INSTANT POWER SPECTRUM ANALYSIS OF HEART-RATE-VARIABILITY DURING ORTHOSTATIC TILT USING A TIME- FREQUENCY-DOMAIN METHOD/

Background Spectral analysis of heart rate (HR) variability (HRV) requ ires, as a rule, some level of stationarity and, as a result, is inade quate to quantify biological transients. A time-/frequency-domain meth od (TF) was developed to obtain an instant spectral power (SP) of HRV during tilt. Methods and Results HR was recorded by Holler monitoring in volunteers and analyzed with a TF, the smoothed pseudo-Wigner-Ville transformation (SPWVT), with the table inclination randomly set or co ntinuously increased while the table rotated in head-up position. (1) The SPWVT assesses, beat by beat, the instant center frequency (ICF) o f the SP. ICF correlates better with instant HR than the ratio of low- (LF) to high-frequency (HF) oscillations. The transient effect of tilt is better characterized as a shift of SP toward lower frequencies tha n by changes in amplitudes. (2) The method evidences variations of HR from one second to another. During the passage to head-up position, th e vagal withdrawal and the sympathetic activation occur nearly simulta neously, as indicated by the instant changes in both LF and HF amplitu des and ICF. (3) The averaged results of the SPWVT give results simila r to those previously obtained with autoregressive algorithms. Conclus ions The SPWVT is a new tool to explore HR transitions such as periods before episodes of arrhythmias on a time scale of one beat and allows quantification of an instant frequency index (ICF) that closely refle cts the instantaneous relationship between sympathetic and vagal modul ations.