Electrophysiologic Changes During Ischemia

During periods of ischemia, blood flow diminishes first and most dramatically in the subendocardium. Epicardial blood flow is preserved until the artery supplying the affected muscle becomes almost completely obstructed. As a result, ischemia usually involves only a partial thickness of the ventricular wall.

Significant metabolic changes occur in the ischemic inner wall (subendo-cardium),whereas the metabolic state of the outer wall (epicardium) remains nearly normal. This creates a difference in electrical potential between ischemic and normal tissue with the net result being that there is a current flow from normal cells in the epicardium toward the ischemic cells in the endocardium (Figure 10.1). This current flow takes place during mechanical systole, which, as you know from Chapter 1, occupies the time interval of the ST segment. Because the current is flowing away from ECG electrodes on the body surface overlying the affected ventricular wall, it is registered on the ECG as a negative needle deflection, resulting in ST depression. As you would surmise, these same electrophysiologic events can also alter T waves.

Figure 10.1. Schematic view of the left ventricle showing the flow of current during systole from healthy cells in the epicardium toward ischemic cells in the subendocardium. Note that the current flow is away from the V5 electrode, producing a negative deflection of the ECG needle, which results in ST depression,

Figure 10.1. Schematic view of the left ventricle showing the flow of current during systole from healthy cells in the epicardium toward ischemic cells in the subendocardium. Note that the current flow is away from the V5 electrode, producing a negative deflection of the ECG needle, which results in ST depression,

When ischemia involves the full thickness of the ventricular wall, that is, both endocardium and epicardium, we say that it is transmural. As noted above, this requires almost total cessation of blood flow through a coronary artery. Transmural ischemia produces ST elevation, with which you are already familiar from your study of AMI (Chapter 9).

Was this article helpful?

0 0

Post a comment