Burst activity and heart rhythm modulation in the sympathetic outflow to the heart

In 13 decerebrate, artificially ventilated cats preganglionic sympathetic outflow to the heart was recorded with ECG and ventilation signal. A novel algorithm was implemented that extracts weighted events representing burst occurrences and their size. A multiple threshold strategy was used to separate bursts. Weighted events yielded count signals. Spectral analysis of the count signal revealed a predominance of a discharge synchronous to heart beat at cardiac frequency (CF≅3 Hz), a ventilation rhythm at the high frequency (HF≅0.3 Hz) of beat-to-beat variability, and in 9 of 13 cats a Mayer's wave lower frequency (LF). The CF component was ∼50% larger in power than both HF and also LF (when present). Spectral analysis at increasing count levels (i.e., with only the events with a weight ≥ the considered level) indicated that all rhythms were carried by burst activity and its modulations. A modulation index of HF over CF, MI_HF/CF was extracted from dynamic (i.e., ventilation cycle by ventilation cycle) folded histograms of counts. MI_HF/CF was significantly higher in the group without LF (0.65±0.20, mean±SD, n=4) than in that with LF (0.41±0.07, n=9). Burst activity can be a key element in the interactions between cardiovascular variability rhythms.