Heart block results from conduction defects. First degree block is characterized by an abnormally long P-R interval, longer than 200 msec (Figure 16). This results from slowed conduction, usually in the AV junctional and AV nodal tissue.
Second degree block can be of two types, the Mobitz I (Wenckebach) and Mobitz II. In Mobitz I, the P-R interval alternates between being of normal length and so long that conduction to the ventricles fails completely, producing the loss of a ventricular contraction, i.e a dropped beat (Figure 17).
In Mobitz II, the P-R intervals are normal, then all of a sudden conduction fails and a beat is dropped.
In third degree block, P waves and QRS complexes are seen, but they bear no relationship to one another (Figure 21). The SA node may be leading atrial contraction, while another pacemaker in the AV node, His bundle, or ventricles may be leading ventricular contraction. Complete dissociation between the SA nodal pacemaker and ventricular depolarization has occurred. Depending upon the site of the ventricular pacemaker, the QRS complex may appear normal, or it may be abnormally wide, double-peaked, or bizzare in shape (Figure 22).
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