[Neural control of the circulation in normal and pathologic conditions]

The concept of sympatho-vagal balance is particularly useful to explore some functional features of cardiovascular neural regulation. The variability of signals like RR interval or systolic arterial pressure, assessed with power spectrum analysis, has provided for the first time, simultaneously, markers of sympathetic and vagal modulation, and hence of their interaction. The relative power of low frequency (LF) oscillation quantifies sympathetic modulation, while the high frequency (HF) oscillation quantifies vagal modulation. LF and HF components are also detectable in the variability of muscle sympathetic nerve activity recorded in man. In numerous physiological and pathophysiological conditions these two oscillations undergo a reciprocal relationship similar to that characterizing sympatho-vagal balance. An increase in sympathetic modulation has been found during different physiological stimuli and in abnormal states such as recovery from myocardial infarction, essential arterial hypertension, and congestive heart failure in its early stage. A correct use of this methodology is prevented by a drastic decrease in variance, often accompanying pathophysiological reductions in responsiveness of target functions. In these cases the decrease in LF power yields an important negative prognostic value.