Baroreflex control of heart rate during exercise: a topic of perennial conflict

The following is the abstract of the article discussed in the subsequent letter:

	ABSTRACT

Raymond, J., G. M. Davis, M. N. van der Plas, H. Groller, and S. Simcox. Carotid baroreflex control of heart rate and blood pressure during ES leg cycling in paraplegics. J. Appl. Physiol. 88: 957-965, 2000.This study investigated control of heart rate (HR) and mean arterial pressure (MAP) at rest and during electrical stimulation (ES) leg cycling exercise (LCE) in paraplegics (Para). Seven men with complete spinal lesions (T₅-T₁₁) and six able-bodied (AB) men participated in this study. Beat-to-beat changes in HR and MAP were recorded during carotid sinus perturbation. Carotid baroreflex function curves were derived at rest and during ES-LCE for Para and during voluntary cycling (Vol) for AB. From rest to ES-LCE, oxygen uptake (O₂) increased (by 0.43 l/min) and HR rose (by 11 beats/min), yet MAP remained unchanged. In AB, Vol increased O₂ (by 0.53 l/min), HR (by 22 beats/min), and MAP (by 8 mmHg). ES-LCE did not alter the carotid sinus pressure (CSP)-MAP relationship, but it displaced the CSP-HR relationship upward relative to rest. No rightward shift was observed during ES-LCE. Vol by AB produced an upward and rightward displacement of the CSP-MAP and CSP-HR relationships relative to rest. These findings suggested that the carotid sinus baroreflex was not reset during ES-LCE in Para.

	LETTER

Baroreflex control of heart rate during exercise: a topic of perennial conflict

To the Editor: The reading of the recent interesting paper by Raymond et al. (8) on arterial baroreflex control of heart rate (HR) (and blood pressure) during exercise in paraplegic and healthy subjects prompted me to offer some comments on this issue.

Throughout that article, it is claimed, by quoting many supporting papers, that the cardiac component of the arterial baroreflex is reset during exercise in healthy humans, apparently disregarding the fact that this is a still controversial issue in human exercise physiology. No mention is given of early (1-3) and recent (4, 5) studies that indicated an attenuation in baroreflex sensitivity (BRS) rather than a resetting.

These conflicting results might be explained, in part, by differences in baroreflex testing methodology. Studies that used the rapid neck suction-pressure method consistently suggest that the arterial baroreflex is reset during exercise (8), whereas studies that used bolus injections of vasoactive drugs (1, 2) or analyzed HR changes in response to spontaneous blood pressure changes (the so-called spontaneous baroreflex method; Refs. 4 and 5) suggest that BRS may be even reduced during specific exercise conditions.

All of the above techniques share the common characteristic of reflecting responses, which are mainly vagally mediated and have inherent advantages and limitations, but it has not been established which method should be considered the "gold standard" (9).

A further source of conflict is the use of R-wave-to-R-wave (RR) interval or HR to evaluate the chronotropic response (1, 3). It has often been claimed that, in terms of HR, reanalysis of studies reporting a reduced BRS during exercise would have shown unchanged BRS. Also, this concept needs to be reconsidered. Using spontaneous baroreflex method, which permits analysis of BRS in terms of either HR or RR interval directly on the same primary data, we showed that BRS is indeed unchanged during static handgrip (6) and during slight-intensity dynamic leg exercise (7) but also that it undergoes a reduction during static leg extension (5) and high-intensity bicycle exercise (4), both in terms of HR and RR-interval.

I believe that these findings represent barriers against a rigid and univocal interpretation of baroreflex control of HR during exercise.

This letter is not a criticism of the study of Raymond et al. (8). My intention is to provide the readers, particularly the nonspecialists, with more complete and balanced information on the issue of baroreflex modulation of HR during exercise and to avoid giving the erroneous impression that a current view represents a general consensus on a topic that is far from fully understood.

	REFERENCES

1.   Bristow, JD, Braun EB, Cunningham DJC, Howson MG, Strange PE, Pickering TG, and Sleight P. Effect of bicycling on the baroreflex regulation of pulse interval. Circ Res 38: 582-593, 1971.

2.   Cunningham, DJC, Strange PE, Peto R, Pickering TG, and Sleight P. Comparison of the effect of different types of exercise on the baroreflex regulation of heart rate. Acta Physiol Scand 86: 444-455, 1972[ISI][Medline].

3.   Eckberg, DL, and Wallin BG. Isometric exercise modifies autonomic baroreflex responses in humans. J Appl Physiol 61: 2325-2330, 1987.

4.   Iellamo, F, Massaro M, Legramante JM, Raimondi G, Peruzzi G, and Galante A. Spontaneous baroreflex modulation of heart rate during incremental exercise in humans (Abstract). FASEB J 12: A692, 1998.

5.   Iellamo, F, Pizzinelli P, Massaro M, Raimondi G, Peruzzi G, and Legramante JM. Muscle metaboreflex contribution to sinus node regulation during static exercise. Insights from spectral analysis of heart rate variability. Circulation 100: 27-32, 1999[Abstract/Free Full Text].

6.   Iellamo, F, Hughson RL, Castrucci F, Legramante JM, Raimondi G, Peruzzi G, and Tallarida G. Evaluation of spontaneous baroreflex modulation of sinus node during isometric exercise in healthy humans. Am J Physiol Heart Circ Physiol 267: H994-H1001, 1994[Abstract/Free Full Text].

