Arterial baroreceptor control of blood pressure in man

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  • Journal of the Autonomic Nervous System, 11 (1984) 115-124 115 Elsevier

    JAN 00379

    Arterial baroreceptor control of blood pressure in man

    Giuseppe Mancia, Gu ido Grassi, G iovanni Bertinieri, A lberto Ferrar i and A lberto Zanchett i

    lstituto di Clinica Medica IV, Universiti~ di Milano and Centro di Fisiologia Clinica e Ipertensione, Ospedale Maggtore, Milano (Italy)

    (Received May 10th, 1984) (Accepted May 20th, 1984)

    Key words: aortic baroreceptors - carotid baroreceptors - heart rate - ageing - blood pressure variability - hypertension

    Abstract

    Information on arterial baroreceptor control of circulation is much more re- stricted in man than in animals, largely because of the limitations in the techniques available in humans for this type of study. However, recent utilization of the neck chamber technique that addresses the primary function of an arterial baroreflex, i.e. blood pressure control, has provided a significant amount of information. This paper describes baroreceptor control of blood pressure as derived from neck chamber studies in normal people and discusses its modifications by exercise and ageing, and its participation in a phenomenon such as spontaneous blood pressure variability. The description given here is focused on aspects of this baroreceptor control that have changed the concept of the overall arterial baroreceptor function developed by means of techniques for studying only baroreceptor influences on heart rate.

    Introduction

    Since the discovery of the aortic nerves by de Cyon and Ludwig [5] and of the carotid sinus nerves by Hering [11], the control of circulation exerted by the arterial

    Correspondence: G. Mancia, Clinica Medica IV, Policlinico, Via F. Sforza 35, 20122 Milano, Italy.

    0165-1838/84/$03.00 1984 Elsevier Science Publishers B.V.

  • 116

    baroreceptors has been one of the major fields pursued in cardiovascular research. Studies performed in both anesthetized and unanesthetized animals have shown

    that (1) this control is vital for insuring blood pressure homeostasis in daily life, (2) it involves to an important degree baroreceptors located in the carotid, aortic and subclavian portion of the arterial tree, (3) it is not similar for these different reflexogenic areas nor does it similarly involve all cardiovascular targets and (4) it is altered in a complex fashion during development of several cardiovascular diseases, the alterations being also different according to the different reflex involved [7,13].

    Information on arterial baroreceptor control of circulation is much more re- stricted in man, largely because of the limitations in the techniques available for this type of study. Techniques such as carotid sinus massage or carotid sinus nerve electrical ~timulation and anesthesia cannot quantify the stimulus applied to the baroreceptor afferent fibers, or quantify it by means of grossly invasive procedures that pose ethical problems [24]. However, other techniques have also drawbacks that range from the inability to limit the stimulus exclusively to the arterial baroreceptors (the Valsalva manoeuver) to the assessment of their influences only on the heart or on few vascular districts (the vaso-active drugs technique, the lower body suction) with no evaluation of the overall function of the arterial baroreflex, i.e. blood pressure control [24].

    Presently, the only technique that permits study of blood pressure control exerted by arterial baroreceptors, is the variable pressure neck chamber, by means of which the increases and reductions in arterial blood pressure that accompany quantifiable stimulations and deactivations of carotid baroreceptors can be easily studied [17,21,22]. This technique has provided a considerable number of new data, some of which have altered the former concept of the overall arterial baroreflex function in normal and abnormal conditions.

    This paper will focus on these data, and discuss them in the light of the differences from the results obtained by other techniques.

    Non-linear properties of the carotid baroreceptor control of blood pressure By use of the neck chamber, Thron et al. [34] first observed that the blood

    pressure fall that accompanied carotid baroreceptor stimulation was less than the blood pressure rise that accompanied carotid baroreceptor deactivation below the level of activity tonically determined by the existing blood pressure. This was confirmed by Stegemann et al. [32] and more recently by us. Fig. 1, which is taken from a study we made in 11 normotensive subjects [21], shows the mean arterial pressure rises and falls that occurred in response to application of positive and negative neck chamber pressures which reduced and increased, respectively, carotid transmural pressure. The slope of the rise in mean arterial pressure that was observed when carotid transmural pressure was reduced, was greater than the slope of the mean arterial pressure fall that was observed when carotid transmural pressure was increased. The difference between the two reflex responses was not only statistically significant in the group as a whole, but also in nearly all individual subjects.

    To explain such difference in magnitude between the blood pressure responses to

  • 117

    carot id baroreceptor st imulat ion and deactivation, several possibil it ies were consid- ered. One, it was thought that this asymmetry might depend on an asymmetry in the carot id baroreceptor engagement by the posit ive and negative neck chamber pres- sures. This possibi l i ty was ruled out by measuring pressure transmission through the neck tissues and calculating the magnitude of the neck chamber- induced posit ive and negative pressure changes outside the carotid sinuses rather than outside the neck [17]. Two, it was thought that the positive neck chamber pressure might st imulate carotid chemoreceptors or cause transient cerebral ischemia by increasing neck venous pressure and reducing blood flow through the carotid bodies or the brain. Tiffs was also ruled out by observing that (1) prevention of carotid barorecep- tor st imulation by pure oxygen breathing did not reduce the neck chamber- induced blood pressure rise and (2) during the posit ive neck chamber appl ication, venous blood from the brain did not show any oxygen desaturat ion as compared to the control condit ion. Final ly, it was argued that the asymmetry in the blood pressure responses could originate from a greater buffering action of the aortic baroreceptors on the blood pressure falls as compared to the blood pressure rises induced by the carot id baroreceptors. This possibi l i ty could not be experimental ly control led as there is no method in man to directly study reflexes stemming selectively from the aort ic reflexogenic areas. However, there are reports that at normal b lood pressure, aortic baroreceptors do not exert a substantial b lood pressure influence in man [10] and, that in animals, both their myel inated and non-myel inated component is usally barely at threshold [26,33]. This should favour their buffering action on blood

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    Fig. 1. Changes in mean arterial pressure induced by changes in carotid transmurat pressure in 11 normotensive subjects. Data are means :i: S.E. of individual regression coefficients relating the carotid transmural pressure alterations to the late or steady-state mean arterial pressure responses. Alterations in carotid transmural pressure were calculated by the algebraic sum of the mean arterial pressure and the positive and negative pressure changes induced by a neck chamber device outside the carotid sinuses (from Mancia et al. [21], redrawn from Circulation Research, by permission).

