Evaluation of spontaneous baroreflex modulation of sinus node during isometric exercise in healthy humans

Abstract

The purpose of this study was to 1) evaluate the baroreflex (BR) modulation of the sinus node during isometric exercise in humans by analyzing the variations in pulse interval (PI) in response to beat-by-beat spontaneous changes in systolic arterial pressure (SAP), thus avoiding external intervention to alter arterial pressure (AP); and to 2) evaluate the specific influence of muscle chemoreflex on the SAP-PI relationship. Sixteen healthy males were studied during rest, handgrip at 30% of maximum voluntary contraction [isometric handgrip (IHG)], post-IHG arrested forearm circulation (AFC), and recovery. AP and PI were measured continuously and noninvasively. A BR response was defined by series of at least three consecutive beats in which SAP and PI of the following beat either increased or decreased in a linear fashion. In nine subjects, forearm (FVR) and calf vascular resistances (CVR) were determined. Mean slope (by linear regression) of SAP-PI sequences did not significantly differ between rest, IHG, and AFC (14.1 +/- 2.0, 12.2 +/- 1.6, and 13.2 +/- 1.4 ms/mmHg, respectively), i.e., BR sensitivity was unchanged. IHG resulted in a rightward shift of the regression line relating SAP to PI. The shift was maintained during AFC, when PI returned to control and AP fell slightly but remained significantly elevated. CVR, which did not change during IHG, significantly increased during AFC, whereas FVR increased during both IHG and AFC, but not significantly. The data indicate that arterial BR is "reset" to a higher operating point during isometric exercise. Muscle chemoreflex appears to be partially involved in this modification.(ABSTRACT TRUNCATED AT 250 WORDS)

Hemodynamic and autonomic correlates of postexercise hypotension in patients with mild hypertension

“…We simultaneously measured forearm (FBF) and calf blood flow (CBF) by a computerassisted venous occlusion plethysmographic device with the patients in the supine position, as previously reported (16). FVR and calf vascular resistance (CVR) were calculated as MBP divided by recorded flows.…”

“…Details of this analysis have been previously described (16,18,19). Briefly, the beat-by-beat time series of SBP and R-R interval were scanned by a computer to identify sequences of three or more consecutive beats in which SBP and R-R interval changed in the same direction [either increasing ("up sequences") or decreasing ("down sequences)].…”

Muscle Metaboreflex Contribution to Sinus Node Regulation During Static Exercise

“…11 The arm with the instrumented finger was held extended at the heart level by means of a pulley arrangement. 5,12 Respiration was assessed by a pneumotachograph-(Fleisch No. 3) pressure transducer set (HP 47304A) connected to a face mask worn by the subjects.…”

“…5,12 Briefly, the beat-by-beat time series of systolic AP (SAP) and RR interval are scanned by a computer to identify sequences of Ն3 consecutive beats in which SAP and RR interval of the following beat change in the same direction (either increasing or decreasing). A linear regression is applied to each individual sequence, and the mean slope of the SAP/RR interval relationship, obtained by averaging all slopes computed within a given test period, is calculated and taken as a measure of the spontaneous baroreflex sensitivity (BRS) for that period.…”

“…2 This general view came from the observation that during postexercise circulatory occlusion, a maneuver that maintains muscle metaboreflex activation while removing the central command, the increases in AP, vascular resistance, and sympathetic nerve activity to resting muscles are kept elevated above resting levels, whereas HR fully recovers. [3][4][5] However, Maciel et al 6 and Martin et al 7 reported a reduced HR response to static exercise after administration of ␤-adrenergic blocking drugs, suggesting an involvement of the sympathetic nervous system in HR regulation, although the mechanism underlying the sympathetic contribution (ie, central versus reflex) has not been determined. More recently, O'Leary 8 provided evidence that sympathetic activation originating from the muscle metaboreflex contributes substantially to the HR increase during exercise in the conscious dog, inasmuch as parasympathetic blockade with atropine did not affect the increase in AP and HR that occurred during exercise but did prevent the fall in HR during postexercise circulatory occlusion.…”