Heart rate and systolic blood pressure variability: The impact of thinness and aging in human male subjects

Background: Life spans are steadily increasing in developing countries where 'thinness' is widely prevalent. However, the interaction of aging and thinness has been poorly studied in terms of its physiological consequences. Objective: To determine the impact of aging and 'thinness' (body mass index (BMI) < 18.5 kg/m2) on resting heart rate and systolic blood pressure (SBP) variability indices in the frequency domain. Subjects: Ninety seven healthy male subjects were divided into two age categories; young; 18-30 yrs and old > 60 yrs. The subjects were further divided on the basis of BMI into young, thin (n=32), young, normal BMI (BMI 18.5-25, n=27), old, thin (n=15) and old, normal BMI (n=23) groups. Methods: Cardiac autonomic nerve function was determined using heart rate variability indices in the frequency domain (low frequency, cardiac sympathetic 0.04-0.15 Hz; high frequency, cardiac parasympathetic 0.15-0.41 Hz). Vasomotor sympathetic activity was determined from the low frequency component of SBP variability. Baroreflex sensitivity was determined from the spectral power of both RR variability and SBP variability between 0.07 to 0.14 Hz. Results: Thinness was associated with a reduction in the absolute total, low and high frequency heart rate power spectrum as compared to individuals of normal BMI, but this difference was only apparent in young adults (P < 0.05) and not in older subjects. The age related decline in heart rate variability (absolute units) was apparent for subjects of both low and normal BMI (P < 0.05). There were no differences in SBP variability either with age or BMI. Conclusions: The data suggest that aging has a pronounced effect on heart rate variability, which may mask differences in heart rate variability related to thinness.