**Question:** In acute pulmonary embolism the most frequent ECG finding is

A. ST segment elevation
B. T wave inversion
C. Atrial fibrillation
D. Right bundle branch block

**Correct Answer:** D. Right bundle branch block

**Core Concept:**
Acute pulmonary embolism (PE) is a life-threatening cardiovascular event caused by the blockage of pulmonary arteries by a blood clot. In such cases, ECG changes are often seen due to the altered electrical activity in the heart caused by the embolism. Among the given options, right bundle branch block (RBBB) is the most frequent ECG finding in acute PE.

**Why the Correct Answer is Right:**
In acute pulmonary embolism, RBBB is predominantly observed due to the following reasons:

1. **Right ventricle strain:** The embolus obstructs the pulmonary arteries, causing right ventricular afterload increase. This strain on the right ventricle leads to a compensatory response, which involves the activation of the autonomic nervous system and release of catecholamines (epinephrine, norepinephrine, and dopamine).
2. **Right ventricular dysfunction:** As the right ventricle struggles to pump blood against the increased afterload, it can lead to right ventricular dysfunction. This dysfunction can manifest as RBBB on the ECG.
3. **Right atrial dilatation and dysfunction:** The increased right ventricular workload can also cause dilatation and dysfunction of the right atrium, leading to the development of RBBB.

**Why Other Options are Incorrect:**
Let's examine the reasons why the other options are incorrect:

1. **ST segment elevation (A):** ST segment elevation typically occurs in inferior wall ischemia or infarction, where there is myocardial ischemia or infarction in the inferior wall of the heart. In acute PE, the most common ECG changes are right axis deviation, right bundle branch block, or both.
2. **T wave inversion (B):** T wave inversion is commonly seen in inferior wall ischemia or infarction, as mentioned above. In acute PE, ST segment elevation or right axis deviation are more common ECG findings.
3. **Atrial fibrillation (C):** Atrial fibrillation is an abnormal heart rhythm characterized by rapid and irregular atrial contractions, which is a separate clinical entity from acute PE. Atrial fibrillation may be associated with PE, but it is not necessarily the most frequent ECG finding in acute PE.

**Clinical Pearl:**
In clinical practice, it is crucial to consider the possibility of acute PE even in the absence of ST elevation or T wave inversion on ECG, as these findings are seen in other conditions too. Right bundle branch block is a specific ECG finding in pulmonary embolism, which can be helpful in pointing towards the diagnosis when seen along with other clinical findings.