A 24-year-old woman presents to clinic for a routine annual exam. She has generally been feeling well, but notes feeling intermittent palpitations over the past few months. Her past medical history is unremarkable and she is not currently taking any medications. She denies drinking alcohol, smoking cigarettes, or using recreational drugs. She attributes her palpitations to recently drinking more caffeine, but would like to obtain an electrocardiogram since her symptoms are worrisome and affecting her ability to concentrate on her career as a fitness instructor. Her temperature is 36.9°C (98.4°F), blood pressure is 116/76 mmHg, pulse is 55/min, respirations are 12/min, and oxygen saturation is 98% on room air. Her electrocardiogram is shown in Figure A. Which of the following is the most likely diagnosis?
First degree atrioventricular block
Sinus bradycardia
Second degree atrioventricular block, Mobitz type II
Second degree atrioventricular block, Mobitz type I
Second degree atrioventricular block, Mobitz type I
This patient's progressively increasing PR intervals until a P wave is not followed by a QRS complex on ECG is consistent with second degree atrioventricular (AV) block, Mobitz type I, also referred to as Wenckebach phenomenon.

Second degree AV block, Mobitz type I can occur in a variety of settings, such as in the context of digoxin, beta blockers, calcium channel blockers, or increased vagal tone. On ECG, there is progressive lengthening of the PR intervals until a P wave is not followed by a QRS complex (a dropped beat); then, the PR interval resets. Patients are typically asymptomatic, but should undergo evaluation for an underlying etiology of the dysrhythmia, such as electrolyte imbalances, medication side effect, or structural heart disease, as well as a follow-up ECG to evaluate for any progression. If the dysrhythmia is caused by medication, the offending agent should be discontinued, but otherwise there are no specific treatments necessary for asymptomatic patients. Patients who are symptomatic and hemodynamically stable can undergo telemetry monitoring and receive atropine with limited efficacy. Patients who are hemodynamically unstable and not responsive to atropine may need cardiac pacing. Generally, there is a low risk of progression to third degree AV block, and a pacemaker is rarely required (in contrast to second degree heart block type II).

Barold and Hayes review second-degree heart block. They note the changes in AV node conduction time on ECG in this condition. They recommend using the correct definition of the different heart blocks for the correct underlying treatment.

Figure/Illustration A is an ECG showing PR intervals that gradually prolong until a P wave is seen not followed by a QRS complex (note the blue bars that show the progressive lengthening of the PR intervals until the dropped beat).

Incorrect Answers:
Answer A: First degree AV block is seen on ECG with prolonged PR intervals greater than 0.2 sec, but a consistent 1:1 ratio of P waves to QRS complexes. This finding is typically found in younger patients with increased vagal tone or in athletes, and does not require treatment.

Answer C: Second degree AV block, Mobitz type II is seen with fixed PR intervals and occasional dropped QRS complexes. Patients with Mobitz II sometimes present with syncope and frequently progress to third degree AV block. This finding may be associated with fibrotic changes to the conduction system or from acute, subacute, or prior myocardial infarction, and should be treated with placement of a pacemaker in most cases.

Answer D: Sinus bradycardia findings on ECG include normal sinus rhythm with a ventricular rate of less than 60 beats per minute. Patients with sinus bradycardia may be asymptomatic, but may also present with syncope, lightheadedness, chest pain, or hypotension. No treatment is required if the patient is asymptomatic, but atropine may be given to increase the heart rate in symptomatic cases. Pacemaker placement is required for patients with chronic symptoms.

Answer E: Third-degree AV block occurs when the atria and ventricles depolarize independently on ECG. There is no relationship between P waves and QRS complexes. Patients may present with syncope, dizziness, acute heart failure, or hypotension. The treatment is pacemaker placement.

Bullet Summary:
Second degree atrioventricular block, Mobitz type I, is characterized by progressively lengthening PR intervals until a P wave is not followed by a QRS complex.