Baroreflex sensitivity: methods, mechanisms, and prognostic value.

A large bulk of data collected over the last 25 years links reflex autonomic activation during acute myocardial ischemia with risk of developing lethal arrhythmias. Specifically, evidence obtained in an experimental preparation in chronically infarcted dogs supported the concept that sympathetic hyperactivity enhances likelihood for ventricular tachyarrhythmias, vagal activation exerts protective effects. Based on this knowledge, it was first proposed by our group that analysis of autonomic control of heart rate could provide information relevant to risk stratification in post-myocardial infarction individuals. Among several possibilities, baroreflex sensitivity was evaluated by correlating blood pressure rise induced by bolus injections of phenylephrine with the consequent beat to beat R-R interval lengthening. Experimental studies involving direct recordings from single neural vagal fibers directed to the heart documented that baroreflex sensitivity closely reproduces cardiac vagal activity. In a large group of conscious dogs it was shown that a depressed baroreflex sensitivity was highly predictive of the risk for ventricular, fibrillation during acute myocardial ischemia. The clinical prognostic value of baroreflex sensitivity has already been confirmed in pilot studies conducted by different groups of investigators. Overall, the phenylephrine test has been performed in several hundred patients with no reports of side effects. An ongoing multicenter study, the ATRAMI (Autonomic Tone and Reflexes After Myocardial Infarction) is aimed to definitively assess the predictive value of baroreflex sensitivity and heart rate variability in patients with a prior myocardial infarction. While the enrollment is still ongoing, this study has already provided an important methodological information about the possibility of using non invasive technique to record blood pressure by means of FINAPRES, to evaluate baroreflex sensitivity. Comparison among 142 tests performed with simultaneous recording from an intraarterial line and from FINAPRES indicated a strong correlation (r = 0.9) between the two methods. ATRAMI is expected to close the enrollment in the near future. To data, baroreflex sensitivity appears to be a safe and non-invasive test likely to provide meaningful information on autonomic balance and consequently on risk profile of patients with a prior myocardial infarction.