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Autonomic mechanisms associated with heart rate and vasoconstrictor reserves

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Abstract

Introduction

Hemorrhage is accompanied by baroreflex-mediated tachycardia and vasoconstriction. The difference between baseline and maximum responses is defined as the heart rate (HR) and vasoconstrictor ‘reserve’.

Objective

To test the hypothesis that higher HR and vasoconstrictor reserves in subjects with high tolerance (HT) to central hypovolemia is associated with greater reserve for sympathoexcitation and vagal withdrawal compared with low tolerant (LT) subjects.

Methods

R–R intervals (RRI), systolic arterial pressure (SAP), estimated stroke volume, and muscle sympathetic nerve activity (MSNA) were measured during lower body negative pressure (LBNP) designed to induce pre-syncope. Subjects with tolerance ≤60 mmHg LBNP were classified as LT (n = 22) while subjects who tolerated LBNP levels >60 mmHg were classified as HT (n = 56). Spontaneous cardiac baroreflex sensitivity (BRS) was assessed via RRI-SAP down–down sequences.

Results

HR reserve in HT subjects (+52 ± 2 bpm) was twofold greater (P < 0.001) than that in LT subjects (+27 ± 3 bpm). Vasoconstrictor reserve in the HT group (+3.4 ± 0.5 pru) was greater (P = 0.04) than that of the LT group (+1.9 ± 0.3 pru). HT subjects demonstrated greater (P ≤ 0.03) BRS reserve (−14.2 ± 1.8 ms/mmHg) and MSNA reserve (+41 ± 2 bursts/min) compared with LT subjects (−7.4 ± 1.7 ms/mmHg and +26 ± 7 bursts/min).

Interpretation

Our data support the hypothesis that greater physiological reserve capacity for tachycardia and vasoconstriction related to high tolerance to central hypovolemia is associated with greater reserves for sympathoexcitation and cardiac vagal withdrawal.

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Acknowledgments

We thank the experimental subjects for their cheerful cooperation, as well as Mr Gary Muniz for his superb technical assistance and Drs John McManus, Girish Sethuraman, Keith Berry, Steven Glorsky and Robert Gerhardt for their assistance with physical examinations and medical monitoring of the subjects. This research was supported by funding from the US Army Combat Casualty Care Research Program. The opinions or assertions contained herein are the private views of the author and are not to be construed as official or as reflecting the views of the US Department of the Army or the US Department of Defense.

Conflict of interest

No conflicts of interest, financial or otherwise, are declared by the authors.

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Authors and Affiliations

  1. US Army Institute of Surgical Research, 3698 Chambers Pass, Fort Sam Houston, TX, 78234-6315, USA

    Victor A. Convertino & Kathy L. Ryan

  2. The University of Texas at San Antonio, San Antonio, TX, 78249, USA

    Caroline A. Rickards

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  1. Victor A. Convertino
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  2. Caroline A. Rickards
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Correspondence to Victor A. Convertino.

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Convertino, V.A., Rickards, C.A. & Ryan, K.L. Autonomic mechanisms associated with heart rate and vasoconstrictor reserves. Clin Auton Res 22, 123–130 (2012). https://doi.org/10.1007/s10286-011-0151-5

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