Cardiac Conduction Abnormalities

AV Block

• 1st degree: every P is followed by QRS complex after prolonged PR interval. Seen in healthy children, Digitalis toxicity, acute rheumatic fever, cardiomyopathies, CHD. Not hemodynamically relevant, but may progress to higher degree AV block.
• 2nd degree:
o Type I (Wenkebach): PR interval becomes progressively prolonged until QRS is dropped; causes same as in 1st degree AV block.
o Type II (Mobitz): beats with normal PR intervals alternating with blocked beats (2-4:1). Seen with cardiac pathology or Digitalis toxicity. Likely to progress to 3rd degree AV block. Tx: pacemaker.
• 3rd degree: every P is completely blocked; independent junctional (narrow QRS)
or ventricular (wide QRS) escape rhythm with slow HR (40-50, in congenital complete AV block 50-80 bpm); congenital in CHD (L-TGA) or maternal connective tissue disease (e.g. SLE). No tx if asymptomatic. Atropine or isoproterenol if acutely symptomatic. Pacemaker if HR <55 bpm, CHF, escape ventricular beats, or long QTc.

RBBB

• RV depolarization is delayed
• Complete:
o Common post-op after right ventriculotomy, ASD, Ebstein’s anomaly
o EKG: QRS duration longer than age limit (100 ms in adults) and rsR’ in RPL
o CAVE: Ventricular hypertrophy, ST segments, T waves cannot be assessed
• Incomplete:
o Normal in children and young adults (rsr’)
o Normal QRS duration
o rsR’ may suggest RVH

LBBB

• LV depolarization is delayed
• Always pathologic
• EKG: Prolonged QRS complex; QS pattern without or with small R in V1; no Q and wide R in I, aVL, and V6

WPW

• Abnormal AV conduction bundle that bypasses AV node. Delta wave seen when conduction antegrade. May lead to retrograde conduction and induce SVT.
• EKG: Short PR interval, delta wave, long QRS complex, often no Q in V5/6