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The faster repolarization of the cardiac action potential causes the tenting of the T waves, and the inactivation of sodium channels causes a sluggish conduction of the electrical wave around the heart, which leads to smaller P waves and widening of the QRS complex.
The abnormality lasts for a very short time, and then elevation of the ST segment occurs. This is the hallmark abnormality of an acute MI. It occurs when the heart muscle is being injured by a lack of blood flow and oxygen and is also called a current of injury. This is followed by T wave inversions. Over time, when the heart muscle cells actually die, these abnormalities are replaced with Q waves. However, not everyone with an MI has an abnormal EKG. In fact the initial EKG may not show ST elevation in up to 40% of patients having an MI.