Heart rate change as a function of age, tidal volume and body position when breathing using voluntary cardiorespiratory synchronization

Voluntary cardiorespiratory synchronization (VCRS), where inspiration and expiration follows a fixed number of heart beats, was used to investigate heart rate change (HRC) in 16 subjects (grouped as 20-30 and 46-57 years of age) as a function of age, body position, and respiratory tidal volumes of 500 and 1000 ml. In phase I, the subjects, following a tone, inspired for two heart beats and expired for three beats. In phase II, the older group's breathing pattern was 2/2, 2/3, 3/3, 4/4, 5/5, and 6/6 inspiratory and expiratory heart beats, respectively, per respiratory cycle. Comparing the younger and older groups, we found a statistically significant (p < 0.05) decrease in HRC for beat 1, 2, and 4 of the respiratory cycle in the supine position, and also for beat 2 in the sitting position, but no significant change with tidal volume. The ratio of the younger to older group's beat 2 HRC was 2.54 sitting and 3.54 supine. Increasing the number of beats per respiratory cycle from four to 12 resulted in the HRC growing from 1.8 to 7.5 beats over the respiratory cycle, showing the importance of respiratory rate in measuring HRC. VCRS can easily observe the influence of each phase of the respiratory cycle on the HRC and show the unique phase dependent changes that occur.