A 72-year-old man with an atrial rate of 80 beats/min develops third-degree (complete) AV block. A pacemaker site located in the A V node below the region of the block triggers ventricular activity, but at a rate of only 40 beats/min. What would be observed?
A. One P wave for each QRS complex B. An inverted T wave
C. A shortened PR interval
D. A normal QRS complex
Answer
D. A normal QRS complex
·
The form of the
QRS will be normal because electrical excitation of the ventricles occurs over
essentially the normal pathway (i.e., A V node to bundle branches to Purkinje
system to myocardium).
·
The T wave will be
normal as well. With complete heart block, P waves and QRS complexes are
completely independent of each other.
·
Some PR intervals
could be shortened by chance, others will be very long; that is, there is no
predictable PR interval.
·
There will not be
a consistent ratio of P waves to QRS complexes because the two are
disassociated, but the average ratio would be 80/40
