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Sinus Bradycardia with Second Degree A-V Block
(Type II) and Intermittent Bundle Branch Block



Look at the QRS complexes: there is a marked change halfway through the strip.
The later complexes might be mistaken for ventricular beats, but they are not ventricular
in origin.  Notice that the R-R interval has not changed:  it is still about 1.18 seconds,
giving a rate of 51.  The QRS has widened from 0.08 to 0.12 seconds, and has changed shape.
Each of these wider complexes is preceded by a P-wave, and the P-R interval is unchanged
at 0.38 seconds.   For all these reasons these beats are not ventricular in origin: they are
still sinus beats. The change in QRS morphology is caused by a bundle branch block
which is intermittent: sometimes present, sometimes not.  It is easy to misinterpret this as VT!
The rhythm here is at first unclear.  It is some kind of A-V block, and at first glance seems to
be first degree.  Look more closely.  If you divide the apparent P-P interval in half, and march
that interval through the strip with calipers or a ruler, you can see that there is a suspicious
bump in the first three T-waves just where that interval falls. These bumps are probably
P-waves, partially hidden in the T-waves, and the rhythm thus is second degree block, two-to-one type.