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Older subjects show no age-related decrease in cardiac baroreceptor sensitivity

Age and Ageing, July, 1999 by Suzanne L. Dawson, Thompson G. Robinson, Jane H. Youde, Alison Martin, Martin A. James, Philip J. Weston, Ronney B. Panerai, John F. Potter

Introduction

The cardiac-baroreceptor reflex arc is one of the many physiological mechanisms involved in the short-term control of arterial blood pressure.

Researchers have examined factors influencing cardiac baroreceptor sensitivity (BRS) [1-5]. Gribbin et al. [1] were the first to identify an age-related decline; others have confirmed this [2-5], but few subjects over 60 years have been investigated. This is partly because of the invasive nature of traditional methods of measuring beat-to-beat changes in arterial blood pressure.

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Increasing age is associated with a rise in blood pressure. There is debate as to which factor influences BRS more. Gribbin et al. [1] felt that age and blood pressure acted independently and that increasing values were inversely associated with BRS. Conversely, work from our department, using pharmacological and non-invasive methods of measuring BRS, found that age contributed only 7% of the variance in cardiac BRS compared with 27-45% for systolic blood pressure (SBP) in subjects over 60 years [4].

Spectral methods of calculating cardiac BRS have several advantages over conventional invasive pharmacological methods and give similar BRS values [6]. Power spectral analysis involves the detection of rhythmicity in computer-derived tachograms of beat-to-beat recordings of blood pressure and pulse interval (PI); various algorithms can then be used to assess the number, frequency and amplitude of the oscillatory components. However, this analysis requires the prior selection of frequency bands for blood pressure and PI. Previous studies favoured either the low-frequency band (0.05-0.15Hz) or the combined low- and high-frequency bands (0.15-0.35 Hz)--the [Alpha] value [7-12].

Two other measures are important in spectral analysis: phase and coherence. Phase, in cardiac BRS settings, is the time relationship between changes in SBP and PI. A negative phase value implies that changes in SBP are leading changes in PI, reflecting a baroreceptor-mediated sequence. A positive phase implies PI leading SBP and reflects system noise or non-baroreceptor-mediated sequences. Coherence is a measure of input-output coupling. It assesses the statistical significance between the dynamic change in arterial blood pressure and the resulting change in Pl. It is similar to the correlation coefficient, with a range of 0 (no relation) to 1.0 (very strongly correlated). A value for squared coherence that is significantly [is less than] 0.5 reflects too much signal noise or an output that is more dependent on variables other than blood pressure input [13, 14]. If there were to be a difference in phase with age and good coherence, this would give important information on baroreceptor activity with age. It may also help determine whether it is better to use low-, high- or combined frequency calculation of BRS. There are no previous reports of the influence of age on these factors (although there are reports of phase differences in the high-frequency bandwidth in patients with coronary heart disease [13]).

Our aims were to assess individual and combined influences of age, blood pressure and other factors on the cardiac BRS measurements derived from spectral analysis, phase IV of the Valsalva manoeuvre and the newer impulse response function (IRF) technique [14]. IRF is similar to spectral analysis but examines data in the time domain. It reflects the dynamic change in an output (PI) in response to an input (SBP). It is thought to be less subject to problems of standardization, as no frequency bands need to be pre-selected, and it may lead to more information on BRS modulation. We have also examined the relation between blood pressure and PI in the various frequency bands used to calculate BRS by spectral methods to see if it was affected by age.

Subjects

We recruited from hospital staff, friends and relatives of patients attending outpatient departments, orthopaedic inpatients awaiting elective procedures and respondents to a newspaper advertisement calling for healthy volunteers.

All subjects were independent in activities of daily living and free from cardiovascular disease (determined by history, examination, 12-lead electrocardiography and blood tests). They were not receiving drugs known to influence the cardiovascular or autonomic systems. We excluded those with a history of hypertension, diabetes mellitus or other diseases affecting the autonomic system, recent myocardial infarction, atrial fibrillation or cerebrovascular disease, and those with a mean of three clinic SBP readings [is greater than] 160 mmHg or diastolic blood pressure (DBP) phase V [is greater than] 95 mmHg.

Methods

All subjects were examined using a standardized protocol [15]. They were asked to attend in the morning after abstinence from caffeine, nicotine or alcohol-containing products for at least 12 h and at least 2 h after breakfast. The study was performed in a quiet laboratory kept at a constant temperature (20-24 [degrees] C), with subdued lighting. Subjects wore loose comfortable clothing and were asked to micturate just before the study.

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Older subjects show no age-related decrease in cardiac baroreceptor sensitivity

Age and Ageing, July, 1999 by Suzanne L. Dawson, Thompson G. Robinson, Jane H. Youde, Alison Martin, Martin A. James, Philip J. Weston, Ronney B. Panerai, John F. Potter

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