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ORIGINAL RESEARCH ARTICLE

New Insight into the Mechanism of Cardiovascular Dysfunction in the Elderly: Transfer Function Analysis

Hong Guo^*,, Frederic Schaller, Nancy Tierney, Scott A. Smith and Xiangrong Shi^*,1

^* Department of Integrative Physiology, University of North Texas Health Science Center, Fort Worth, Texas 76107; College of Sports Science, South China Normal University, Guangzhou, China 510631; Department of Internal Medicine, University of North Texas Health Science Center, Fort Worth, Texas 76107; and Department of Health Care Sciences, University of Texas Southwestern Medical Center, Dallas, Texas 75390

To whom requests for reprints should be addressed at ¹ Department of Integrative Physiology, University of North Texas Health Science Center, 3500 Camp Bowie Boulevard, Fort Worth, TX 76107. E-mail: xshi{at}hsc.unt.edu

This study sought to test the hypothesis that alterations in the relationships between (i) mean arterial pressure (MAP) and heart rate (HR), (ii) cardiac output (CO) and MAP, and (iii) total peripheral resistance (TPR) and MAP variability contribute to the diminished dynamic control of cardiovascular function with advanced age. Six-minute hemodynamic data were continuously recorded in 11 elderly (70 ± 2 years) and 11 young (26 ± 1 year) healthy volunteers under supine resting condition and during lower body negative pressure–induced orthostatic challenge. The data were converted using fast Fourier transform, and the ratio of cross-spectra to auto-spectra between two signals (i.e., MAP-HR, CO-MAP, TPR-MAP) was computed for transfer function analysis. In the low-frequency ranges (LF; 0.04–0.14 Hz) and high-frequency ranges (0.15–0.30 Hz), the gain and coherence of the transfer function describing the relationship between MAP-HR signals were significantly greater in younger than in older adults. The phase degree was significantly >0 in both groups under all conditions, suggesting that the MAP variability preceded the HR variability. In contrast, the coherence between CO-MAP signals in both age groups was <0.5, indicating that the beat-to-beat MAP variability was not significantly related to the CO signals. However, the transfer function gain and coherence of TPR-MAP signals were significantly greater in the young group (coherence 0.5 in the LF range), suggesting a more effective dynamic vasomotor control. In conclusion, the oscillations in CO-MAP signals are not significantly synchronized or not related in a simply linear fashion in both age groups. The MAP variability is more related to the oscillation of TPR signals in the young group only. Advanced age not only diminishes MAP-HR transfer function gain, but also weakens its coherence. Thus, alterations in the relationship between MAP-HR variability and TPR-MAP variability may significantly contribute to the diminished dynamic control of cardiovascular function manifest in the elderly.

Key Words: cardiac output variability • coherence • regional cerebral O₂ saturation • LBNP