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Third Degree Block example three



This example of third degree block has some unusual features.   The ventricular rate is 54 throughout this strip, and the QRS complexes show a bundle branch block on every other beat.  Apparently, the bundle that is not conducting has a refractory period greater than the interval between beats (1.12 seconds).   The atrial rate is 68, and the P-waves are doubly peaked, indicating left atrial enlargement.  After the fourth QRS, with no change in P-P interval, there is a P-wave with markedly different morphology.  This is followed by a PAC with a new shape.   An identical PAC occurs between the sixth and seventh QRS complexes.   Of course, these variations in atrial activity have no effect on the ventricles:  none of them are conducted.