We propose a network representation of time series which consists of time intervals between subsequent heart contractions, called RR intervals,  as a way to qualify and quantify short-term  patterns of the overall complex heart rate dynamics. In particular, a directed network of increments in RR-intervals is constructed based on a Holter recording. The vertices are labeled by the values of the observed RR increments, the edges link consecutive increments. The network is weighted - the edge widths show how often a given  pair of RR-increments  occurs in  the signal. The proposed representation allows to identify the arrhythmic events in a patient oriented way.  

As an example, a network is constructed from the nocturnal Holter recording of a male heart-transplant patient, who was 17 years following surgery, and had a good overall health status. Different rhythm patterns are found when scanning the recorded signal, thanks to the noticeable transition in the network structure: the basic rhythm which has a mean time interval about 900 ms, and another rhythm, a slower one, which is strongly variable, however well-pronounced during sleep.  It occurs that the second rhythm is beginning with a large deceleration of 400 ms,  which is followed by two compensating accelerations. Cardiologists refer to such rhythm as Mobitz type I atrioventricular  block. This block is a relatively benign form of  atrioventricular block because it is often related to periods of high vagal activity what does not need much care. However, it can also occur in generalized disease of the conducting tissue what needs further observation by a doctor.

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