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Premature Ventricular Contraction



This strip begins with sinus rhythm with a notched and slightly
prolonged QRS complex,with no positive deflections in the V1 lead characteristic
of a left bundle branch block.  This rhythm is interrupted by a wide (0.16 second)
and differently shaped complex that is not preceded by a P-wave.  The
depolarization phase of the complex is opposite the isoelectric line from the
repolarization phase, and in most leads, the depolarization phase slurs into the
repolarization phase without a pause.  These are hallmarks of a ventricular beat.
The pacemaker of this beat is an area of myocardium in the ventricles that is for
some reason irritable.  Because the wave of depolarization initiated by this
irritable focus propagates through the heart without benefit of the specialized
conduction system, depolarization is prolonged: this is why the QRS is widened.
This beat is premature:  it occurs before the next expected sinus beat.  Thus, it is
called a premature ventricular contraction, or PVC.  Notice that the R-R interval
between the two sinus beats surrounding the PVC is exactly twice the R-R
intervals between the other sinus beats.  This is called a compensatory pause.
The atria have not been disturbed by the PVC, and the sinuatrial node continues
its rhythm. The missing P-wave is buried in the repolarization phase of the PVC.