The working heart is the source of electrical signal which carries the basic information about homeostasis process of human being. After appropriate acquisition and preprocessing electrocardiogram (ECG) signal may be analyzed in different ways revealing not only current status of atrial and ventricular electrical activity of heart muscle but also information about sympathetic and parasympathetic nervous system, thermoregulation and vasomotor tone, enin-angiotensin control systems, baroreceptor refle and blood pressure regulation, respiratory activity etc. Heart rate variability (HRV) defined as a timing of the sequence of QRS complexes is one of the most promising quantitative markers of autonomic activity. Different mathematical indices in time and frequency domain were designed to extract full details valuable for diagnostic purposes. Presented paper is the first one of the series of articles describing mathematical methodology of heart rate (HR) signal processing and further classification of obtained markers. The ultimate goal is to find optimal set of HR descriptors which support diagnosis in different cardiac pathologies.