Autonomic nervous system influence on arterial baroreflex control of heart rate during exercise in humans

Abstract

A combination of sympathoexcitation and vagal withdrawal increases heart rate (HR) during exercise, however, their specific contribution to arterial baroreflex sensitivity remains unclear. Eight subjects performed 25 min bouts of exercise at a HR of 90, 120, and 150 beats min −1 , respectively, with and without metoprolol (0.16 ± 0.01 mg kg −1 ; mean ± S.E.M.) or glycopyrrolate (12.6 ± 1.6 µg kg −1 ). Carotid baroreflex (CBR) function was determined using 5 s pulses of neck pressure (NP) and neck suction (NS) from +40 to −80 Torr, while transfer function gain (G TF ) was calculated to assess the linear dynamic relationship between mean arterial pressure and HR. Spontaneous baroreflex sensitivity (SBR) was evaluated as the slope of sequences of three consecutive beats in which systolic blood pressure and the R-R interval of the ECG either increased or decreased, in a linear fashion. The β-1 adrenergic blockade decreased and vagal cardiac blockade increased HR both at rest and during exercise (P < 0.05). The gain at the operating point of the modelled reflex function curve (G OP ) obtained using NP and NS decreased with workload independent of β-1 adrenergic blockade. In contrast, vagal blockade decreased G OP from −0.40 ± 0.04 to −0.06 ± 0.01 beats min −1 mmHg −1 at rest (P < 0.05). Furthermore, as workload increased both G OP and SBR, and G OP and G TF were correlated (P < 0.001), suggesting that the two dynamic methods applied to evaluate arterial baroreflex (ABR) function provide the same information as the modelled G OP . These findings suggest that during exercise the reduction of arterial baroreceptor reflex sensitivity at the operating point was a result of vagal withdrawal rather than an increase in sympathetic activity.

Spontaneous baroreflex control of heart rate versus cardiac output: altered coupling in heart failure

“…Several previous studies have examined spontaneous baroreflex control of HR during rapid, transient changes in blood pressure under various experimental and pathological conditions (1,3,11,13,18,21,25,32). However, whether the spontaneous baroreflex HR responses functionally translate into effective blood pressure regulation has often been uncertain, since the extent to which HR responses cause changes in CO can be variable.…”

“…This aspect is particularly relevant in HF conditions, in which, for example, sympathostimulatory reflexes by stretch of cardiac chambers after phenylephrineinduced increase of afterload or a direct ␤-adrenergic stimulation at the sinus node level by high doses of the drug may affect baroreflex sensitivity determination. On the other hand, the autonomic mechanisms mediating these rapid baroreflex-induced changes in HR and CO are likely confined to the parasympathetic component of baroreflex (20,21). Furthermore, this approach only examines the baroreflex sensitivity over a relatively modest range of pressure, which therefore does not allow the calculation of the entire sigmoidal baroreflex stimulus-response relationship.…”

“…Changes in arterial blood pressure result in arterial baroreflex-mediated reciprocal changes in heart rate (HR) and total peripheral resistance. Arterial baroreflex sensitivity (cardiac component) has often been assessed via techniques that analyze normally occurring spontaneous changes in blood pressure and the reciprocal changes in HR (3,12,21,32). This technique has been used as an index of cardiac baroreflex function in a number of species, including humans, and in many experimental and pathological conditions (1,3,11,13,18,21,25,32).…”

“…Arterial baroreflex sensitivity (cardiac component) has often been assessed via techniques that analyze normally occurring spontaneous changes in blood pressure and the reciprocal changes in HR (3,12,21,32). This technique has been used as an index of cardiac baroreflex function in a number of species, including humans, and in many experimental and pathological conditions (1,3,11,13,18,21,25,32). Sinoaortic baroreflex denervation virtually abolishes baroreflex sensitivity assessed via this method, indicating that the spontaneous reciprocal changes in HR that occur as a result of changes in arterial pressure are indeed mediated by the baroreflex (1,19).…”

Thoracolumbar spinal manipulation and the immediate impact on exercise performance

“…53,54 This is followed by further concomitant vagal withdrawal and increased sympathetic stimulation through the cardiac plexus and release of epinephrine and norepinephrine from the adrenal medulla. [55][56][57] The increase in the chronotropic and inotropic events of the heart that follow directly raises baseline cardiac output as cardiac reserve is used. The heart rate will continue to increase to support exercise demands until an athlete attains his or her maximum heart rate and reach exhaustion.…”