Cardiac Conduction System: Anatomy and ECG Correlation

The cardiac conduction system is a network of specialized cardiomyocytes that initiate and propagate the electrical impulses driving each heartbeat. The signal originates in the sinoatrial (SA) node in the right atrium near the superior vena cava, propagates across the atria producing the P wave on the ECG, reaches the atrioventricular (AV) node at the inferior interatrial septum where it pauses (giving the PR interval), then descends rapidly through the bundle of His and bundle branches before fanning out across the ventricular myocardium via the Purkinje fibers — producing the QRS complex. Repolarization of the ventricles produces the T wave. The autonomic nervous system modulates rate (sympathetic faster, parasympathetic slower) but does not change the basic anatomic pathway. Disease at any node, bundle, or fascicle produces predictable ECG abnormalities — bradycardia, heart blocks, bundle branch blocks, or fascicular blocks — each with characteristic ECG patterns.

The SA (sinoatrial) node is a cluster of pacemaker cells in the right atrial wall near the junction with the superior vena cava. Its cells exhibit spontaneous depolarization (automaticity) driven by the funny current (If, a mixed Na/K inward current that depolarizes the cell during phase 4 of the action potential). The SA node's intrinsic rate is 60-100 beats per minute in the absence of autonomic input. Blood supply: SA nodal artery branches from the right coronary artery (RCA) in 60% of individuals or from the left circumflex (LCx) in 40%. Inferior wall MIs (RCA territory) often produce SA node ischemia and sinus bradycardia. Autonomic modulation: Sympathetic stimulation (β1 receptors) increases the slope of phase-4 depolarization, raising firing rate. Parasympathetic stimulation (vagus nerve, M2 muscarinic receptors) decreases the slope, lowering firing rate. The vagus tonically slows the SA node — atropine (muscarinic blockade) raises heart rate by removing this brake. Clinical correlations: Sick sinus syndrome (intrinsic SA node dysfunction in elderly) produces inappropriate bradycardia or tachy-brady alternation. SA node ischemia produces sinus bradycardia in inferior MI. Sinus arrhythmia (variation with respiration) is a normal phenomenon in young healthy individuals from waxing and waning vagal tone.

The AV (atrioventricular) node sits in the inferior interatrial septum near the coronary sinus. It performs two critical functions: it delays the impulse (giving the atria time to contract before the ventricles) and it acts as the only normal electrical connection between atria and ventricles (the rest of the AV junction is electrically insulating fibrous tissue). Intrinsic rate (if pacing alone, e.g., complete AV block from above): 40-60 bpm. PR interval on ECG (0.12-0.20 s) primarily reflects AV nodal delay. Blood supply: AV nodal artery from the posterior descending artery (PDA) — which arises from RCA in 80% of people (right-dominant) and LCx in 20% (left-dominant). Inferior MIs (RCA territory) commonly produce AV nodal ischemia and varying degrees of AV block. The bundle of His arises from the AV node and descends through the membranous interventricular septum, then divides into the right and left bundle branches. The left bundle branch further divides into anterior and posterior fascicles. The bundle of His is the only normal connection between atrial and ventricular electrical activity outside the AV node. Clinical correlations: 1st-degree AV block (PR > 0.20 s) — usually AV nodal slowing, often benign. 2nd-degree Mobitz I (Wenckebach) — progressive PR lengthening then dropped beat — usually AV nodal level. 2nd-degree Mobitz II — fixed PR with intermittent dropped beats — typically below the AV node (His-Purkinje system); higher risk of progression to complete heart block. 3rd-degree (complete) AV block — atria and ventricles dissociated; pacemaker is escape rhythm from below the block.

