OBJECTIVES: 1) To analyze heart rate variability (HRV) changes, reflecting the sympathovagal balance with secondary hypertension caused by sleep disordered breathing (SDB), compared to healthy controls and essential hypertension without SDB; 2) to compare HRV changes between various degrees of SDB severity; and 3) to test the modification of HRV indices by continuous positive airway pressure (CPAP) in SDB patients. BACKGROUND: Differentiation of secondary hypertension caused by SDB from essential hypertension and healthy controls by ambulatory blood pressure measurement (ABPM) and its modification by CPAP, requires an analysis of HRV changes, as frequently used for the prediction of cardiovascular risk. METHODS: HRV changes were analyzed in 48 adults divided into six groups according to the apnoea/hypopnoea index (AHI), i.e. three groups with various degrees of SDB, a group with severe SDB after CPAP application, a group with essential hypertension without SDB, and a group of healthy controls. Night-time and daytime values of low frequency (LF) and high frequency (HF) bands and the LF/HF ratio were compared in the six groups. RESULTS: The night-time values of LF bands were higher in severe than in moderate and mild degrees of SDB, and the correlation of LF/HF ratio with AHI (r = 0.3511) suggests the gradual increase of sympathetic predominance with the severity of SDB. The high sympathetic activity substantially decreased after application of CPAP in severe SDB. CONCLUSION: The increased nocturnal values of the LF band and the LF/HF ratio, caused by frequent apnoea/ hypopnoea episodes, support the usefulness of HRV spectral analysis for the prediction of cardiovascular risk in patients with SDB (Tab. 1, Fig. 3, Ref. 36).