Fusion beats

The arrow indicates a fusion beat on this ECG.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Synonyms and keywords: Capture beats; fusion complexes; fusion QRS complexes

Overview

A fusion beat occurs when electrical impulses from different sources act upon the same region of the heart at the same time. If it acts upon the ventricular chambers, it is called a ventricular fusion beat, wheres colliding currents in the atrial chambers produce atrial fusion beats.

Pathophysiology

Ventricular fusion beats can occur when the heart's natural rhythm and the impulse from a pacemaker coincide to activate the same part of a ventricle at the same time, causing visible variation in configuration and height of the QRS complex of an electrocardiogram reading of the heart's activity. This contrasts with the pseudofusion beat wherein the pacemaker impulse does not affect the complex of the natural beat of the heart. Pseudofusion beats are normal. Rare or isolated fusion beats caused by pacemakers are normal as well, but if they occur too frequently may reduce cardiac output and so can require adjustment of the pacemaker.

In some cases of VT with AV dissociation the SA node may transiently "capture" the ventricles, producing a capture beat, which has a normal QRS duration, or a fusion beat, in which the sinus and ventricular beats coincide to produce a hybrid complex.

Electrocardiogram

In the example below, "late" (end-diastolic) PVCs are illustrated with varying degrees of fusion with normal conduction. For a fusion beat to occur the sinus P wave must successfully make its way to the ventricles to start the activation sequence, but before ventricular activation is completed by the late PCV. The resulting QRS complex resembles both the normal QRS, as well as the PVC and hence the term fusion QRS or fusion beat. The fusion beats are marked by an F in the illustration below:

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