14th International Congress
THE "NEW FRONTIERS"
OF ARRHYTHMIAS 2000

Jan. 29 - Feb. 5, 2000
Marilleva, Trento, Italy

RT-27

Role of baroreflex sensitivity in the identification of post-MI patients candidates to implantable cardioverter defibrillator

Maria Teresa La Rovere, Andrea Mortara, Marco Gnemmi, Peter J. Schwartz*.
Centro Medico Montescano, Fondazione “Salvatore Maugeri” IRCCS, Montescano, *Dipartimento di Cardiologia, Universita di Pavia e Policlinico S. Matteo IRCCS, Pavia, Italy

Introduction

Following the results of MADIT¹ showing that in a well defined population of patients at increased risk the prophylactic use of the ICD significantly improves survival, other populations need to be defined who may benefit of this therapeutic option². Accordingly, primary prevention trials have been recently launched^3,4. However, in MADIT-II³ and SCD-HeFT⁴ risk stratification is based only on severely depressed left ventricular function and clinical symptoms of heart failure.

Although there is a growing awareness that the autonomic nervous system (particularly its imbalances characterized by an increase in sympathetic and/or a decrease in vagal activity) plays an important role in the chain of events leading to cardiovascular mortality⁵, measures of autonomic control have not yet entered in the process of risk stratification on a routine basis.

Increased sympathetic activity links several important risk factors for coronary artery disease, and the deleterious effects of the circadian surge in sympathetic activity on platelet aggregability and on vasoconstriction have been recognised to account for the increased incidence of myo-cardial infarction and sudden death⁶.

Myocardial infarction can affect the function of the autonomic nervous system which through its sympathetic and parasympathetic outflows and their complex influences can modify cardiac electrophysiology thus playing a role in the development of life-threatening arrhythmias⁷. As the arterial baroreflex is an important control mechanism regulating vagal and sympathetic activity, the measure of the sensitivity of the arterial baroreflex (BRS) does inform on the sympatho-vagal balance to the heart. The underlying pathophysiology implies that a reduced BRS has to be interpreted as the result of a decresased capability to activate vagal reflexes thus limiting the antifibrillatory effect of vagal activity. Clinical data support previous experimental observations⁸. In the study by Mitrani et al⁹ patients with previously implanted cardioverter defibrillators had induction of ventricular fibrillation (VF) both in the control state and after infusion of phenylephrine which by activation of the baroreceptors produces a reflex increase in vagal activity. It was observed that in the majority of patients who had a vagal response (identified from the increase in sinus cycle length) following phenylephrine injection there was an increase in the energy required to induce VF, while this was observed in only 20% of patients who did not show signs of vagal activation.

Two distinct aspects of the value of BRS will be considered: a) the role of baroreceptors in the hemodynamic outcome of sustained ventricular arrhythmias and b) the role of the analysis of baroreceptive responses in risk stratification.