Resonance frequency is not always stable over time and could be related to the inter-beat interval

Abstract

Heart Rate Variability Biofeedback (HRVB) is based on breathing at an optimal rate (or resonance frequency, RF) corresponding to the respiratory sinus arrhythmia (RSA). Our aim is to check whether the RF is a stable factor and analyse the HRV parameters individually per each breathing rate, comparing it with free slow breathing. A sample of 21 participants were trained in a test-retest HRVB protocol. The results indicated that RF changed between Test and Retest sessions in 66.7% of participants. This instability could be related to the average of interbeat interval (IBI). HRV time domain parameters (SDNN and RMSSD) were significantly higher for RF than for other breathing rates, including 6 breath/min and free slow breathing. Free slow breathing showed a lower heart rate averages than RF and other slow breathing rates. Overall, our study suggests the relevance of assessing RF individually and before each HRVB session, because the maximum cardiovascular benefits in terms of increasing HRV were found only at RF. Thus, breathing at the individualized and momentary frequency of resonance increases cardiac variability.

Figure 1

Schema of the study procedure.

Figure 2

Comparative box plots of RF individual values (breaths per minute) at Test and Retest.

Figure 3

Representation of the spectrogram analysis (STFT, 120 s window, 1.5 s resolution, Hann window) of the IBI series (bottom trace) for each breathing rate (7, 6.5, 6, 5.5, 5 b/m) during the HRVB of Participant 15 during the Test session. Hotter color means more respiration energy of the signal and colder less, represented in arbitrary units. During this session, P15 breathed following the metronome with a clear pattern, hotter in 7 b/m (RF). (RR interval: IBI).

Figure 4

The mean interval time (ms) between heart beats (IBImean or RRmean) is represented for Test (A), ReTest (B) and RF-RFE comparison (C). “RF” represents the optimal resonance frequency, “Other Rates” represents the mean of the other breathing rates, “Lowest RSA” represents the lowest RSA amplitude, “6 b/m” represents the rate of 6 breathes per minute, “Free” represents the free slow breathing session only in retest, and “RFE” represents the expanded RF that consists on taking the IBImean of the nearest below and above rates from the RF. (Non-parametric Wilcoxon test has been performed when Friedman test showed significant differences.). * (p < 0.05) and ** (p < 0.01) mean differences with respect to RF in the same session. ^## (p < 0.01) means differences with respect to Free Respiration in the same session.

Figure 5

HRV time domain parameters: SDNN (A–C) and RMSDD (D–F) for Test (A,D), ReTest (B,E) and RF-RFE comparison (C,F). “RF” represents the optimal resonance frequency, “Other Rates” represents the mean of the other breathing rates, “Lowest RSA” represents the lowest RSA amplitude, “6 b/m” represents the rate of 6 breathes per minute, “Free” represents the free slow breathing session only in retest, and “RFE” represents the expanded RF that consists on taking the SDNN/RMSSD of the nearest below and above rates from the RF. Non-parametric Wilcoxon test has been performed. * (p < 0.05), ** (p < 0.01) and *** (p < 0.001) mean differences with respect to RF in the same session.