Fig. 3. PQ maturation defect and QRS prolongation in Tbx5^del/+ mice. (A) Schematic representation of electrical impulse propagation through the mammalian heart correlated with surface ECG and in-vivo electrophysiology intervals. PQ intervals include impulse propagation throughout the atria and atrioventricular node (proximal AH interval) and the atrioventricular bundle and proximal bundle branches (distal HV interval). P-wave duration represents atrial depolarization. QRS intervals represent ventricular activation, and include bundle branch and Purkinje conduction. Bundle-branch block causes QRS prolongation with characteristic ECG wave front morphology. SAN, sinoatrial node; AVN, atrioventricular node; AVB, atrioventricular bundle; RBB, right bundle branch; LBB, left bundle branch. (B) Representative ECGs from wild-type and Tbx5^del/+ newborns and adult mice. Note comparable PQ intervals (atrial plus atrioventricular canal conduction time) in neonatal wild-type and Tbx5^del/+ mice. Adult wild-type mice had significantly shorter PQ intervals than those of Tbx5^del/+ mice. QRS intervals of newborn and adult Tbx5^del/+ mice were longer than in wild-type mice (Table 1). (C) Representative ECG recordings from right precordial leads (V1) in wild-type and Tbx5^del/+ adult mouse. Wild-type mice had normal QRS complexes. Tbx5^del/+ mice had QRS prolongation with a RSR' wave front pattern indicative of RBB. RBB occurred in 9 of 11 of adult Tbx5^del/+ mice versus 3 of 27 adult wild-type mice (P<0.001).