Normal range of PR interval is 120 to 200 msec. In this case the PR interval is 280 msec, indicating first degree atrioventricular block (first degree AV block). The P wave is almost encroaching on the T wave of the previous cycle. If the heart rate goes up a bit, the P wave will fall on the T wave of the previous cycle. This will result in atrial contraction occurring during ventricular systole when the AV valves are closed. The forceful atrial contraction against a closed atrioventricular valve (tricuspid valve) will produce a cannon wave in the jugular venous pulse. First degree AV block with sinus tachycardia is thus a cause for regular cannon waves. PR interval of 280 msec or above is conventionally considered as a situation in which beta blockers are avoided for fear of worsening AV conduction. Digoxin can also cause worsening of AV conduction in this case.