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Heart rate variation was studied in 56 healthy subjects from ages 20-81 while supine and standing during spontaneous and metronome breathing. Time domain analyses revealed no age effect on supine heart rate but standing heart rate decreased with aging (p less than .008). The SD of R-R intervals decreased with increasing age in both positions (p less than .001). Frequency domain analysis (Fast Fourier Transformation of R-R interval data) showed total spectral content to decrease with aging (p less than .001). Both high frequency (0.20-0.32 Hz) and low frequency (0.04-0.12 Hz) content were reduced with aging (p less than .001). The ratio of high to low frequency content, however, was not affected by age in the supine position nor were changes in this ratio in response to standing during spontaneous breathing. Although the absolute increase in low frequency content upon standing was less with aging (p less than .001), the percent increase in low frequency content was not related to age. Metronome breathing decreased total spectral content (p less than .001) but increased high frequency content, especially in younger subjects (p less than .03). In response to standing, greater proportional and absolute decreases in high frequency content occurred in younger subjects resulting in age differences in the changes in ratios of high to low frequency content with standing during metronome breathing. In summary, decreased heart rate variation was seen with aging, but the balance between parasympathetic and beta-adrenergic input at rest and in response to postural changes does not appear to be influenced by age during spontaneous breathing. Metronome breathing altered spectral content within subjects and produced age-related differences in responses to postural maneuvers not seen during spontaneous breathing.
Schwartz et al. (Wed,) studied this question.