Basic Cardiac Anatomy and Physiology
Core concepts
Know the conduction pathway in order: SA node → internodal/atrial pathways → AV node → bundle of His → right and left bundle branches → Purkinje network. The SA node sits at the junction of the superior vena cava and right atrium and normally fires at 60–100 bpm; the AV node (40–60 bpm) and Purkinje system (20–40 bpm) are subsidiary pacemakers. Understand the coronary supply to the conduction system: the AV node is supplied by the right coronary artery in most people, the SA node by the right or circumflex. This explains why inferior MI commonly produces AV block and sinus bradycardia.
Correlate the ECG with mechanical events: the P wave is atrial depolarization, the QRS is ventricular depolarization (atrial repolarization is hidden within it), and the T wave is ventricular repolarization. The PR interval reflects AV conduction time; the QT interval spans depolarization plus repolarization and varies inversely with heart rate. Cardiac output = heart rate × stroke volume, and stroke volume depends on preload, afterload, and contractility - the framework for explaining why bradycardia or loss of atrial kick reduces output.
Question traps
- Confusing the pacemaker hierarchy rates (SA 60–100, AV 40–60, ventricular 20–40).
- Assuming the left coronary artery supplies the AV node.
- Forgetting that atrial contraction (atrial kick) contributes meaningfully to ventricular filling, especially in stiff ventricles.
Rule of thumb
If a stem describes an inferior MI with bradycardia and AV block, think "right coronary artery ischemia affecting the SA/AV node" and anticipate a transient, often vagally mediated, conduction problem.