Dynamic cardiorespiratory interaction during head-up tilt-mediated presyncope

S. Krishnamurthy, X. Wang, D. Bhakta, E. Bruce, J. Evans, T. Justice, A. Patwardhan

Research output: Contribution to journalArticlepeer-review

21 Scopus citations


In 28 healthy adults, we compared the dynamic interaction between respiration and cerebral autoregulation in 2 groups of subjects: those who did and did not develop presyncopal symptoms during 70° passive head-up tilt (HUT), i.e., nonpresyncopal (23 subjects) and presyncopal (5 subjects). Airflow, CO2, cerebral blood flow velocity (CBF), ECG, and blood pressure (BP) were recorded. To determine whether influences of mean BP (MBP) and systolic SP (SBP) on CBF were altered in presyncopal subjects, coherencies and transfer functions between these variables and mean and peak CBF (CBF m, and CBFp) were estimated. To determine the influence of end-tidal CO2 (ETCO2) on CBF, the relative CO2 reactivity (%change in CBFm per mmHg change in ETCO2) was calculated. We found that in presyncopal subjects before symptoms during HUT, coherence between SBP and CBFp was higher (P = 0.02) and gains of transfer functions between BP (MBP and SBP) and CBFm, were larger (MBP, P = 0.01; SBP, P = 0.01) in the respiratory frequency region. In the last 3 min before presyncope, presyncopals had a reduced relative CO2 reactivity (P = 0.005), likely a consequence of the larger decrease in ETCO 2. We hypothesize that the CO2-mediated increase in resistance attenuates autoregulation such that the relationship between systemic and cerebral hemodynamics is enhanced. Our results suggest that an altered cardiorespiratory interaction involving cerebral hemodynamics may contribute in the cascade of events during tilt that culminate in unexplained syncope.

Original languageEnglish
Pages (from-to)H2510-H2517
JournalAmerican Journal of Physiology - Heart and Circulatory Physiology
Issue number6 56-6
StatePublished - Dec 2004


  • Cerebral autoregulation
  • Respiratory frequency region

ASJC Scopus subject areas

  • Physiology
  • Cardiology and Cardiovascular Medicine
  • Physiology (medical)


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