Significance of Downsloping ST- Segment Depression Induced by Low-level Exercise in Severe Coronary Artery Disease. Assessment with Myocardial Ischemia and Collateral Perfusion.

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Abstract
Exercise-induced downsloping ST-segment depression is a common manifestation of severe myocardial ischemia. Although greater downsloping ST-segment depression is suspected to indicate more severe ischemia, its exact relationship to regional myocardial blood flow (RMBF) has not yet been clarified. We investigated the relationship between the magnitude of downsloping ST-segment depression and exercise-induced changes in RMBF and collateral perfusion. Nitrogen-13 ammonia positron emission tomography was performed in 6 healthy volunteers and 72 patients with angiographically proven coronary artery disease. The left ventricle was divided into 11 regions of interest, and RMBF in each region was measured at rest and during low-level supine bicycle exercise. Downsloping ST-segment depression of 0.1 mV or more at 80 milliseconds after the J point was accepted as significant. Low-level exercise induced downsloping depression of 0.1 to 0.2 mV in 10 patients (group D1) and downsloping depression of 0.2 mV or more in 8 patients (group D2). Multivessel disease was common in both group D1 (80% of patients) and group D2 (88% of patients). Collateral circulation was significantly more frequent in group D1 (90%) than in group D2 (13%, p < 0.01). Ischemic areas were larger and cardiac function was worse in group D2 than in group D1. The RMBF increased sufficiently in all regions (56 +/- 30%) with exercise in the healthy group. In group D1, RMBF was unchanged or decreased in ischemic areas (10 +/- 23%) but increased sufficiently in surrounding areas (50 +/- 32%). In group D2, RMBF was unchanged in ischemic areas (17 +/- 24%) and increased insufficiently in surrounding areas (41 +/- 21%). Therefore, exercise-induced downsloping ST-segment depression of 0.1 to 0.2 mV may reflect an underlying change in blood flow in viable myocardium with collateral perfusion, and downsloping depression of 0.2 mV or more may reflect more severely impaired myocardium without collateral perfusion.

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