Anatomy Thorax Abdomen · Year 1 · from Anatomy Thorax Abdomen
Case 3: Complete Heart Block
Clinical Presentation
A 78-year-old female is brought to the emergency department after a syncopal episode at home. She reports several weeks of progressive fatigue and lightheadedness. She has no chest pain. On examination, blood pressure is 90/60 mmHg and heart rate is 35 beats per minute with regular rhythm. Jugular venous pressure is elevated with intermittent cannon A waves (occurring when the atrium contracts against a closed tricuspid valve).
ECG demonstrates complete atrioventricular (third-degree) block with a regular atrial rate of 75 beats per minute and an independent ventricular escape rhythm at 32 beats per minute with widened QRS complexes. There is no relationship between P waves and QRS complexes. Troponin is mildly elevated. She is diagnosed with complete heart block likely due to age-related fibrosis of the conduction system. A temporary pacing wire is placed, followed by permanent dual-chamber pacemaker implantation with resolution of symptoms.
Key Anatomical Points
- The AV node lies in the interatrial septum in the triangle of Koch, bounded by the coronary sinus orifice, the tendon of Todaro, and the tricuspid annulus
- The bundle of His is the only electrical connection between atria and ventricles, penetrating the cardiac skeleton at the right fibrous trigone
- The AV node receives blood supply from the AV nodal artery (from RCA in 80%, from LCx in 20%)
- Complete heart block occurs when conduction through the AV node or bundle of His is completely interrupted
Key Learning Points
- The cardiac skeleton electrically insulates the atria from the ventricles, ensuring orderly activation sequence
- The SA node (60-100 bpm) is the primary pacemaker; the AV node (40-60 bpm) and Purkinje fibers (20-40 bpm) serve as backup pacemakers
- In complete heart block, the ventricles are driven by an escape rhythm below the block - the lower the origin, the slower and wider the QRS
- The right bundle branch travels in the moderator band (septomarginal trabecula) to the anterior papillary muscle