[Baroreflex sensitivity].

Abstract

Arterial baroreceptors play an important role among the large number of physiological mechanisms governing the adjustment of cardiovascular system to several surrounding conditions. By baroreceptor stimulation, arterial pressure changes can modulate both sympathetical and vagal activity and, as a consequence, heart rate, contractility and vascular resistance. In the last years, many experimental and clinical observations have shown that ischemic heart disease and heart failure can change baroreceptor reflex sensitivity and cause excessive or inappropriate activity of the sympathetic system. Several methods have been developed to measure baroreceptor sensitivity by estimating the extent of change in heart rate following blood pressure oscillations being them spontaneous or brought about by application of pharmacological or mechanical stimuli. Under normal clinical conditions these measurements can be taken as the ability to activate a sympathetic answer (hypotension) or a parasympathetic one (hypertension), with the interplay of tonic vagal or sympathetic activity. The methodology most extensively used in the clinical setting relies on intravenous administration of phenylephrine, a pure alpha-agonist drug that activates arterial baroreceptors and leads to a reflex bradycardia, which can be measured as RR interval prolongation. Baroreflex sensitivity is quantified in ms of RR interval prolongation for each mmHg of arterial pressure increase. Compared to values obtained in normal subjects (average 15 ms/mmHg) baroreflex sensitivity is significantly depressed in post-infarction patients and in patients with heart failure. The application of a mechanical stimulus is carried out by means of a positive or negative pneumatic pressure through a collar around the neck. A decrease in neck chamber pressure, by stretching carotid receptors, is sensed as an arterial pressure increase and activates reflex bradycardia at the sinus node. Finally, the analysis of spontaneous oscillations of arterial pressure and heart rate can also provide information about baroreflex control of the cardiovascular system: indeed, even small physiological variations in arterial pressure can evoke a reflex heart rate response brought about by arterial baroreceptor. The potential clinical interest of these measurements (completely non-invasive) must be still studied in large populations to define both range of normality and prognostic significance.