A 63-Year-Old Man Was Admitted to the Hospital 1 Hour

A 63-year-old man was admitted to the hospital 1 hour after experiencing substernal chest pain.He was weak and diaphoretic,and his heart was beating rapidly at 90 beats/minute as a consequence of 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; his arterial pulse rate was about 45 beats/minute,and yet the ECG revealed an atrial rate of 90 beats/minute.A cardiac pacemaker was inserted because the AV node had been damaged by ischemia.With the implanted pacemaker set at 75 beats/minute,he felt somewhat better,and drug therapy was initiated. While the heart was being paced,the cardiologist discontinued ventricular pacing periodically to test the patient's cardiac status.The cardiologist found that the ventricles did not begin beating spontaneously until about 5 to 10 seconds after cessation of pacing because the preceding period of pacing led to:

A) Overdrive suppression of the automatic cells in the ventricles.
B) Release of norepinephrine from the cardiac sympathetic nerves.
C) Release of neuropeptide Y from the cardiac sympathetic nerves.
D) Fatigue of the ventricular myocytes.
E) Release of acetylcholine from the cardiac parasympathetic nerves.