First-Degree Block PROLONGATION of the PR interval beyond 0.20 second may occur in many disorders including acute myocardial infarction, aortic stenosis, severe aortic insufficiency, adrenal insufficiency, rheumatic fever and a number of systemic infectious diseases. An occasional normal person will have first-degree heart block in the absence of any apparent disease. Although an optimal role for atrial contraction may be provided by a normal PR interval, the delayed onset of ventricular contraction in first-degree block rarely produces significant changes in heart function. A prolonged PR interval will usually return to normal with atropine or sympathomimetic agents, but treatment of this . . .