First Degree AV block

ECG Quiz 19

Normal range of PR interval is 120 to 200 ms. In this case the PR interval is 280 ms, indicating 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 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 ms or above is conventionally considered as a situation in which beta blockers are avoided for fear of worsening AV conduction. Digoxin also can cause worsening of AV conduction in this case.