New insights in measurement of myocardial ischemia

Myocardial ischemia may be defined as myocellular dysfunction resulting from hypoxia usually due to limited coronary blood flow. The methods commonly used to make a diagnosis of myocardial ischemia employ either clinical findings (e.g., angina, myocardial infarction) or signals from iahoratory tests. Since ischemia is often clinically silent and since clinical events related to ischemia may he catastrophic (i.e., myocardial infarction and sudden death), physicians are dependent on tests using various targeted signals. These signals, however, do net actually provide quantitative measurements of the degree of ischemia or related myocardial dysfunction. Nevertheless, the functional abnormalities reflected by these signals can identify patients at high or low risk for adverse outcomes related to ischemia. So, in this sense, these signals can be used to support the diagnosis of ischemia as well as evaluate its importance in a given patient. The most commonly used signal is an ST-segment shift evident on the electrocardiogram (ECG). When this is horizontal or downsloping and [greater than or equal to]1.0 mm, this is often, but not always, due to myocardial ischemia. Although assessment of the exercise-stress ECG offers several advantages over assessment of the resting ECG, the standard Bruce protocol is associated with notable shortcomings that become apparent when an attempt is made to assess the effects of a treatment on the ST-segment signal. These might be surmounted by use of a continuous ramp-type protocol. Ambulatory ECG monitoring is growing in importance in the wake of increasing awareness of the different daily life circumstances that are associated with ischemia. In addition, some differences have become noticeable when ischemia that occurs during exercise testing and ischemia that occurs during daily activities are compared. Results of ambulatory ECG studies have underscored the importance of tailoring diagnostic procedures and therapies to the observed circadian variation in myocardial ischemia as well as to daily life experiences. Assessments of ischemia can also be based on a blood flow signal. The one most commonly used is a reversible perfusion defect detected with injection of certain radionuclides. Anether signal is a reversible left ventricular regional wall-motion abnormality detected with radionuclide angiocardiography, echocardiography, or contrast angiography. At present the information gained from these studies is more important from the standpoint of risk stratification than as a guide to numerically accurate measurements of myocardial ischemia. (Am J Cardiol 1992;70:19G-25G)