299 Evidence for Central Organization of Cardiovascular Rhythms NICOLA MONTANO, ALBERTO PORTA, AND ALBERTO MALLIANI Centro Ricerche Cardiovascolari, CNR, Dipartimento di Scienze Precliniche LITA di Vialba, Medicina Interna II, Ospedale “L. Sacco,” Università degli Studi di Milano, Milano, Italy ABSTRACT: Spectral analysis of heart rate and arterial pressure variabilities is a powerful noninvasive tool that is increasingly used to infer alterations of cardiovascular autonomic regulation in a variety of physiological and patho- physiological conditions such as hypertension, myocardial infarction, and con- gestive heart failure. A most important methodological issue to properly interpret the results obtained by the spectral analysis of cardiovascular vari- ability signals is represented by the attribution of neurophysiological correlates to these spectral components. In this regard, recent application of spectral techniques to the evaluation of the oscillatory properties of sympathetic effer- ent activity in animals as well as in humans offers a new approach to a better understanding of the relationship between cardiovascular oscillations and autonomic regulation. The data so far collected seem to suggest the presence of a centrally organized neural code, characterized by excitatory and inhibitory neural mechanisms subserving the genesis and the regulation of cardiovascular oscillations concerning the major variables of autonomic regulation. KEYWORDS: Heart rate variability; Sympathetic activity; Spectral analysis; Baroreflex mechanisms The neural regulation of circulatory function is mainly effected through the interplay of the sympathetic and vagal outflows, which are tonically and phasically modulated by means of the interaction of at least three major factors: (1) central integration, (2) peripheral inhibitory reflex mechanisms with negative feedback characteristics, and (3) peripheral excitatory reflex mechanisms with positive feedback characteristics. Although there are situations, such as diving reflex, in which sympathetic and vagal drive may change in the same direction, in most physiological conditions, the activation of one outflow is accompanied by the inhibition of the other. 1–3 This is true for reflexes arising predominantly not only from the arterial baroreceptive areas, but also from the heart. This interaction can be explored by assessing cardiovascular rhythmicity with an appropriate methodology. Application of spectral analysis to beat-to-beat cardiovas- cular variability has emerged as a powerful tool to study the dynamic interaction of sympathetic and vagal regulatory mechanisms. 1–3 The possibility of quantifying the Address for correspondence: Nicola Montano, Centro Ricerche Cardiovascolari, CNR, DiSP LITA di Vialba, Università degli Studi di Milano, Ospedale “L. Sacco,” Via G. B. Grassi 74, 20157 Milano, Italy. Voice: +39-02-38210535; fax: +39-02-38210533. nicola.montano@unimi.it