Heart rate variability and baroreflex sensitivity in myocardial infarction.

The mortality rate during the year after myocardial infarction (MI) is in the range of 5 % to 15% .la 2 Survival curves after an MI generally demonstrate that the highest attrition rate occurs immediately after the event and then declines so that by 1 to 2 years after the infarction, the slope of the survival curve remains constant. In one study 50% of the deaths that occurred within 4 years after an MI occurred within the first 6 months.3 Approximately 50 % of all deaths after an MI occur with little or no warning.4-7 These deaths are usually referred to as sudden deaths; however, the definition of sudden death has not been uniform.s1° The incidence ranges from 3 % to 8 % in the first year and 2 % to 4% per year thereafter.6q 11-14 The majority of these events begin as ventricular tachycardia, which degenerates into ventricular fibrillation; and such events are seldom associated with either acute infarction or significant ischemia.i53 l6 Bradycardia and asystole are infrequently the cause of sudden death in patients with coronary artery disease but account for 13 % to 20% of deaths among patients with end-stage congestive heart failure.lsls Extensive research has been focused on risk stratification after MI during the last 2 decades. This research has been complicated by difficulty in accurately determining the mechanisms of death. Many clinical variables, as well as noninvasive and invasive tests, have proved to be of value in risk stratification. However, the ability of individual tests to accurately predict arrhythmic events and/or sudden death is limited. The most important determinant for both shortand long-term survival is left ventricular function. A progressive increase in l-year mortality rate is seen as left ventricular ejection fraction falls below 40 % .2 However, left ventricular dysfunction