abstract

PURPOSE Primary ST depression in the ECG is thought to be caused by subendocardial ischemia. The classical explanation is that a partially blocked coronary artery causes only the inner part of the ventricular wall to become ischemic. This idea was supported by mathematical studies which, however, ignored the anisotropic electrical conductivity of the myocardium. Recent numerical studies that did account for this anisotropy have shown that regional subendocardial ischemia may well lead to ST elevation in overlying leads, instead of ST depression. The meaning of primary ST depression has therefore become uncertain. The purpose of this study was to find scenarios that could lead to primary ST depression.

METHODS Ischemia was simulated in a detailed bidomain reaction-diffusion model of the human heart and torso. Myocardial anisotropy and the major inhomogeneities in torso conductivity were accounted for. Subendocardial ischemia with 30%, 50%, 70%, and 90% transmural extent was simulated, as well as transmural ischemia. For each transmural extent, 4 different ischemic zones with 75mm diameter were simulated, as well as one ischemic zone affecting the whole left ventricle. The severity of ischemia was also varied. Significant ST depression was defined as >1mm (0.01mV).

RESULTS Regional subendocardial ischemia of any transmural extent only caused significant ST depression when it was severe enough to cause activation block in the ischemic zone. In contrast, a zone that affected the whole left ventricle caused several mm ST depression even for moderate ischemia without changes in activation.

CONCLUSIONS Two explanations for primary ST depression remain: severe regional subendocardial ischemia, and global subendocardial ischemia (i.e. affecting the whole left ventricle). The latter hypothesis is attractive because it explains the clinical observation that the pattern of primary ST depression cannot predict the location of a coronary stenosis.

funding

Computational resources were provided by the Réseau québécois de calcul de haute performance (RQCHP). This work was supported by a postdoctoral research award from the Groupe de recherche en sciences et technologie biomédicale (GRSTB), École Polytechnique and Université de Montréal; and by the Research Center of Sacré-Coeur Hospital, Montréal, Québec, Canada.