Effects of parasympathetic blockade on nonlinear dynamics of heart rate in mice

The complexity of the heart rate series has been assessed by 3 kinds of nonlinear estimates: correlation dimension (CD), Lyapunov exponents (LE), and approximate entropy (ApEn), in a population of young normal mice before and after IP injection of atropine. The resulting parasympathetic blockade produced an increase in the complexity of the heart rate (RE) series as measured by CD, the first LE and ApEn, and a decrease in heart rate variability, especially in the low frequency component of the RR spectrum. This last feature is a notable difference from the effects of atropine on the RR spectrum, where it is mainly the high frequency peak which is erased. These findings suggest that: (1) in general, one should not mistake complexity, as measured by these nonlinear estimates, for variability; (2) the autonomic nervous system of mice, a species of growing interest thanks especially to the recent use of murine transgenic models as tools of cardiovascular exploration, behaves under parasympathetic blockade in a way which is quite different from the human case; (3) it may be quite hazardous to take without precautions transgenic animals as models in human pathology, since in the control (non transgenic) group, one may encounter very unusual physiological characteristics, as shown on this species