A 69-year-old man is brought to the emergency department with sudden onset palpitations and dyspnea. His medical history is significant for hypertension and gastroesophageal reflux disease. ECG reveals a heart rate of 120/min with an irregularly irregular rhythm, narrow QRS complexes, and no organized P waves. Which of the following physiologic factors most likely determines the ventricular contraction rate in this patient?

Question

A 69-year-old man is brought to the emergency department with sudden onset palpitations and dyspnea.  His medical history is significant for hypertension and gastroesophageal reflux disease.  ECG reveals a heart rate of 120/min with an irregularly irregular rhythm, narrow QRS complexes, and no organized P waves.  Which of the following physiologic factors most likely determines the ventricular contraction rate in this patient?

Quizplus · Accepted Answer

This patient's ECG findings are diagnostic for atrial fibrillation with rapid ventricular response. Atrial fibrillation is the most common cardiac arrhythmia and is characterized by absent P waves, irregularly irregular R-R intervals, and narrow QRS complexes. Rapid, irregular, low amplitude fibrillatory waves are also frequently seen and represent chaotic, continuous atrial depolarizations. Atrial fibrillation is initiated by aberrant electrical impulses that arise within regions of heightened atrial excitability (most often the pulmonary veins). Once triggered, atrial fibrillation induces electrical remodeling of the atria with the development of shortened refractory periods and increased conductivity. This facilitates the creation and persistence of multiple ectopic foci and reentrant impulses within the atria, increasing the risk and chronicity of subsequent episodes (atrial fibrillation begets atrial fibrillation). Ventricular response in atrial fibrillation is dependent on the transmission rate of abnormal atrial impulses through the atrioventricular (AV) node. Each time the AV node is excited, it enters a refractory period during which additional atrial impulses cannot be transmitted to the ventricles; consequently, the majority of atrial impulses never reach the ventricles (Choice A). The average ventricular rate in atrial fibrillation usually ranges between 90-170/min. Because atrial excitation is chaotic, the ventricular rate is irregular with no set intervals between contractions. (Choice C) Bundle branch conductivity determines the duration of the QRS complexes, which are widened when there is bundle branch block. It does not affect the ventricular contraction rate. (Choice D) The Purkinje system can assume pacemaker activity in patients with severe bradycardia (<40 beats per minute), but it would not generate impulses in atrial fibrillation since the ventricular rate is typically elevated. (Choice E) During atrial fibrillation, the atria are in a state of chaotic, continuous depolarization that is independent of sinoatrial node activity. (Choice F) The ventricular muscle refractory period does not limit the ventricular contraction rate during AF (the ventricular heart rate can approach 300/min in ventricular tachycardia). Educational objective: Atrial fibrillation occurs due to irregular, chaotic electrical activity within the atria and presents with absent P waves, irregularly irregular R-R intervals, and narrow QRS complexes. The atrioventricular node refractory period regulates the number of atrial impulses that reach the ventricles and determines the ventricular contraction rate in conditions where the atria undergo rapid depolarization.

A 69-Year-Old Man Is Brought to the Emergency Department with Sudden