Cardiology In Critical Care

Arrhythmia Recognition

First Degree AV Block

Rhythmic impulses originating in the SA node are conducted abnormally slow through the AV junction to the ventricles.  The rhythm pattern is similar to normal sinus rhythm except that the PR interval is prolonged beyond 0.20 sec in adults and 0.18 sec in children.  With a severe (i.e. greater than 0.5 sec) lengthening of the PR interval or with a rapid rate, the P wave may be superimposed on the preceding T wave.  Occasionally the rate may be slower than 60/minute.  Frequently associated with vagotonia (in which case it disappears after exercise or atropine).

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