Frequency domain of heart rate and blood pressure variability in essential hypertensive patients during sleep: Differences between dippers and non-dippers

Methods: Autonomic nervous function was evaluated by means of power spectral analysis of heart rate and blood pressure variability in dipper (n = 10) and non-dipper (n = 9) essential hypertensive subjects during sleep. The non-dipper subjects were defined as those in whom the nocturnal decrease in blood pressure was <10% of the daytime blood pressure. We measured beat-to-beat blood pressure by using a Finapres device and all stages of sleep by simultaneous polysomnographic recording during spontaneous nocturnal sleep. We analysed the pattern of changes in blood pressure for random periods of 4 min duration while the patient was awake and during all stages of sleep. For each period (waking, stages 2, 3 and 4 of sleep) a segment of 256 stationary data points was analysed. In the frequency domain, the spectral characteristics of the stationary segments were estimated by fast Fourier transformation over three frequency bands: low frequency (0.025-0.07 Hz), mid-frequency (0.07-0.14 Hz) and high frequency (0.14-0.35Hz). Results: Pulse-interval power spectral analysis did not reveal any difference between dippers and non-dippers during waking. In dipper patients, the low-frequency pulse interval (LFPI) decreased during sleep whereas the high-frequency pulse interval increased; the mid-frequency systolic blood pressure and diastolic blood pressure (DBP) decreased significantly and the high-frequency DBP increased during sleep. In non-dipper patients, the LFPI increased from wakefulness to stages 2 and 3 of sleep and the high-frequency pulse interval decreased during sleep; the mid-frequency systolic blood pressure and DBP increased in stage 4 sleep and the high-frequency DBP decreased during sleep. Conclusions: These findings indicate that non-dipper hypertensive subjects are characterized by increased LFPI and mid-frequency blood pressure during sleep compared with dipper subjects. This alteration in the autonomic nervous function may explain the non-dipper phenomenon in essential hypertension.