ECG Quiz 19

ECG Quiz 19

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.

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