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CLINICAL STUDY: CARDIAC ARRHYTHMIAS

Simultaneous heart rate and blood pressure variability analysis

Insight into mechanisms underlying neurally mediated cardiac syncope in children

Jeffrey P. Moak, MD, FACC^*,*, James J. Bailey, MD, MSc and Fairouz T. Makhlouf, BS

^* Children’s National Medical Center, Washington, DC, USA
Center for Information Technology, National Institutes of Health, Bethesda, Maryland, USA
Department of Statistics, American University, Washington, DC, USA

Manuscript received December 12, 2001; revised manuscript received June 3, 2002, accepted July 2, 2002.

^* Reprint requests and correspondence: Dr. Jeffrey P. Moak, Department of Cardiology, Children’s National Medical Center, 111 Michigan Avenue, NW, Washington, DC 20010, USA.
jmoak{at}cnmc.org

OBJECTIVES: The purpose of our investigation was to examine serial changes in autonomic nervous system activity along with measurements of hemodynamics and cardiac contractility, in assessing the mechanism(s) that underlie neurally mediated cardiac syncope (NMCS) in children.

BACKGROUND: Previous research that used heart rate variability analysis alone to understand changes in autonomic activity that result in NMCS has provided conflicting results. We performed simultaneous heart rate and blood pressure variability analyses to characterize dynamic alterations in sympathetic and vagal tone during tilt-table testing in 23 children with a history of syncope or frequent dizziness.

METHODS: Power spectra of heart rate and blood pressure variability were analyzed using autoregressive modeling. Maximum dP/dT of systolic blood pressure and the electrical-mechanical activation time were used to assess cardiac contractility.

RESULTS: Tilt-table testing was positive in 12 children and negative in 11. Syncope was associated with decreased heart rate, blood pressure and low-frequency (LF) power. Before episodes of syncope, systolic blood pressure dP/dT decreased, and the electrical-mechanical activation time was prolonged. The decrease in blood pressure LF power exceeded and occurred before the decrease in heart rate LF power. Despite similar early increases in LF power to the initial stress of upright tilting, no significant decline in LF power (heart rate or blood pressure) was observed during negative tilt-table tests.

CONCLUSIONS: All of these changes considered in total provide evidence supporting the hypothesis of sympathetic withdrawal/failure, resulting in a decrease in peripheral vascular tone and cardiac contractility, which results in profound hypotension in children with NMCS.

Abbreviations and Acronyms   BP   blood pressure   BPV   blood pressure variability   HPV   heart period variability   HRV   heart rate variability   HUT   head up tilt   LF   low frequency   Max   maximum   NMCS   neurally mediated cardiac syncope   SVR   systemic vascular resistance

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