In the conditions of sinus rhythm and low heart rate (bradycardia), a correlation was established between such important characteristics of cardiac activity as the RR interval, the QT interval and its correction by Fridericia QTcF. It is shown that the duration of the QTcF interval is almost uniquely determined by the adhesion coefficient AC, which is the SDQT / SDRR ratio of the standard deviations of the QT and RR intervals calculated from some sequence of cardiac cycles. In other words, AC is the ratio of the temporary instability of the electric systole and the total power of regulation of the heart rhythm. Therefore, the same elongation (shortening) of the corrected QTcF interval can be associated with various combinations of SDQT and SDRR. Analysis of such combinations may be useful in the study of the nature of the pathology associated with changes in the duration of QTcF.