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. Author manuscript; available in PMC: 2012 Mar 7.
Published in final edited form as: J Am Coll Cardiol. 2008 Jan 1;51(1):77–84. doi: 10.1016/j.jacc.2007.06.060

Figure 1. Rhythms in 2° Fetal AVB.

Figure 1

Rhythm strips from second-degree (2°) atrioventricular block (AVB) subjects showing: (A) intermittent pre-excitation with QRS normalization in fetus #24 at 32 weeks’ gestation; (B) varying PR interval due to transient atrioventricular dissociation in fetus #24 at 37 weeks’ gestation; (C) period of low atrial rhythm (PR = 55 ms) in fetus #27 with 2 atrial rhythms at 24 weeks’ gestation; in the high rhythm (not shown), PR interval was approximately 107 ms; and (D) transition of sinus rhythm (1:1 conduction) from stable RP/PR to alternating RP/PR in fetus #25 at 33 weeks’ gestation. After several beats with relatively constant RP/PR, the fourth atrial beat conducts slowly, shortening RP of the succeeding beat. Rather than blocking, the fifth atrial beat not only conducts, it does so more rapidly than the prior beat, prolonging RP of the succeeding beat. The alternating (short–long) RP intervals of the ensuing beats continue to show PR prolongation during the “long” phase of the cycle and conduction at short RP during the “short” phase until block occurs, notably, during the “long” phase. The tracings were processed to remove maternal interference but were not signal averaged.