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Chapter 29. Origin of the Heartbeat & the Electrical Activity of the Heart

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An older patient who had previously recovered from a heart attack was experiencing slight chest discomfort, dizziness, palpitations and shortness of breath. He was taken to the emergency room via ambulance where an electrocardiogram (ECG) showed increased heart rate (> 100 beats/min) and a long QRS (> 0.12 s). He was diagnosed with wide complex tachycardia with a ventricular origin and properly treated. Which part of his ECG corresponds to ventricular repolarization?

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A. The P wave

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B. The QRS duration

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C. The T wave

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D. The U wave

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E. The PR interval

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The correct answer is C. The deflection that characterizes the T wave is dominated by a collection of ventricular myocyte repolarizations. In a typical ECG, the deflections observed in the P wave (A) and the QRS duration (B) are primarily produced by atrial depolarization and ventricular depolarization, respectively. Thus, these two answers are ruled out. Although understanding of the inconsistent U wave (D) is incomplete, it has been attributed to Purkinje fiber repolarization, ventricular myocytes with long action potentials, or electrolyte imbalances and can be excluded. The PR interval (E) represents the time between atrial and ventricular depolarization, and can be ruled out.

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A 50-year-old woman was recently prescribed disopyramide for cardiac arrhythmia. In a follow-up examination, the woman complained of faintness and confusion. A brief examination showed a low heart rate with sick sinus syndrome. Her disopyramide dosage was adjusted and a subsequent follow-up examination showed a normal ECG. In which of the following regions in the heart would you normally find a slowly depolarizing “prepotential”?

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A. Sinoatrial node

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B. Atrial muscle cells

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C. Bundle of His

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D. Purkinje fibers

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E. Ventricular muscle cells

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The correct answer is A. Cells in the sinoatrial node or atrioventricular node have pacemaker potential that is associated with a slowly depolarizing prepotential. Atrial muscle cells (B), cells in the His Bundle (C), Purkinje fiber cells (D) and ventricular muscle cells (E) all maintain a stable depolarization that is quickly polarized following activation of voltage-gated Na+ channels.

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An elite athlete came in for a routine physical examination. It was noticed on his ECG that there were progressive increases in the PR interval which were eventually followed by a skip of the QRS complex. Further tests determined the observed second-degree heart block ...

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