A 50-year-old man has been having fainting “spells” for about 2 weeks. During the episodes, his ECG shows a ventricular rate of 25 beats/min and 100 P waves per minute. After about 30 seconds of fainting, a normal sinus rhythm recurs. What is his likely diagnosis?
## **Core Concept**
The patient's symptoms and ECG findings during episodes suggest a condition characterized by transient, complete heart block, leading to a significant reduction in ventricular rate. This results in decreased cardiac output, causing symptoms like fainting (syncope). The key ECG feature here is the presence of **atrial dissociation** or **AV block**, indicated by a high number of P waves not followed by QRS complexes.
## **Why the Correct Answer is Right**
The patient's presentation with a ventricular rate of 25 beats/min and 100 P waves per minute during episodes indicates **third-degree atrioventricular (AV) block**. In third-degree AV block, there is a complete dissociation between atrial and ventricular contractions. The atria and ventricles beat independently, leading to a situation where the ventricular rate can be very low (as seen in this case), while the atrial rate remains within a relatively normal range or even faster. This condition can cause symptoms like syncope due to decreased cardiac output.
## **Why Each Wrong Option is Incorrect**
- **Option A:** Without specifics on the options, we can't directly address each; however, conditions like **first-degree AV block** involve a prolongation of the PR interval without dropped beats, which wouldn't cause such severe symptoms.
- **Option B:** **Second-degree AV block** involves some but not all P waves being followed by QRS complexes. While it can cause symptoms, the pattern described (e.g., Mobitz type II) might not fully align with the complete heart block suggested by the patient's presentation.
- **Option C:** This option would be incorrect if it suggested a condition not consistent with the ECG findings of complete AV dissociation.
- **Option D:** Similarly, this would be incorrect if it did not align with the diagnosis of third-degree AV block.
## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that patients with **third-degree AV block** may present with **Adams-Stokes syndrome**, characterized by episodes of syncope or near-syncope due to abrupt changes in heart rate. These patients often require **permanent pacemaker** implantation as a definitive treatment to prevent recurrent episodes.
## **Correct Answer:** C. Third-degree AV block.