The purpose of this study was to evaluate the differences in the circadian variations of blood pressure (BP) and heart rate (HR) among three subject groups (hypertensive, normal/high normal and optimal blood pressure). The ambulatory BP and HR of 385 subjects, without clinical evidence of hypertension-related complications, were acquired using a Holter Blood Pressure Monitor and their circadian patterns were analyzed. Systolic, Diastolic and Mean BP showed four different well-defined trends in specific time intervals of the day, similar for the three BP measures and among the three subject groups. Both BP and HR signals presented a decrease between 10:00 and 14:30 and from 19:00 to 2:00 and an increase from 5:00 to 10:00. Between 14:30 and 19:00, BP and HR presented an opposite relationship with decreasing HR and increasing BP, not yet reported in the literature. The behaviors of BP and HR were well approximated in each of the four periods by linear trends in all the three subject groups. On the contrary, in the period between 2:00 and 5:00 both BP and HR showed a quite constant trend. Results support the hypothesis of an independent vagal control during 24-h in respect of HR and BP mean levels, the latter depending on the specific subject group. Moreover, linear approximation in the identified four intervals could be used to quantify the circadian changes for all kind of subjects.
