Frequency-dependent baroreflex control of blood pressure and heart rate during physical exercise

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Abstract

Background

It is widely recognised that during exercise vagal heart rate control is markedly impaired but blood pressure control may or may not be retained. We hypothesised that this uncertainty arose from the differing responses of the vagus (fast) and sympathetic (slow) arms of the autonomic effectors, and to differing sympatho–vagal balance at different exercise intensities.

Methods and Results

We studied 12 normals at rest, during moderate (50% maximal heart rate) and submaximal (80% maximal heart rate) exercise.

The carotid baroreceptors were stimulated by sinusoidal neck suction at the frequency of the spontaneous high- (during moderate exercise) and low-frequency (during submaximal) fluctuations in heart period and blood pressure. The increases in these oscillations induced by neck suction were measured by autoregressive spectral analysis. At rest neck stimulation increased variability at low frequency (RR: from 6.99 ± 0.24 to 8.87 ± 0.18 ln-ms2; systolic pressure: from 3.05 ± 1.7 to 4.09 ± 0.17 ln-mm Hg2) and high frequency (RR: from 4.67 ± 0.25 to 6.79 ± 0.31 ln-ms2; systolic pressure: from 1.93 ± 0.2 to 2.67 ± 0.125 ln-mm Hg2) (all p < 0.001). During submaximal exercise RR variability decreased but systolic pressure variability rose (p < 0.01 vs rest); during submaximal exercise low-frequency neck stimulation increased the low-frequency fluctuations in blood pressure (2.35 ± 0.51 to 4.25 ± 0.38 ln-mm Hg2, p < 0.05) and RR. Conversely, neck suction at high frequency was ineffective on systolic pressure, and had only minor effects on RR interval during moderate exercise.

Conclusion

During exercise baroreflex control is active on blood pressure, but the efferent response on blood pressure and heart rate is only detected during low frequency stimulation, indicating a frequency-dependent effect.

Introduction

The fact that blood pressure and heart rate both increase during exercise suggests that baroreflex control is greatly reduced [1], [2]. Impaired reflex control has been shown by drug-induced changes in blood pressure [3], carotid occlusion in dogs [4], or neck suction during exercise [5], [6]. In contrast the control of arterial blood pressure during exercise has been more controversial [7], [8]. Some recent studies have demonstrated persistent modulation of blood pressure by the baroreflex during exercise, despite resetting [9], [10].

We previously found that, at rest in the supine position, the response to stimulation of the carotid baroreceptors at different frequencies in humans is dependent not only on the stimulus frequency, but also on the ability of the effectors to respond to these frequencies [11]. Although the sinus node is capable of responding to both slower and faster frequencies, the vascular system responds only up to 0.1 Hz stimuli (i.e. in the low-frequency [LF] range) [11]. These findings support the concept that higher frequency stimulation (HF) acts primarily by the faster responding vagus, while lower frequency stimulation acts both by modulation of sympathetic (on RR interval and blood pressure) and vagal activity (on RR interval only).

Exercise presents first a withdrawal of vagal tone and then a progressive increase in sympathetic activation. Non-neural HF fluctuations in blood pressure are caused by the faster and deeper respiration during exercise which results in greater changes in venous return [12], [13]. In addition, small fluctuations in RR interval are reflexly induced by direct stretch of the sinus node [12], [14] subsequent to this increased variability in venous return.

We reasoned that changes in both baroreflex control and also in sympatho–vagal balance induced by exercise, might modify the responses to autonomic (baroreflex) stimulation of the cardiovascular system at different precisely defined frequencies, so that:

  • 1)

    The rapid withdrawal of vagal tone could be responsible for the disappearance of the response to faster (in the respiratory range) stimulation of the carotid baroreceptors on RR interval: thus, even at low levels of exercise, we expected that the heart period would not respond well to fast (0.2 Hz) baroreflex stimulation;

  • 2)

    The increase in sympathetic tone with increasing exercise could be responsible for maintaining a discernible response of blood pressure (and, to a lesser extent of RR interval) to baroreceptor stimulation, but only at the frequency at which the whole sympathetic efferent pathway could respond, i.e., only if the stimulation was slow enough (up to 0.1 Hz) to allow these responses. Thus, with progression of exercise, we expected that blood pressure and heart period could still respond if the baroreflex was stimulated at a slow frequency (up to 0.1 Hz).

We therefore assessed (with the newer techniques of neck suction and power spectral analysis) the baroreflex control of blood pressure and heart rate during 2 levels of sitting bicycle exercise, particularly examining the power spectral response to slower versus faster stimuli applied to the carotid baroreceptors, during moderate and submaximal exercise.

Section snippets

Subjects and protocol

We studied twelve healthy men (colleagues in the hospital), familiar with the laboratory but unaware of the aims of the study. Their mean age was 27.7 ± 1.6 (SEM) years (range 20–42), height 180 ± 2 cm, weight 73.7 ± 2.2 kg, body mass index 22.8 ± 0.6 kg/m2. All subjects were physically fit, but not competitive athletes. All gave informed consent; the protocol was approved by the local Ethics committee.

The subjects sat on a cycle ergometer (MEDIFIT, Mijnhardt, Netherlands) in a room at 22 °C, 60%

Results

Complete results are presented in Table 2. Examples of the time series and of power spectra obtained during moderate and submaximal exercise, and of the neck suction stimulations are given in Fig. 2, Fig. 3, Fig. 4, Fig. 5.

Main findings

The novel finding in this study is that the changes in autonomic nervous control during exercise markedly affect the frequency response of the baroreflex arc. Faster baroreflex stimulation (in the frequency range of respiration) was no longer able to modulate heart period even at moderate exercise levels. This was associated with the almost total disappearance of the high frequency fluctuations in heart period, (typical of respiratory sinus arrhythmia and mainly due to parasympathetic

Conclusions

We have found that during exercise the sympathetic arm of the baroreflex remains relatively intact, based on the evidence that LF-neck suction induced LF oscillations in blood pressure; in addition, the spontaneous LF did not decrease during exercise, whereas the response of the cardiovascular system to fast perturbations (through the vagus) is severely blunted. As a consequence, during physical exercise the heart is not able to buffer rapidly enough the increased fast (mechanical) changes in

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