AKA ventricular ectopics, ventricular extrasystoles, ventricular premature beats, ventricular premature depolarisations.
- A premature beat arising from an ectopic focus within the ventricles.
Origin of Ectopic Beats
- Groups of pacemaker cells throughout the conducting system are capable of spontaneous depolarisation.
- The rate of depolarisation decreases from top to bottom: fastest at the sinoatrial node; slowest within the ventricles.
- Ectopic impulses from subsidiary pacemakers are normally suppressed by more rapid impulses from above.
- However, if an ectopic focus depolarises early enough — prior to the arrival of the next sinus impulse — it may “capture” the ventricles, producing a premature contraction.
- Premature contractions (“ectopics”) are classified by their origin — atrial (PACs), junctional (PJCs) or ventricular (PVCs).
- Ectopic firing of a focus within the ventricles bypasses the His-Purkinje system and depolarises the ventricles directly.
- This disrupts the normal sequence of cardiac activation, leading to asynchronous activation of the two ventricles.
- The consequent interventricular conduction delay produces QRS complexes with prolonged duration and abnormal morphology.
PVCs have the following features:
- Broad QRS complex (≥ 120 ms) with abnormal morphology.
- Premature — i.e. occurs earlier than would be expected for the next sinus impulse.
- Discordant ST segment and T wave changes.
- Usually followed by a full compensatory pause.
- Retrograde capture of the atria may or may not occur.
“Discordance” describes a pattern of repolarisation abnormality (typically seen with left bundle branch block, paced rhythms, VT) in which the ST segment and T wave are directed opposite to the main vector of the QRS complex:
- ST depression and T wave inversion in leads with a dominant R wave.
- ST elevation with upright T waves in leads with a dominant S wave.
With a full compensatory pause, the next normal beat arrives after an interval that is equal to double the preceding R-R interval (see diagram below).
Retrograde capture describes the process whereby the ectopic impulse is conducted retrogradely through the AV node, producing atrial depolarisation. This is visible on the ECG as an inverted P wave (“retrograde P wave”), usually occurring after the QRS complex.
PVCs are said to be “frequent” if there are more than 5 PVCs per minute on the routine ECG, or more than 10-30 per hour during ambulatory monitoring.
PVCs may be either:
- Unifocal — Arising from a single ectopic focus; each PVC is identical.
- Multifocal — Arising from two or more ectopic foci; multiple QRS morphologies.
The origin of each PVC can be discerned from the QRS morphology:
PVCs often occur in repeating patterns:
- Bigeminy — every other beat is a PVC.
- Trigeminy — every third beat is a PVC.
- Quadrigeminy — every fourth beat is a PVC.
- Couplet — two consecutive PVCs.
- Triplet — three consecutive PVCs.
- PVCs are a normal electrophysiological phenomenon not usually requiring investigation or treatment.
- Frequent PVCs may cause palpitations and a sense of the heart “skipping a beat”.
- In patients with underlying predispositions (e.g. ischaemic heart disease, WPW), a PVC may trigger the onset of a re-entrant tachydysrhythmia — e.g. VT, AVNRT/AVRT.
Frequent PVCs are usually benign, except in the context of an prolonged QTc, when they may predispose to malignant ventricular arrhythmias such as Torsades de Pointes by causing “R on T” phenomenon (read more about this here).
Frequent or symptomatic PVCs may be due to:
- Sinus rhythm with PVCs of two different morphologies (arrows).
- Note the appropriately discordant ST segments / T waves.
- The pause surrounding the PVC is equal to double the preceding R-R interval (= a full compensatory pause).
- Sinus rhythm with frequent PVCs in a pattern of bigeminy
- Chan TC, Brady WJ, Harrigan RA, Ornato JP, Rosen P. ECG in Emergency Medicine and Acute Care. Elsevier Mosby 2005.
- Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice (6th edition), Saunders 2008.
- Wagner, GS. Marriott’s Practical Electrocardiography (11th edition), Lippincott Williams & Wilkins 2007.