Normative values for circadian and ultradian cardiovascular rhythms in childhood

To assess the prevalence and characteristics of physiological circadian (24-hour) and ultradian (12-, 8-, and 6-hour) rhythms of mean arterial blood pressure (BP) and heart rate (HR), we analyzed 24-hour ambulatory BP profiles from 938 healthy school children aged 5 to 18 years. Cosine harmonics were fitted by Fourier analysis, and an amplitude and acrophase ( time of peak) were calculated for each rhythm. Ninety percent of children displayed circadian rhythmicity of BP, independent of age, whereas circadian HR rhythmicity decreased with puberty from 96% to 87% ( P < 0.0001). Puberty had marked effects on the prevalence of ultradian rhythmicity: 12- and 6-hour rhythms increased for BP (27% to 47%, P < 0.0001; 18% to 25%, P = 0.01) and HR (36% to 47%, 17% to 31%, both P = 0.001), whereas 8- hour BP rhythms decreased (34% to 23%, P = 0.002). Median amplitudes were 10.1, 5.9, 5.6, and 5.2 mm Hg for the 24-, 12-, 8-, and 6-hour BP rhythms, respectively, and 13.4, 7.7, 6.8, and 6.4 bpm for HR. The acrophase occurred at approximately 14: 00 hours, 8: 00 hours, 5: 30 hours, and 2: 00 hours ( military time) for the four BP rhythms, and at 13: 30 hours, 08: 30 hours, 01: 50 hours, and 02: 00 hours for HR. For the combined curve, the peak - trough difference was 25.9 mm Hg and 35 bpm for BP and HR, respectively, with the peaks occurring at 13: 50 hours and 13: 10 hours. There was marked association between BP and HR rhythms, both for prevalence ( P < 0.0001 for coupling of BP and HR rhythms of the same period length) and timing, with a median time lag of BP after HR acrophase of only 21, 16, 13, and 5 minutes for the four rhythms, respectively.