[Pathophysiology of painful and silent myocardial ischemia]

Abstract

Regardless of the factor assumed responsible for precipitation of myocardial ischemia - varying from coronary occlusion in acute myocardial infarction to increased oxygen demand in exertional angina pectoris and reduced myocardial oxygen supply due to plaque rupture or changes in vasomotor tone in unstable angina - its incurrence may or may not be associated with pain. In the vast majority of cases, silent myocardial ischemia is observed in patients with established symptomatic coronary artery disease. Interindividual comparisons have not enabled reliable differentiation between those with painful and those with silent ischemia based on the anatomic extent of coronary artery disease, left ventricular function or previous myocardial infarction. Similarly, functional parameters such as exercise capacity, exercise duration, time to onset of ST-segment depression during exercise as well as heart rate and blood pressure both at rest and during exercise have failed to reveal differences between the symptomatic and the asymptomatic patients. Intraindividual differences have also been noted, but not consistently corroborated, and postulated as responsible for the fact that ischemia in a given patient alternates in its presence with and without pain. Since most patients with silent ischemia either have, or at some time in the past have experienced, painful ischemia, the integrity of the appropriate nervous system function can be assumed to be intact and neurocardiologic factors seem most likely to account for apparent discrepancies in pain perception. Prior to precipitation of pain, myocardial ischemia must elicit an adequate stimulus. According to some investigators, the adequate stimulus is that associated with a duration of the ischemic episode of at least three minutes and with increase in left ventricular filling pressure of more than 7 mm Hg. This threshold, consequently, represents a prerequisite but not invariably sufficient criteria for the occurrence of pain. The next step in the sequence of pain is generation of an action potential, that is, transduction by means of chemical or mechanical stimuli. During this process, a latency of 20 to 40 seconds is incurred such that the appearance of pain usually has its onset after derangement of relaxation and contraction, increased filling pressure and the observation of ECG changes. Through conduction, the information is forwarded to the central nervous system after coding of the details with regard to intensity. The intensity, in turn, is determined by the number of receptors (free nerve endings) in the field activated by the ischemic event.(ABSTRACT TRUNCATED AT 400 WORDS)