Cardiac Conduction System: SA Node, AV Node, Bundle of His, and Purkinje Fibers Explained
Direct Answer: The Electrical Pathway in 60 Seconds
The cardiac conduction system generates and distributes the electrical impulses that trigger each heartbeat. The pathway starts at the sinoatrial (SA) node in the upper right atrium, spreads across both atria causing atrial contraction, converges at the atrioventricular (AV) node where conduction slows to let the atria finish filling the ventricles, then races down the Bundle of His, splits into right and left bundle branches, and terminates in the Purkinje fibers that depolarize the ventricular myocardium from apex to base. Each component has a unique intrinsic firing rate that serves as a backup if the upstream pacemaker fails. SA node fires at 60-100 beats per minute (the dominant pacemaker). AV node fires at 40-60 bpm. Bundle of His and ventricular conduction system fire at 20-40 bpm. This hierarchy is why a patient with complete heart block can still have a slow ventricular rhythm — the ventricular myocardium is generating its own impulses. Clinical relevance is enormous. Every ECG finding, every arrhythmia, and every heart block type maps directly to a specific component of this system. SA node dysfunction causes sick sinus syndrome. AV node disease causes first-, second-, and third-degree heart blocks. Bundle branch blocks cause widened QRS complexes with characteristic morphologies. Purkinje fiber abnormalities contribute to ventricular tachycardia and fibrillation.
SA Node: The Dominant Pacemaker
The sinoatrial node is a banana-shaped cluster of specialized cardiomyocytes located in the upper lateral right atrium near the junction of the superior vena cava and the right atrial appendage. In most people it sits subepicardially, about 1 mm deep to the epicardial surface, making it vulnerable to inflammation (pericarditis) and surgical injury. Blood supply comes from the SA nodal artery, which arises from the right coronary artery in approximately 60% of people and the left circumflex artery in 40%. This variability matters clinically — a right coronary artery occlusion (inferior MI) can knock out the SA node in those 60% of patients, causing sinus bradycardia or arrest as an acute finding. Innervation is autonomic. Parasympathetic fibers from the right vagus nerve slow the rate via acetylcholine acting on M2 muscarinic receptors (which open K+ channels, hyperpolarizing the cell). Sympathetic fibers from the cervical ganglia speed the rate via norepinephrine acting on beta-1 adrenergic receptors. At rest, parasympathetic tone dominates — which is why vagal maneuvers (carotid massage, Valsalva) can temporarily slow the rate enough to break certain reentry arrhythmias. The SA node's automaticity comes from the funny current (If) — a sodium influx through HCN channels that slowly depolarizes the cell until it reaches threshold. Ivabradine, a medication used for heart failure and stable angina, selectively blocks this current to slow the rate without affecting contractility.
AV Node and the Conduction Delay
The atrioventricular node sits in the triangle of Koch, bounded by the tricuspid annulus, the coronary sinus ostium, and the tendon of Todaro. It is the only normal electrical connection between the atria and ventricles — the annulus fibrosus is an electrical insulator everywhere else. Conduction through the AV node is deliberately slow — about 0.12 seconds — and this delay has two critical functions. First, it gives the atria time to finish pumping blood into the ventricles (atrial kick contributes roughly 20-30% of ventricular filling at rest, more in patients with stiff ventricles). Second, it acts as a filter that protects the ventricles from supraventricular tachyarrhythmias. A patient in atrial fibrillation with atrial rates of 400+ bpm is protected from ventricular rates that high because the AV node cannot conduct every impulse. The AV delay shows up on the ECG as the PR interval (normal 120-200 ms). First-degree AV block is defined as PR > 200 ms — the delay is abnormally prolonged but every P wave still gets through. Second-degree block comes in two flavors: Mobitz I (Wenckebach) shows progressive PR lengthening until a P wave is dropped (AV node disease — usually benign); Mobitz II shows constant PR intervals with sudden dropped beats (disease below the AV node — often requires pacing). Third-degree (complete) block shows total AV dissociation. Blood supply is from the AV nodal artery, which in 80-90% of people arises from the right coronary artery at the crux of the heart. An inferior MI can cause AV block as a direct consequence of AV nodal ischemia.
