13th International Congress
THE "NEW FRONTIERS"
OF ARRHYTHMIAS 1998

January 24-31, 1998
Marilleva, Trento, Italy

RT-213

Physiological interpretation of spectral components of heart rate variability

Alberto Malliani, Stefano Guzzetti.
Centro Ricerche Cardiovascolari, CNR, L.I.T.A. di Vialba, Medicina Interna II, Ospedale "L. Sacco", Universita degli Studi, Milan, Italy

Methodology

Numerous exhaustive accounts have been recently published on power spectrum analysis of HRV^1,6,7. In this context suffice it to say that in our studies we have mainly used autoregressive algorithms that indicate the existence in both the heart period and arterial pressure variabilities of three spectral components, respectively centered around 0. Hz (the very low frequency component, VLF), 0.1 Hz (the LF component) and 0.20-0.35 Hz (the HF component). Usually only the LF and HF components can be adequately studied in time series of 200-500 events. With this approach each spectral component is assessed in terms of its center frequency and of its absolute power (i.e. the area), expressed in squared units (e.g., msec²).

The normalization procedure

In view of the physiological model¹ of a reciprocal relation between LF and HF components, their estimation independent of total power has proven to be crucial. This can be easily obtained by computing their values in normalized units (nu), which are obtained by dividing the power of a given component by the total power (from which VLF has been subtracted) and multiplying by 100. Usually the sum of LF and HF falls short of 100 because of the frequent presence of some small noise components. As an example of the importance of the normalization procedure for the interpretation of data, one should consider that the sympathetic excitation that leads to tachycardia is often accompanied by a reduction in total power of HRV: this can cause a paradoxical reduction of LF in absolute units, while this component appears clearly increased if expressed in nu.