The aim of the work was to study the changes of total power (ТР) and lowtohigh frequencies ratio (LF/HF) of heart rate variability (HRV) in patients with paroxysmal and persistent atrial fibrillation (AF) during episodes of sinus rhythm (SR) in different classes of QRS complex duration. A total of 59 patients with paroxysmal (31) and persistent (28) AF (36 men and 23 women), mean age (62±11) years, were studied during episodes of SR. Duration of the QRS complexes was measured in leads II, V1, V5, V6 (three consecutive complexes) with a choice of maximum value from the lead and registered complexes. To perform the orthostatic test of the HRV, ECG was recorded in lead II, consistently, in supine and standing position at 7 minutes intervals. Depending on the QRS complex duration (physiologic (NQRS) 60–100 ms), and prolonged (LQRS) > 100 ms), patients were divided into 2 subgroups. Patients were examined before, after 2 weeks, 1 month, 6 months and 1 year of treatment. Patients with NQRS had adequate reactions of TP and LF/HF to orthostasis, patients with LQRS –
adequate reaction of LF/HF and inadequate reaction of TP. AF therapy positively influenced HRV indices in patients with NQRS and didn’t influence or led to deterioration of the TP and LF/HF initial deviations in patients
with LQRS, that requires careful monitoring of the antiarrhythmic therapy in such patients.