The TQ segment depression and the ST segment elevation in the electrocardiogram during acute myocardial ischemia are caused by flow of injury current. This current flows between potential gradients across the ischemic border. The initial change is the TQ segment depression, which is brought about by a positive shift of the resting membrane potential of the ischemic cells. After 1 to 2 minutes ST segment elevation develops as a consequence of the action potential shortening and loss of plateau. The loss of potassium ions and ensuing extracellular K+ accumulation is the major cause of the alterations in action potential. After 15 to 20 minutes of ischemia, electrical cell-to-cell uncoupling occurs and interrupts the flow of injury current (decrease of TQ segment depression and ST segment elevation), producing conduction block.