A 63-year-old man was admitted to hospital 1 hour after experiencing substernal chest pain
He was weak and diaphoretic, and his heart was beating rapidly at 90 beats/min as a consequence of having a coronary artery occlusion that impaired blood flow to the left ventricle. From an ECG, it was determined that the tachycardia originated in the SA node. Before therapy could be started, the man became much weaker and his arterial pulse rate was about 45 beats/min, yet the ECG revealed an atrial rate of 90 beats/min. A cardiac pacemaker was inserted because the AV node had been damaged by ischemia. With the implanted pacemaker set at 75 beats/min, he felt somewhat better and drug therapy was initiated.
The most likely mechanism responsible for the patient's arterial pulse rate of about 40 beats/min after impulse conduction through the AV junction was blocked is:
A. excitation of the ventricles via an AV bypass tract
B. conversion of ventricular myocardial fibers to automatic cells
C. firing of ventricular ectopic cells that have the same electrophysiolgical characteristics as SA node cells
D. firing of automatic cells (Purkinje fibers) in the specialized conduction system of the ventricles
E. excitation of ventricular cells by the rhythmic activity in the autonomic neurons that innervate the heart
ANS: D
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