Sequential spectral analysis of 24-hour blood pressure and pulse interval in humans

Blood pressure and pulse interval are characterized not only by erratic variations but also by rhythmic fluctuations at low-, mid-, and high-frequency (0.025-0.07, 0.07-0.14, and 0.14-0.35 Hz, respectively). However, information on these phenomena has largely been derived from analysis of short-term recordings taken in standardized laboratory conditions. In seven normotensive and 10 untreated mild essential hypertensive subjects, power spectrum analysis was performed on the intra-arterial blood pressure and pulse interval signal collected over a 24-hour period using the fast Fourier transform algorithm and splitting the recording into contiguous segments of 256 beats. About 70% of the segments were suitable for the analysis; the segments excluded for a nonstationary signal amounted to only 30%. All powers were characterized by a high segment-to-segment variability, but in each subject the mid- and high-frequency powers of diastolic blood pressure and the mid-frequency power of systolic blood pressure were markedly reduced during the night as compared with the daytime period, whereas the opposite occurred for the low- and high-frequency powers of the pulse interval. Over the 24-hour period, mid- and high-frequency powers of blood pressure were positively correlated to each other, but both accounted for less than 25% of the 24-hour blood pressure variance. No difference between mean normalized power values of normotensive and hypertensive subjects was observed. Thus, at both normal and mildly elevated blood pressure, rhythmic blood pressure and pulse interval oscillations falling in the low-, mid-, and high-frequency ranges are not masked by the erratic environmental stimuli of daily life but can be identified over most of the 24-hour time period. The magnitude of these phenomena is only a fraction of the overall variability and is subjected to large moment-to-moment modifications. In several circumstances, however, the nighttime period was accompanied by systematic and pronounced changes that may help in determining the mechanisms that underlie these events.