Bundle of His, Bundle Branches, and Purkinje Fibers
Once past the AV node, conduction accelerates dramatically. The Bundle of His (atrioventricular bundle) runs through the membranous interventricular septum and within about 1-2 cm splits into the right bundle branch and left bundle branch. The right bundle branch is a single thin cord that runs down the right side of the interventricular septum toward the right ventricular apex. The left bundle branch is broader and divides into two main fascicles — the left anterior fascicle (supplying the anterior and superior left ventricle) and the left posterior fascicle (supplying the inferior and posterior left ventricle). Some texts also describe a septal fascicle. This fascicular anatomy explains why left bundle branch blocks, left anterior fascicular blocks, and left posterior fascicular blocks produce distinct ECG patterns. At the terminal ends, the bundle branches split into Purkinje fibers — a dense network of fast-conducting cells that penetrate the ventricular myocardium from the subendocardial surface outward. Conduction velocity in the Purkinje system is roughly 4 m/s, compared to 0.05 m/s in the AV node — an 80-fold difference. This allows the ventricles to contract nearly simultaneously from apex to base, which is mechanically efficient. Conduction abnormalities map to ECG findings with high specificity. Right bundle branch block produces an RSR' pattern in V1 with a wide QRS. Left bundle branch block produces a broad monophasic R wave in V6 with a wide QRS and obscures the normal Q waves used to diagnose MI. Left anterior fascicular block produces left axis deviation (-45 to -90 degrees) without significant QRS widening.
Heart Block Types and Clinical Correlations
Heart blocks are graded by severity and localized by ECG morphology. Understanding the anatomy makes the classification intuitive. First-degree AV block: PR interval > 200 ms, every P wave conducted. Usually benign, often found on routine ECGs. Causes include increased vagal tone (athletes), AV nodal medications (beta-blockers, calcium channel blockers, digoxin), and mild AV node disease. Second-degree Mobitz I (Wenckebach): progressive PR lengthening until a P wave is dropped. The dropped beat has the longest PR before it and the shortest PR after. The block is in the AV node — usually benign, commonly seen during sleep or with high vagal tone. Second-degree Mobitz II: constant PR intervals with sudden dropped beats. The block is below the AV node (usually in the His-Purkinje system). More ominous — can progress to complete heart block without warning. Usually requires pacemaker. Third-degree (complete) heart block: total AV dissociation. Atria and ventricles beat independently. Escape rhythm origin determines rate and stability — junctional escape (40-60 bpm, narrow QRS) is more stable than ventricular escape (20-40 bpm, wide QRS). Permanent pacemaker is the definitive treatment. Bundle branch blocks are a separate category — the conduction block is below the bifurcation, so AV conduction is preserved but ventricular depolarization is abnormal. RBBB alone has a relatively benign prognosis; new LBBB in the setting of chest pain is treated as a STEMI equivalent.
Clinical Scenarios for Exam Preparation
Scenario 1: Inferior MI with bradycardia. A 62-year-old presents with chest pain and ST elevation in leads II, III, and aVF. Heart rate drops to 38 bpm with new first-degree AV block. Why? Right coronary artery occlusion ischemizes the SA node (60% of the time) and the AV node (80-90% of the time). The AV block is usually at the AV node level, responds to atropine, and often resolves with reperfusion. Scenario 2: Ventricular tachycardia after anterior MI. A 55-year-old had a large anterior STEMI one week ago and now has runs of wide-complex tachycardia. Why? The left anterior descending artery supplies the anterior interventricular septum, which contains the right bundle branch and left anterior fascicle. Scar tissue from the infarct becomes a reentrant circuit for ventricular arrhythmias. Scenario 3: Syncope with bifascicular block. A 70-year-old with LBBB plus right bundle branch pattern on an old ECG now presents with syncope. Bifascicular block plus syncope is high-risk for progression to complete heart block — an electrophysiology study or empiric pacemaker is often warranted. Scenario 4: Wolff-Parkinson-White. A 25-year-old has a short PR interval (< 120 ms) and a delta wave (slurred QRS upstroke). Why? An accessory pathway (Bundle of Kent) bypasses the AV node, allowing atrial impulses to preexcite the ventricle. Can cause supraventricular tachycardia with risk of rapid ventricular conduction in atrial fibrillation — avoid AV nodal blockers, which can paradoxically increase conduction through the accessory pathway.
Study Strategy and High-Yield Points
Memorize the pathway as a linear sequence: SA node → atrial conduction → AV node (delay) → Bundle of His → right and left bundle branches → fascicles (on the left) → Purkinje fibers → ventricular myocardium. Every ECG finding and every arrhythmia mechanism builds on this framework. Link each component to its blood supply: SA node and AV node are both primarily supplied by the right coronary artery, which is why inferior MI causes sinus node dysfunction and AV block. The Bundle of His and bundle branches are supplied by the left anterior descending artery, which is why anterior MI causes bundle branch blocks and ventricular arrhythmias. Pair anatomy with pharmacology. AV node-blocking drugs (beta-blockers, non-dihydropyridine calcium channel blockers, digoxin, adenosine) exploit the AV node as a therapeutic target for rate control and termination of supraventricular reentry tachycardias. Class I antiarrhythmics slow sodium-dependent conduction (affects atrial, ventricular, and His-Purkinje tissue). Class III antiarrhythmics prolong action potential duration and refractory period (affects the entire myocardium). For practical exam preparation, work through normal ECGs first to cement the P-QRS-T correspondence to the conduction sequence, then layer in increasingly complex arrhythmia strips. Most USMLE and nursing exam questions about the conduction system come back to pattern recognition built on the underlying anatomy.
Frequently Asked Questions
Common questions about cardiac conduction system
The AV node deliberately slows conduction (about 0.12 seconds — the normal PR interval) for two reasons. First, the delay allows the atria to finish contracting and pumping blood into the ventricles before ventricular systole begins. The atrial kick contributes 20-30% of ventricular filling at rest, and more in patients with stiff ventricles. Second, the AV node acts as a protective filter that limits the ventricular rate during supraventricular tachyarrhythmias — a patient in atrial fibrillation with atrial rates of 400 bpm is protected from dangerous ventricular rates because the AV node cannot conduct that fast.
SA node fires at 60-100 beats per minute at rest and is the dominant pacemaker. AV node has an intrinsic rate of 40-60 bpm. The Bundle of His and Purkinje system fire at 20-40 bpm. This hierarchy matters clinically — if the SA node fails, the AV node takes over (junctional rhythm). If the AV node also fails, ventricular escape rhythms emerge at 20-40 bpm with wide QRS complexes. This is why complete heart block is survivable in the short term — the ventricles can generate their own slow rhythm.
The right coronary artery supplies the SA node in about 60% of people and the AV node in 80-90% of people. An inferior wall MI — caused by right coronary artery occlusion — ischemizes these structures. The resulting sinus bradycardia, first-degree AV block, and Wenckebach (Mobitz I) pattern are common early findings. The block is typically at the AV node level, responds to atropine, and often resolves with successful reperfusion. Inferior MI-related AV block is usually transient and does not require permanent pacing. This contrasts with anterior MI, where block tends to be below the AV node and more ominous.
Mobitz I (Wenckebach) shows progressive PR prolongation with each beat until a P wave is dropped, then the cycle resets. The block is at the AV node level and is typically benign — commonly seen in athletes, during sleep, or with AV nodal medications. Mobitz II shows constant PR intervals (whether normal or prolonged) with sudden dropped beats. The block is below the AV node in the His-Purkinje system and is more ominous — it can progress without warning to complete heart block. Mobitz II usually requires pacemaker evaluation.
Wolff-Parkinson-White syndrome features an accessory conduction pathway (the Bundle of Kent) that directly connects atrial and ventricular myocardium, bypassing the AV node. Because the accessory pathway lacks the AV node's deliberate conduction delay, atrial impulses preexcite part of the ventricle — producing the classic ECG findings of a short PR interval (less than 120 ms) and a delta wave (slurred QRS upstroke). Patients are prone to reentrant tachycardias that circle between the AV node and the accessory pathway. In atrial fibrillation, the accessory pathway can conduct rapidly and produce dangerously fast ventricular rates — a reason to avoid AV nodal blocking drugs in WPW patients with atrial fibrillation, which can paradoxically increase accessory pathway conduction.
Yes. Snap a photo of any ECG strip, conduction diagram, or clinical vignette and AnatomyIQ maps the findings back to the underlying anatomy — SA node, AV node, Bundle of His, bundle branches, fascicles, and Purkinje fibers. It walks through normal conduction, heart block types, bundle branch blocks, and preexcitation syndromes with appropriate depth for pre-med, medical school, nursing, and USMLE preparation. This content is for educational purposes only and does not constitute medical advice.