2026 Volume 63 Issue 2 Pages 38-47
Spectral analysis of heart rate variability (HRV) is a widely used non-invasive method for assessing cardiovascular autonomic function. Conventional HRV spectral analysis is typically performed on five-minute HRV time-series data, assuming stationarity. However, this approach has inherent limitations in elucidating the physiological and pathophysiological significance of individual frequency components. A deeper understanding of HRV requires analysis of transient and dynamic fluctuations that emerge over periods of several seconds to several tens of seconds. We have therefore applied time–frequency analysis based on the short-time Fourier transform (STFT) to perform continuous spectral analysis of HRV time-series data, focusing on non-stationary short-term fluctuations and their associated power changes across frequency bands. Furthermore, we developed a novel cross-spectral analysis program and applied it to the arterial baroreflex (ABR), a representative reflex mechanism underlying transient blood pressure and heart rate regulation. ABR-related cardiovascular responses typically occur over a short time scale of approximately 10 seconds, making them poorly suited to conventional stationary spectral approaches. By combining continuous short-time frequency analysis using STFT with cross-spectral analysis, we were able to characterize rapid, non-stationary changes in the dynamic coupling between blood pressure and heart period. Using this approach, we demonstrated that orthostatic stress induced by postural change from the supine to the standing position markedly reduces the transfer function gain of the ABR. In this review, we present these findings as illustrative examples of short-term dynamic changes in HRV time-series data and discuss the significance of analyzing transient fluctuations in HRV research.
