonstrate a profound reduction in baroreflex sensitivity in older individuals with additional blunting associated with hypertension. The mechanisms responsible for this age-associated change remain unclear. Traditional thinking has been that increased blood vessel rigidity impairs the function of the afferent baroreceptors in the carotids and aortic arch, through either structural changes (due to atherosclerosis) or functional changes (such as reduced vascular nitric oxide activity). However, since baroreflex sensitivity can be altered acutely after stroke, altered central processing may be an additional factor. Clinical relevance What is the relevance of these changes to clinical care of the older patient? First, it is surprising that these profound changes in baroreflex sensitivity result in no apparent adverse consequences in most healthy subjects in middle age and beyond. Postural hypotension remains uncommon in normotensive older individuals despite this blunting of the baroreflex [2]. However, reduced baroreflex function modifies the response of older subjects to vasodilator drugs. The reflex tachycardia and increase in stroke volume in response to vasodilatation is reduced, resulting in more marked falls in blood pressure [3], which increases the likelihood that older patients will experience druginduced orthostatic hypotension. This common clinical observation has yet to be confirmed in well-designed studies comparing middle-aged and older subjects with similar cardiovascular conditions. Other adverse consequences of impaired baroreflex function are now being increasingly recognized. Impaired baroreflex function may be the underlying pathophysiological abnormality in vasovagal syncope [4]. Changes in heart rate variability and blunting of the baroreflex following
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Editorial acute stroke may contribute to cardiac dysrhythmias and cardiac deaths seen in this condition [5]. Reduced baroreflex sensitivity after myocardial infarction is associated with increased cardiac mortality [6]. Optimization of use of cardiovascular therapy in older people would be improved through a better understanding of drug effects on baroreflex function and its relationship to common adverse effects (such as syncope). The development of interventions which improve baroreflex function in older subjects may offer treatment opportunities for the difficult problems of orthostatic hypotension and vasovagal syncope. Gender, hormone replacement therapy and exercise influence baroreflex sensitivity in middle-aged subjects, but effects in older subjects may differ [7]. Further work is required to elucidate the effect of these and other environmental influences on baroreflex function in older subjects. Given the importance of cardiovascular disease as a cause of premature death and disability in older people, research which increases our understanding of ageing changes in cardiovascular physiology—and their relevance to cardiovascular and cerebrovascular risk—should yield rich dividends in improving cardiovascular therapy for the older patient.
References 1. Dawson SL, Robinson TG, Youde JH et al. Older subjects show no age-related decrease in cardiac baroreceptor sensitivity. Age Ageing 1999; 28: 347–53. 2. Mader SL, Josephson KR, Rubenstein LZ. Low prevalence of postural hypotension among community dwelling elderly. JAMA 1987; 285: 1511–4. 3. Robertson DRC, Waller DG, Renwick AG et al. Age-related changes in the pharmacokinetics and pharmacodynamics of nifedipine. Br J Clin Pharmacol 1988; 25: 297–305. 4. Thomson HL, Wright K, Frenneaux M. Baroreflex sensitivity in patients with vasovagal syncope. Circulation 1997; 95: 395–400. 5. Robinson TG, James MA, Youde J et al. Cardiac baroreceptor sensitivity is impaired after acute stroke. Stroke 1997; 28: 1671–6. 6. LaRovere MT, Bigger JT Jr, Marcus FI et al. Baroreflex sensitivity and heart-rate variability in prediction of total cardiac mortality after myocardial infarction. ATRAMI Investigators. Lancet 1998; 351: 1436–7. 7. Bowman AJ, Clayton RH, Murray A et al. Effects of aerobic exercise training and yoga on the baroreflex function in healthy sedentary normotensive elderly persons. Eur J Clin Invest 1997; 27: 443–9.
GARY A. FORD Department of Medicine, University of Newcastle, Newcastle upon Tyne NE2 4HH Email:
[email protected]
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