Figure 3.

Electrocardiographic phenotypes. (a) Lead II rhythm strip from proband (age 6 years) of family SS1 showing absent P waves and prolonged QRS duration. (b and c) Lead II rhythm strip and intracardiac electrophysiologic tracings from proband family SS2 (III-2) at age 9 years. No P waves were evident on surface tracings, and no atrial activity was documented with intracardiac recording. Atrial pacing could not be achieved even with high output (stimuli of 7 mA, 3 milliseconds). Both QRS duration (100 milliseconds, normal for age is less than 85 milliseconds) and His-ventricular interval (80 milliseconds, normal is less than 50 milliseconds) are prolonged. HBE, His bundle electrogram; H, potential recorded from the His bundle; RA, right atrium; RV, right ventricle. (d and e) Lead II tracing from members of SS1, heterozygous for G1408R, showing (d) first-degree heart block with prolonged QRS (III-7) (e) first-degree heart block (IV-8). (f and g) In a compound heterozygote from family SS1 (IV-9), lead II tracings showing transition from first-degree heart block (f) to sinus arrest (g) at age 17 months and 25 months, respectively. (h) Lead II tracing showing first-degree heart block and prolonged QRS duration in individual II-5 family SS2, heterozygous for R1623X. A 1-second time scale is shown in c; electrocardiographic traces were obtained at standard recording conditions of 25 mm/s and 10 mm/mV.