  • 118

    pressure rises and oppose rather than originate the asymmetry observed with the neck chamber studies.

    The most likely explanation is therefore that the asymmetric blood pressure responses to carotid baroreceptor stimulation and deactivation depend on the inherent properties of the carotid baroreflex, namely on the location of its set-point towards the saturation limit of the curve relating the carotid baroreceptor stimulus to the blood pressure effect. Although this eccentrical set-point may depend on several mechanisms, it is possible that one factor responsible is a high basal activity of the carotid sinus mechanoreceptors because of their sensitivity to pulsatile as well as mean blood pressure stimuli [2] and of their location in a thin, elastic and distensible vascular wall [27]. Regardless of the mechanism, however, this set-point should make the carotid baroreflex system of normal human beings physiologically more effective for buffering a reduction rather than an increase in blood pressure.

    A discussion of the normal set-point of the human carotid baroreflex must include the results of Eckberg [6], who has observed in normotensive subjects large bradycardic and slight tachycardic responses to neck chamber-induced carotid baroreceptor stimulation and deactivation respectively. It is difficult to reconcile our conclusions with these observations, which suggest that the normal set-point position does not approach the reflex saturation but rather the reflex threshold. However, such position does not account for the large pressor and tachycardic responses that have been reported in normotensive man following anesthesia of the carotid sinus nerves (Fig. 2, Ref. 12), and occlusion of the common carotid arteries (Fig. 3, Ref. 29). These studies suggest that carotid baroreceptors have a large tonic influence on both blood pressure and heart rate, and that therefore the set-point of this reflex is normally well above threshold.

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    The carotid baroreflex during exercise and ageing The arterial baroreceptor control of heart rate has been studied under a variety of

    behaviors and found to be often increased and reduced as compared to that measured at rest [3,4,31]. Fig. 4 shows the bradycardia induced by an arterial baroreceptor stimulation obtained through an i.v. bolus of phenylephrine injected at rest and during dynamic exercise [3]. The slope, relating the magnitude of the R -R

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  • 120

    lengthening to the phenylephrine-induced blood pressure rise became progressively less and finally reached zero as the exercise intensity progressed indicating that this behavior markedly impaired the sensitivity of the baroreceptor-heart rate control.

    Reductions in this control have been observed not only during behaviors that depart from rest, but also during a physiological event, such as ageing [9,14]. Fig. 5 shows the results of a study performed by Gribbin et al. [9] on a large number of normotensive and hypertensive subjects whose age ranged from 20 to 60 years. In both cases, the sensitivity of the arterial baroreceptor-heart rate reflex (measured by the same technique employed by Bristow et al. [3]), was maximal and minimal in the 20- and in the 60-year old subjects, respectively, with a steady reduction of about 10% every 10 years. These findings highlight the fact that, in each subject, the effectiveness of reflex homeostatic mechanisms undergo large short- and long-term variations. Thus, this function (as any biological function) must not be represented by a single but rather by a spectrum of values.

    Without challenging the truth of the previously mentioned concept, it must be emphasized that studies employing the neck chamber technique have found the carotid baroreceptor-blood pressure reflex to be much less variable over short-term and long-term time bases. Fig. 6 shows the results we obtained by studying the bradycardic and the hypotensive response to carotid baroreceptor stimulation at rest and during an isometric exercise (hand-grip) in a population of 19 essential hyper- tensive subjects [23]. In line with the data described previously, during exercise, the lengthening in R -R interval, induced by the baroreceptor stimulation, was markedly reduced as compared to that obtained at rest. Calculation of the reflex response as heart rate rather than R-R interval, also showed a reduction during exercise. In contrast, the hypotensive response to baroreceptor stimulation was similar at rest

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  • 121

    and during exercise, this being the case either in a mild normotensive (n = 10) and a more severe hypertensive (n - -9 ) group separately analyzed. Similar findings had been reported in an earlier study limited to normotensive people [16]. This leaves no doubt upon the fact that the exercise-induced impairment of arterial baroreflexes may be largely limited to their heart rate component, the ability of this mechanism to control blood pressure being substantially preserved in this condition.

    In our experience this appears to be the case also as far as ageing is concerned. Fig. 7 shows the pressor and the depressor responses to neck chamber-induced carotid baroreceptor deactivation and stimulation in 4 groups of subjects with similar blood pressure values but progressively increasing age. When the pressor response was calculated early (i.e. within 15 s) after the alteration in the barorecep- tor stimulus, its magnitude was indeed progressively less with the increasing age. However, when the pressor response was calculated during the late or steady-state phase of the stimulus alteration, its magnitude was not significantly affected by the age factor. This was the case also for both the early and the late or steady-state depressor responses to baroreceptor stimulation. Thus, ageing impairs the speed of the baroreceptor-dependent blood pressure homeostasis against hypotensive dis- turbances, which may account for the larger prevalence of orthostatic hypotension in elderly people. This c...

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