Time-independent indices of circadian blood pressure and heart rate regulation from ambulatory blood pressure monitoring

Objectives We hypothesized that addressing the 24-h relationship between RR interval and systolic arterial pressure (SAP) during ambulatory blood pressure monitoring could detect alterations in circadian arterial pressure and heart rate profile and could also be linked to the ambulatory arterial stiffness index. Methods We analyzed retrospectively 676 ambulatory blood pressure monitoring 24-h recordings. Individuals were median split into low (n = 338) and high (n = 338) SAP groups (mean 24 h of 120 ± 7, 140 ± 10 mmHg, respectively). We estimated for each individual ambulatory blood pressure monitoring, the slope (ms/mmHg) of the regression of RR interval on SAP over 24-h (defined as theta index). Results Theta index was significantly (P<0.001) steeper in individuals with lower pressure than those with higher pressure, whereas day-night difference in SAP did not differ significantly. Theta index was also less negative in nondipper than in dipper individuals and displayed a significant interaction (P<0.001) with hypertension. The theta index correlated significantly with age and ambulatory arterial stiffness index. Moreover, approximately 14-month reproducibility, as assessed by intraclass correlation coefficient, for theta index was similar to SAP (0.703 and 0.757, respectively). Conclusion These results indicate that theta can detect differences in circadian arterial pressure profile that could assess the efficiency of arterial pressure (and heart rate) regulatory mechanisms over the whole day in individuals with different arterial pressure levels. Theta can be easily obtained under ambulatory conditions without any assumption on underlying mathematical models or operator-dependent attribution of arterial pressure and heart rate values to daytime or night-time subperiods. Cross-sectional and prospective studies relating theta to target organ damage and cardiovascular outcomes will inform us about its clinical relevance.