The monograph systematizes and summarizes data on bradycardia-tachycardia syndrome. Results of own study of heart rate reactivity, heart rhythm variability, electrophysiological parameters of heart work with application of increasing, reducing and programmed transesophageal stimulation of atria in patients with bradycardia-tachycardia syndrome and other forms of sinus node weakness syndrome are presented. There are presented additional diagnostic criteria for sinus weakness syndrome by daily heart rate trend, new sinus node automatism values determined in transesophageal programmed atrial stimulation. Clinical and electrocardiographic classification of bradycardia-tachycardia syndrome is proposed.