  Signals from heart transplant recipients can be considered to be a natural
source of information for a better understanding of the impact of the autonomic
nervous system on the complexity of heart rate variability. Beat-to-beat heart
rate variability can be represented as a network of increments between
subsequent $RR$-intervals, which makes possible the visualization of short-term
heart period fluctuations. A network is constructed of vertices representing
increments between subsequent $RR$-intervals, and edges which connect adjacent
$RR$-increments. Two modes of visualization of such a network are proposed. The
method described is applied to nocturnal Holter signals recorded from healthy
young people and from cardiac transplant recipients. Additionally, the analysis
is performed on surrogate data: shuffled RR-intervals (to display short-range
dependence), and shuffled phases of the Fourier Transform of RR-intervals (to
filter out linear dependences). Important nonlinear properties of autonomic
nocturnal regulation in short-term variability in healthy young persons are
associated with $RR$-increments: accelerations and decelerations of a size
greater than about 35 ms. They reveal that large accelerations are more likely
antipersistent, while large decelerations are more likely persistent. Changes
in $RR$-increments in a heart deprived of autonomic supervision are much lower
than in a healthy individual, and appear to be maintained around a homeostatic
state, but there are indications that this dynamics is nonlinear. The method is
fruitful in the evaluation of the vagal activity - the quantity and quality of
the vagal tone - during the nocturnal rest of healthy young people. The method
also successfully extracts nonlinear effects related to intrinsic mechanisms of
the heart regulation.