Below the bundle of His, the conduction system splits into right bundle branch (single fascicle) and left bundle branch (which divides into anterior and posterior fascicles). The bundle branches descend along the interventricular septum and then arborize into Purkinje fibers, which spread across the subendocardial surface of both ventricles. The Purkinje system conducts at 4 m/s — the fastest conduction velocity in the heart, ensuring near-simultaneous depolarization of both ventricles. The QRS complex on ECG (normally <0.12 s wide) reflects this rapid synchronized ventricular depolarization. When a bundle branch is blocked, the affected ventricle depolarizes via slow muscle-to-muscle conduction (~0.4 m/s), producing a wide QRS (≥0.12 s) with a characteristic pattern. Patterns: - Right bundle branch block (RBBB): wide QRS with rsR' pattern in V1 ("rabbit ears") and broad slurred S in lateral leads (I, V6). - Left bundle branch block (LBBB): wide QRS with broad notched R in lateral leads and broad QS in V1. - Left anterior fascicular block (LAFB): left axis deviation (more negative than -45°); qR pattern in I and aVL, rS in II/III/aVF. - Left posterior fascicular block (LPFB): right axis deviation (more positive than +90°); qR in II/III/aVF, rS in I/aVL. Blood supply: The right bundle branch and left anterior fascicle are supplied primarily by the LAD; the left posterior fascicle has dual supply (LAD + PDA), making it the most resistant to ischemic injury. The left main bundle branch is supplied by both LAD and LCx. Clinical correlations: New LBBB in an MI patient is treated as a STEMI equivalent (urgent reperfusion). Bifascicular block (RBBB + LAFB or RBBB + LPFB) carries elevated risk of progression to complete heart block, particularly perioperatively. Trifascicular block (bifascicular + first-degree AV block on ECG) often warrants prophylactic pacing in some clinical contexts.

The heart receives both sympathetic and parasympathetic input that modulates rate, conduction, and contractility — but does not change the underlying conduction pathway. Sympathetic innervation: arises from T1-T4 sympathetic chain ganglia (cervical and upper thoracic). Releases norepinephrine acting on β1 receptors. Effects: increases SA node firing rate (positive chronotropy), accelerates AV node conduction (positive dromotropy), and increases ventricular contractility (positive inotropy). Functions through Gs → adenylyl cyclase → cAMP → PKA pathway. Parasympathetic innervation: arises from medullary nuclei (nucleus ambiguus and dorsal motor nucleus of vagus) and travels via cranial nerve X (vagus). Releases acetylcholine acting on M2 muscarinic receptors. Vagal innervation is densest at the SA and AV nodes; ventricular muscle is largely free of parasympathetic input. Effects: decreases SA node rate (negative chronotropy), slows AV node conduction (negative dromotropy). Functions through Gi → ↓adenylyl cyclase → ↓cAMP plus direct activation of K+ channels (hyperpolarization). Clinical correlations: Vasovagal syncope = exaggerated parasympathetic outflow causing bradycardia and vasodilation. Carotid sinus hypersensitivity = excessive vagal response to carotid pressure → asystole or AV block. Beta-blockers slow heart rate by blocking sympathetic input. Atropine accelerates heart rate by blocking parasympathetic input. Adenosine briefly stops AV nodal conduction (used to terminate SVT) by hyperpolarizing AV node cells via A1 receptors.

Specific conduction structures are at risk during specific MIs because of their blood supply. Inferior MI (RCA territory): - SA node ischemia → sinus bradycardia (60% of inferior MIs since RCA supplies SA node 60% of time) - AV node ischemia → various degrees of AV block (since PDA from RCA supplies AV node in right-dominant systems) - Both typically resolve as MI heals or with reperfusion; transient pacing may be needed Anterior MI (LAD territory): - Right bundle branch ischemia → RBBB (LAD supplies RBB) - Left anterior fascicle ischemia → LAFB - Combined RBBB + LAFB (bifascicular) is common in large anterior MIs - New LBBB in MI context = STEMI equivalent → urgent reperfusion - Higher risk of complete heart block; prophylactic pacing may be warranted The left posterior fascicle, with its dual blood supply (LAD + PDA), is the most ischemia-resistant element of the conduction system. Isolated LPFB without underlying disease is rare; when present, it suggests significant coronary disease. Clinical pattern: The combination of RBBB + LAFB after anterior MI commonly precedes complete heart block. The combination of inferior MI + Mobitz II AV block is unusual (Mobitz II is typically below the AV node, supplied by LAD); presence suggests anterior involvement.

Cardiac conduction anatomy and ECG correlation is heavily tested on USMLE Step 1, NBME shelf exams, and most medical school cardiology blocks. The challenge is integrating anatomic structure, electrophysiological function, and ECG appearance. Snap a photo of any conduction-system diagram or ECG strip and AnatomyIQ identifies the structure, traces its functional role, and links it to the ECG component it generates. For arrhythmia ECGs, AnatomyIQ produces the corresponding anatomic localization of the lesion. This content is for educational purposes only and does not constitute medical advice.