Autonomic mechanisms associated with heart rate and vasoconstrictor reserves

Victor A. Convertino, Caroline A. Rickards, Kathy L. Ryan

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64 Scopus citations


Introduction Hemorrhage is accompanied by baroreflexmediated 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 RRISAP 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 B 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.

Original languageEnglish
Pages (from-to)123-130
Number of pages8
JournalClinical Autonomic Research
Issue number3
StatePublished - Jun 2012


  • Cardiac vagal control
  • Hemorrhage
  • Lower body negative pressure
  • Sympathetic nerve activity


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