Fusion beat
Let’s talk about fusion beat.
A fusion beat or complex represents the simultaneous activation of either the atria or ventricles by two, or on rare occasions, more impulses originating from the same or different chambers of the heart.
Atrial Fusion
Fusions of sinus node (SA) impulses with atrial premature complexes (APCs) are infrequent due to the sinus node being discharged by the APCs, making dual excitation of the atria unlikely. Moreover, the low amplitude of the P wave and frequent lack of configuration detail make identifying atrial fusions challenging. Atrial fusion is more commonly observed with an atrial parasystole and, less frequently, a junctional rhythm. In theory, atrial fusion could result from the collision of two atrial ectopic impulses, but distinguishing it from multifo-cal APCs is not feasible.
Ventricular Fusion
Ventricular fusions are frequent and generally point towards a ventricular arrhythmia, with rare exceptions. A ventricular fusion can occur from the collision of a ventricular impulse with either a sinus impulse, a supraventricular ectopic impulse, or another ventricular impulse. The latter could be a single late ventricular premature complex (VPC), an idioventricular rhythm, ventricular parasystole, ventricular tachycardia (VT), or a paced ventricular rhythm. Usually, ventricular fusion presents an intermediate morphology (hybrid complex).
Beat #3 is a ventricular fusion beat.
Exceptions to the rule
1- The QRS complex in Wolff-Parkinson-White (WPW) represents a fusion of a supraventricular impulse, typically an SA nodal impulse, simultaneously activating the ventricle through the accessory pathway and the normal AV pathway. This stands as an exception to the stated rule that fusion results from the collision of impulses arising from two different foci.
2- Fusion between a supraventricular impulse conducting through an accessory pathway and one originating in the AV junction has been documented and serves as another exception to the rule that ventricular fusion indicates a ventricular arrhythmia.
History
Ventricular fusion was first noted by Thomas Lewis in humans in 1913 and in the canine model in 1920.
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