A patient known to have an AV pacemaker has been brought to the emergency department of the hospital where you work. The patient is pale, is having difficulty breathing, and is barely conscious. At the physician's request, you perform an immediate ECG. A portion of the tracing is shown here. What may be causing the patient's signs and symptoms? 
A. The pacemaker is oversensing, so it is generating impulses at a slower rate than it should, resulting in a slow heart rate and low cardiac output.
B. The pacemaker has stopped functioning, and the patient is in cardiac arrest.
C. The pacemaker is sending too many impulses, and the patient's heart cannot respond quickly enough.
D. The pacemaker is functioning, but the patient's heart is not depolarizing after every impulse, leading to low cardiac output.
Answer: D
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