This patient has marked bradycardia on exam with a borderline low blood pressure. These vital sign abnormalities are likely the cause of the patient’s dizziness. Bradycardia is defined as any heart rate under 60 beats/min. The most common cause of bradycardia is sinus bradycardia (Choice A). Other types of bradycardia include conduction blocks (i.e. type 2 or type 3 AV blocks), junctional rhythms (lack of P waves with slow SA nodal conduction), idioventricular rhythms (wide QRS complex rhythms that originate from the ventricles, not atria), or low atrial fibrillation or atrial flutter. About 80% of all bradycardias are caused by factors external to the cardiac conduction system, such as hypoxia, drug effects (i.e., beta block or calcium channel blocker use or overdose), or acute coronary syndromes.

Sinus bradycardia (Choice A) occurs when the electrical impulse originates from the SA node in the atria. Signs of sinus bradycardia on EKG are the presence of a P wave prior to every QRS complex. This EKG shows P waves prior to each QRS complex, but there are extra P waves that are not followed by QRS complexes. Some P waves are “buried” within QRS complexes or within T waves. The EKG below marks each P wave with a red line and each QRS complex with a blue line.

First-degree AV Block (Choice B) is a benign arrhythmia characterized by a prolonged PR interval. This patient’s EKG has variable PR intervals (some prolonged, some normal). This is a result of a more severe AV conduction block. Second-Degree AV Blocks are divided into Mobitz type I and Mobitz Type II. Mobitz type I, also known as Wenckebach, is characterized by a progressive lengthening PR interval followed by a dropped QRS complex. This can be remembered by the phrase, “longer, longer, longer, drop.” Wenckebach is a benign arrhythmia that does not typically require any treatment. Mobitz type II (Choice C) is characterized by a normal PR interval with random intermittent dropping of QRS complexes. This patient’s EKG has consistent spacing between each QRS complex (blue lines) and consistent spacing between each P wave (red lines). However, the P waves and QRS complexes are not associated with each other. This phenomenon is known as AV dissociation. These EKG changes are signs of a complete heart block, also known as Third-Degree AV Block (Choice D). Both Second-Degree AV block- Mobitz type II (Choice C) and Third-Degree AV Block (Choice D) are more serious conduction blocks that require cardiac pacemakers. Correct Answer: D