Frequency Coupling and Sensitivity Spectral Measures of the Respiratory Sinus Arrhythmia System in Response to Increasing Respiratory Frequency

In 19 healthy subjects we assessed the effects of chirped respiratory frequency (RF) from 0.05 to 0.8 Hz, and of standing (STC) on the 130-s time courses of the central frequency and power of the high frequency components of RR (CFEHFRR, PEHFRR) and of respiration (CFEHFRES, PEHFRES), estimated by a time-frequency distribution. We took as indexes of respiratory sinus arrhythmia (RSA) frequency coupling (RSAFC) the CFEHFRES-CFEHFRR relation, their difference (ΔCFEHF) and coherence (RSAco), and the alpha index as RSA sensitivity (RSAs). The effects of RF on RSA measures were distinctive in three RF ranges, with precise limits at 0.09±0. 005, 0.18±0.03, 0.51±0.10 and 0.81±0.03 Hz. In the low, mid and high RF ranges, respectively: CFEHFRR was first unchanged, proportional to RF (r=0.97±0.03), then constant again; RSAco was 0.73±0.06, 0.97±0.03 and 0.78±0.08; RSAs was 135±34 ms/l, proportional to RF (r=-0. 79±0.08), and 62±30 ms/l; ΔCFEHF was greater than 0.02 Hz in the three RF stages. STC decreased mean RSAs (p<0.02) in all RF stages. RSAFC and RSAs measures vary as function of RF, showing three stages with precise RF limits and distinctive functionality, respectively: low for RSAFC but high for RSAs, optimal and linear for both, and reduced for both measures. Baroreflex activation significantly depresses RSAs.