7.   Iellamo, F, Legramante JM, Raimondi G, and Peruzzi G. Baroreflex control of sinus node during dynamic exercise in humans: effects of central command and muscle reflexes. Am J Physiol Heart Circ Physiol 272: H1157-H1164, 1997[Abstract/Free Full Text].

8.   Raymond, J, Davis GM, van der Plas MN, Groller H, and Simcox S. Carotid baroreflex control of heart rate and blood pressure during ES leg cycling in paraplegics. J Appl Physiol 88: 957-965, 2000[Abstract/Free Full Text].

9.   Rudas, L, Crossman AA, Morillo CA, Halliwill JR, Tahvanainen KUO, Kuusela TA, and Eckberg DL. Human sympathetic and vagal baroreflex responses to sequential nitroprusside and phenylephrine. Am J Physiol Heart Circ Physiol 276: H1691-H1698, 1999[Abstract/Free Full Text].

Ferdinando Iellamo, Dipartimento di Medicina Interna-Centro di Riabilitazione Cardiologica S. Raffaele Università di Roma "Tor Vergata" E-mail: iellamo{at}med.uniroma2.it

	REPLY

To the Editor: We thank Dr. Iellamo for the interest he has shown in this research. The issues raised by Dr. Iellamo are quite stimulating, and we support his comments concerning the limitations of the various techniques employed when testing baroreflex control of HR during exercise. We also acknowledge his work addressing R-R interval vs. HR using the spontaneous baroreflex (SBR) method. Dr. Iellamo has raised an important point concerning the controversy surrounding the change, or lack thereof, in baroreflex sensitivity from rest to exercise, citing several studies that suggest that baroreflex sensitivity is attenuated during exercise. The findings of these studies contrast with the findings of other research, predominantly using the neck pressure-suction technique, which suggest that sensitivity is unaltered (e.g., Refs. 2-4).

Although the issue of altered baroreceptor sensitivity during exercise remains unresolved, it is arguable whether similar controversy extends to the concept of baroreflex resetting. Baroreflex sensitivity, as assessed by the pharmacological and the SBR methods, and resetting, as quantified by the neck pressure-suction method, are quite different features of the arterial baroreflex. Sensitivity refers to the magnitude of change in the output of the reflex (in this case HR) in response to the magnitude of change in the input (in this case blood pressure) and is defined by the slope of the curve relating the input and output variables (6). In contrast, resetting, in this context, refers to a lateral shift of the curve relating the input and output variables (5, 6). To quantify such a shift, researchers statistically compare the input value of certain landmarks (e.g., threshold, saturation, centering point) of one curve to another. If the landmarks occur at higher input values, then the entire reflex has been reset. This concept of resetting during exercise has been demonstrated in several studies in both animals and humans (e.g., Refs. 1-4).

On the issue of baroreflex sensitivity, we do not agree that we alluded to a general consensus of baroreflex sensitivity during exercise (whether changed or unchanged). From the data, which did not provide any statistical evidence of a change in maximal gain, it was simply concluded that baroreflex sensitivity remained unchanged. However, on the issue of baroreflex resetting, the arguments in the manuscript were heavily weighted toward the assumption that the baroreflex would be reset during dynamic exercise, and this view was supported by the conclusions of several research papers (e.g., Refs. 1, 3, 4). Certainly, the literature does present evidence to the contrary, but the purpose of the paper was not to examine whether baroreflex resetting occurs (or does not) in able-bodied individuals. Rather, our study investigated the behavior of baroreflex-controlled HR and blood pressure at rest and during electrical stimulation-induced leg cycling. Therefore, we presented what was interpreted from the literature as the most current and well-supported view.

	REFERENCES

1.   DiCarlo, SE, and Bishop VS. Onset of exercise shifts operating point of arterial baroreflex to higher pressures. Am J Physiol Heart Circ Physiol 262: H303-H307, 1992[Abstract/Free Full Text].

2.   Norton, KH, Boushel R, Strange S, Saltin B, and Raven PB. Resetting of the carotid arterial baroreflex during dynamic exercise in humans. J Appl Physiol 87: 332-338, 1999[Abstract/Free Full Text].

3.   Papelier, Y, Escourrou P, Gauthier JP, and Rowell LB. Carotid baroreflex control of blood pressure and heart rate in men during dynamic exercise. J Appl Physiol 77: 502-506, 1994[Abstract/Free Full Text].

4.   Potts, JT, Shi XR, and Raven PB. Carotid baroreflex responsiveness during dynamic exercise in humans. Am J Physiol Heart Circ Physiol 265: H1928-H1938, 1993[Abstract/Free Full Text].

5.   Raven, PB, Potts JT, and Shi X. Baroreflex regulation of blood pressure during dynamic exercise. Exer Sport Sci Rev 25: 365-389, 1997[Medline].

6.   Rowell, LB, O'Leary DS, and Kellog DL. Integration of cardiovascular control systems in dynamic exercise. In Handbook of Physiology, Exercise: Regulation and Integration of Multiple Systems. Bethesda, MD: Am. Physiol. Soc, 1996, sect. 12, chapt. 17, p. 770-840.

Jacqui Raymond Glen M. Davis Herbert Groeller, Rehabilitation Research Centre The University of Sydney Lidcombe, NSW 2141, Australia E-mail: J.Raymond{at}cchs.usyd.edu.au