A 62-year-old man with a past medical history of bladder cancer and recurrent hematuria comes to the emergency department after "passing out." The patient was standing up to urinate shortly after waking when he felt dizzy. He was able to return to his bed before losing consciousness for several minutes. The patient's electrocardiogram (ECG) at the time of admission is shown in the Exhibit. Which of the following most likely accounts for the observed ECG changes?

Question

A 62-year-old man with a past medical history of bladder cancer and recurrent hematuria comes to the emergency department after "passing out."  The patient was standing up to urinate shortly after waking when he felt dizzy.  He was able to return to his bed before losing consciousness for several minutes.  The patient's electrocardiogram (ECG) at the time of admission is shown in the Exhibit.  Which of the following most likely accounts for the observed ECG changes?

Quizplus · Accepted Answer

Explanation: This patient's electrocardiogram (ECG) shows progressive prolongation of PR interval leading to a non-conducted P wave and a 'dropped' QRS complex consistent with Mobitz type I second degree atrioventricular (AV) block. Mobitz type I AV block (often referred to as Wenckebach block) is an intermittent AV block due to impaired conduction within the AV node. Characteristic features of Mobitz type I AV block include: ECG Constant P-P interval Increasing PR interval Decreasing R-R interval Group beating (repeating clusters of beats followed by a dropped QRS) The PR interval prolongation is best appreciated by measuring PR intervals just before and after the 'dropped' QRS complexes. Mobitz type I AV block is often asymptomatic and has a benign prognosis in young patients with good ventricular function. However, it may occasionally cause significant bradycardia with symptoms of inadequate cardiac output (eg, fatigue, lightheadedness, angina, syncope, heart failure), especially in older patients with acute ischemia or additional cardiac comorbidity. (Choice A) In atrial fibrillation, there is no organized atrial activity, and impulses are conducted in a random and unpredictable manner. This leads to an absence of any discernible P waves and irregularly irregular rhythm with varying R-R interval. atrial fibrillation (Choice B) Atrial premature beats indicate depolarization of the atria originating in a focus outside the sinoatrial (SA) node. They are seen on ECG as a P wave with abnormal morphology, often occurring earlier in the cardiac cycle than a normal P wave. Atrial premature beats (Choice D) Impaired SA node automaticity leads to sick sinus syndrome, which is often due to degeneration and/or fibrosis of the SA node and surrounding atrial myocardium. Patients may present with fatigue, lightheadedness, palpitations, presyncope, or syncope. ECG often shows bradycardia, sinus pauses/arrest, SA exit block, or alternating bradycardia and atrial tachyarrhythmias (tachycardia-bradycardia syndrome). (Choice E) Ventricular preexcitation (Wolff-Parkinson-White syndrome) is due to an accessory conduction pathway that directly connects the atria and ventricle, bypassing the AV node. The ECG shows a characteristic pattern of short PR interval (< 0.12 seconds), widened QRS complex, and "slurred" upstroke of QRS complex ("delta wave"). Wolff-Parkinson-White syndrome Educational objective: In Mobitz type I atrioventricular block, there is progressive prolongation of PR interval leading to a non-conducted P wave and a 'dropped' QRS complex. The PR interval prolongation is best appreciated by measuring PR intervals just before and after the 'dropped' QRS complexes. References: Long-term follow-up of older patients with Mobitz type I second degree atrioventricular block. (http://www.ncbi.nlm.nih.gov/pubmed/23086974) Is Mobitz type I atrioventricular block benign in adults? (http://www.ncbi.nlm.nih.gov/pubmed/14729789)

A 62-Year-Old Man with a Past Medical History of Bladder