fusion beat, in an electrocardiogram, a P wave or QRS complex resulting from the concurrent activation of the atria or the ventricles by two stimuli in the same chamber. An atrial fusion beat results when the sinus beat coincides with an atrial ectopic beat, when two atrial ectopic beats coincide, or when an atrial or sinus beat coincides with retrograde conduction from a junctional focus. A ventricular fusion beat results when a ventricular beat coincides with a sinus beat, a ventricular ectopic beat, or a junctional beat.
