A 16-year-old boy developed dizziness. Objectively, his heart rate is 35 / min., his blood pressure is 85/45 mm Hg, the heart borders are not enlarged, the heart sounds are loud and clear. ECG shows P waves disconnected from QRS complexes, dissociation, and different rhythm of atria and ventricles are accompanied by varying location of P wave in relation to QRS complex. This presentation is most characteristic of the following disease: