Purkinje Fibers
The Purkinje fibers are the terminal, highly specialized conduction fibers of the ventricular conduction system. Forming a dense subendocardial network in both ventricles, they conduct impulses at the fastest velocity in the heart — up to 4 m/s — enabling nearly simultaneous activation of the entire ventricular endocardium. Purkinje fibers also possess slow automaticity and can serve as a ventricular escape pacemaker at 20–40 bpm.
Also known as: Purkinje Network, Purkinje System, Subendocardial Purkinje Network, His-Purkinje System (terminal)
Anatomy & Physiology
| Location | Purkinje fibers are located in the subendocardial layer of both the right and left ventricles, forming a complex branching network that fans out from the terminal ends of the right bundle branch and left anterior and posterior fascicles. They penetrate a few millimeters into the myocardium before transitioning into ordinary ventricular working myocardium. The network is densest at the apex and papillary muscles. |
| Function | Purkinje fibers rapidly distribute the electrical wavefront from the bundle branches across the entire ventricular endocardium simultaneously, producing rapid, synchronous, and coordinated ventricular contraction from endocardium to epicardium and from apex to base. This near-simultaneous endocardial activation ensures the ventricle contracts efficiently as a single unit, maximizing stroke volume. Without the Purkinje network, impulse spread would occur only cell-to-cell through working myocardium at much slower velocities (0.3–0.5 m/s), producing disorganized, inefficient contraction. |
| Conduction Velocity | 2.0–4.0 m/s (fastest in the heart) |
| Blood Supply | The Purkinje network receives blood from penetrating endocardial branches of both the LAD (anterior and apical) and the posterior descending artery (posterior and inferior). The subendocardial location makes Purkinje fibers vulnerable to ischemia in states of reduced coronary perfusion pressure, since the subendocardium is the last territory perfused. |
Clinical Relevance
Purkinje fiber disease underlies several important arrhythmias. Purkinje fiber reentry is the mechanism of some forms of idiopathic ventricular tachycardia (fascicular VT). Purkinje fibers are highly sensitive to ischemia and represent the substrate for reentrant ventricular arrhythmias in the peri-infarct period. Early afterdepolarizations (EADs) in Purkinje fibers are responsible for triggering torsades de pointes in long QT syndrome. Purkinje network disease also underlies the broad QRS in bundle branch blocks and idioventricular rhythms, and may contribute to exercise-induced PVCs.
Associated Pathologies
- Fascicular ventricular tachycardia (Purkinje reentry VT)
- Idioventricular rhythm (Purkinje escape pacemaker)
- Torsades de pointes (early afterdepolarizations in Purkinje fibers)
- PVCs arising from Purkinje fiber ectopic foci
- Bundle branch reentrant ventricular tachycardia
- Subendocardial ischemia affecting Purkinje network
Frequently Asked Questions
Why are Purkinje fibers so important for ventricular function?
Purkinje fibers conduct at 2–4 m/s — the fastest conduction velocity in the heart — enabling near-simultaneous activation of the entire ventricular endocardium within milliseconds. This coordinated activation produces efficient, synchronous ventricular contraction that maximizes stroke volume. Without Purkinje fibers, the impulse would spread only through working myocardium at 0.3–0.5 m/s, producing a much slower, disorganized wavefront and a broad, bizarre QRS complex — exactly what is seen in idioventricular rhythms.
Can Purkinje fibers act as a pacemaker?
Yes. Purkinje fibers possess slow spontaneous automaticity, with an intrinsic firing rate of 20–40 bpm. When all higher pacemakers (SA node, AV node, Bundle of His) fail, Purkinje fibers generate an idioventricular escape rhythm characterized by a very slow rate, a wide and bizarre QRS, and hemodynamic instability. This is the pacemaker of last resort and indicates severe conduction system disease.
What is fascicular ventricular tachycardia?
Fascicular VT (also called idiopathic left ventricular tachycardia or Belhassen tachycardia) is a form of VT arising from a reentrant circuit within the left posterior fascicle and the Purkinje network. It produces a relatively narrow QRS (compared to typical VT) with right bundle branch block morphology and left axis deviation. It typically occurs in young patients without structural heart disease and responds well to verapamil — distinguishing it from most other VTs.
How do Purkinje fibers contribute to torsades de pointes?
In long QT syndrome (congenital or acquired), prolonged action potential duration leads to early afterdepolarizations (EADs) — abnormal depolarizations during the repolarization phase. Purkinje fibers are particularly prone to EADs because of their long action potential duration and high expression of the channels that mediate them (I_Kr). When an EAD reaches threshold, it can trigger a premature beat that initiates the twisting, polymorphic VT pattern of torsades de pointes.
Why is the subendocardium — including Purkinje fibers — vulnerable to ischemia?
The subendocardium is the last territory perfused by coronary blood flow because the perfusion pressure gradient through the myocardial wall is lowest near the endocardium. During exercise, tachycardia, or reduced perfusion pressure (aortic stenosis, hypotension), subendocardial blood flow is compromised first. This explains why subendocardial ischemia produces ST depression (rather than ST elevation), and why it can impair Purkinje fiber conduction and trigger ventricular arrhythmias.
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