  Cardiac and respiratory rhythms are known to exhibit a modest degree of phase
synchronization, which is affected by age, diseases, and other factors. We
study the fine temporal structure of this synchrony in healthy young, healthy
elderly, and elderly subjects with coronary artery disease. We employ novel
time-series analysis to explore how phases of oscillations go in and out of the
phase-locked state at each cycle of oscillations. For the first time we show
that cardiorespiratory system is engaged in weakly synchronized dynamics with a
very specific temporal patterning of synchrony: the oscillations go out of
synchrony frequently, but return to the synchronous state very quickly (usually
within just one cycle of oscillations). Properties of synchrony depended on the
age and disease status. Healthy subjects exhibited more synchrony at the higher
(1:4) frequency-locking ratio between respiratory and cardiac rhythms, while
subjects with coronary artery disease exhibited relatively more 1:2 synchrony.
However, multiple short desynchronization episodes prevailed regardless of age
and disease status. The same average synchrony level could alternatively be
achieved with few long desynchronizations, but this was not observed in the
data. This implies functional importance of short desynchronizations dynamics.
These dynamics suggest that a synchronous state is easy to create if needed,
but is also easy to break. Short desynchronizations dynamics may facilitate the
mutual coordination of cardiac and respiratory rhythms by creating intermittent
synchronous episodes. It may be an efficient background dynamics to promote
adaptation of cardiorespiratory coordination to various external and internal
factors.
