The 24 hour blood pressure-R-R interval relation in ambulatory monitoring

The present study was aimed at investigating whether the blood pressure-R–R interval relation obtained by ABPM may give useful information about autonomic control in the 24 h period. To this purpose ABPM was performed in 60 healthy young subjects (30 females and 30 males, mean age 21.8 ± 1.0 years) and the collected variables were copied to a software program to convert heart rate into R–R interval values, for statistical analysis and graphic representation. The following data were calculated: 1) day and night means ± SD; 2) difference and percent difference in mean night less mean day R–R interval (Δy), diastolic and systolic blood pressures (Δx) and their Δy/Δx ratios; 3) intercept (a_24 h), slope (b_24 h) and r coefficient (r_24 h) of the linear regressions of 24 h R–R interval over diastolic and systolic blood pressure values. In all subjects night, with respect to day, was characterized by R–R interval lengthening and blood pressure lowering. Despite this common pattern, day and night means and SDs, night and day differences, Δy/Δx ratios, a_24 h and b_24 h were different from individual to individual, but they were characteristic and reproducible in 20 out of the 21 subjects in which ABPM was repeated twice. Subjects could thus be classified according to their Δy/Δx ratios and slope (b_24 h). The 24 h blood pressure-R–R interval relation as calculated from ABPM yields individually characteristic indices of circadian sympatho-vagal reciprocity. This novel approach may be helpful in characterizing the 24 h autonomic control of several groups of patients.