Cardiovascular Physiology Concepts
                                    Richard E. Klabunde, Ph.D.

Electrophysiological Changes During Cardiac Ischemia

Ischemia/hypoxia causes an elevation in extracellular K⁺.  This occurs because K⁺ leaks out through K_ATP channels (normally inhibited by ATP) and because of decreased activity of the Na⁺/K⁺-ATPase pump. These changes occur because ATP levels decline in hypoxic cells.

Increases in extracellular K⁺ cause membrane depolarization in accordance with the Nernst equation (although very small increases in extracellular K⁺ may cause hyperpolarization by increasing g_K). Depolarization will inactivate fast Na⁺⁺ channels and thereby decreased action potential upstroke velocity (fast Na⁺ channels inhibited). This will lead to decreased conduction velocity. These changes often lead to arrhythmias that may require the use of antiarrhythmic drugs.

Injury currents flowing from the depolarized ischemic regions to normal regions result in the appearance of ST segment elevation or depression, depending upon whether the ischemic region is non-transmural, subendocardial (ST depression) or transmural (ST elevation). The former is usually associated with demand ischemia (e.g., exertional angina), whereas the latter is associated with supply ischemia (e.g., coronary occlusion).

For non-transmural ischemia, the ST segment depression occurs because when the ventricle is at rest and repolarized states, the depolarized, ischemic region generates electrical currents that are recorded by an overlying electrode. If the depolarizing currents are traveling toward the positive electrode, the baseline voltage prior to the QRS complex (which is normally isoelectric - i.e., zero volts) will be elevated. In contrast, when the ventricle becomes depolarized, all the muscle is depolarized so that zero voltage is recorded by the electrode as usual. Therefore, the net effect of the elevated baseline voltage is that the ST segment appears to be depressed relative to the baseline.

For transmural ischemia, the ST segment elevation occurs because when the ventricle is at rest and repolarized, the depolarized, ischemic region generates electrical currents that are  traveling away from the positive electrode; therefore the baseline voltage prior to the QRS complex will be depressed. When the ventricle becomes depolarized, all the muscle is depolarized so that zero voltage is recorded by the electrode. Therefore, the net effect of the depressed baseline voltage is that the ST segment appears to be elevated relative to the baseline.

RK Revised 04/02/2007

DISCLAIMER: These materials are for educational purposes only, and are not a source of medical decision-making advice.

© 1999-2008 Richard E. Klabunde, all rights reserved.