Origins of the power of the low frequency heart rate variability bandwidth

Purpose: We hypothesized that heart period (HP) variability in the low frequency (LF) band is due to transient fluctuations of about 10 s in HP sequences, associated with fluctuations in blood pressure. Methods: 10 healthy subjects, mean age 36 y, had HP and blood pressure acquired for 10 min each. Nonrandom HP fluctuations (ripples) lasting 6.7-20 s were detected using time-scrambled surrogate sequences as controls. Results: Ripples were 99 +/- 40 ms in amplitude, in concatenates 23.4 +/- 7.4 s long. They co-occurred with similar blood pressure ripples with amplitudes 5 +/- 5 mm Hg, correlating with them with r(2) = 0.68 +/- 0.10, slope 23.9 +/- 10.8 ms/mm Hg, at an optimum lag of 2.16 +/- 0.63 beats. A second HP structure consisting of transient tachycardias of 140 +/- 53 ms lasting 15.1 +/- 6.1 occurred singly. Together the two structures contributed 84% +/- 8% of the total power in the LF band. Conclusion: The LF band is caused by two types of HP structures that occur at discrete times. (C) 2018 Elsevier Inc. All rights reserved.