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Ventricular ectopic beats
Like SVEBs, VEBs are common and, by themselves, generally harmless. Past attempts to suppress
them as harbingers of malignant arrhythmias have caused more harm than good. Nevertheless,
recognition of their electrocardiographic morphology and behaviour remain important.
VEBs that have a fixed coupling to the preceding beats are thought to represent a localised
ventricular re-entry and are said to be extra-systolic (extra-systoles). In the top strip of they each replace a sinus beat, with a sinus P wave buried inside the ectopic QRS; their
pauses are exactly (fully) compensatory. In the bottom strip, the pause cannot be quantified
but, unlike the aberrant beats of Ashman’s phenomenon in , there is hint of a compensatory
pause, even during AF. Also unlike aberrant beats in Figure 3.33, the VEBs do not
come after the longest cycles in AF.
VEBs with same morphology but variable coupling intervals usually represent a continuous
discharge from an ectopic focus, like a fixed-rate electronic pacemaker. They capture the
ventricles whenever the latter are not refractory and, when they occur at the right time, produce
ventricular fusion beats. Fusion beats occur when impulses from two origins, in the case seen
in from the sinus node and the parasystolic focus, occur at almost the same time.
The resultant QRS complex has features of both types of beat. These VEBs are called parasystolic