Welcome to Day 13! Today, we’re diving into Bundle Branch Blocks (BBB), specifically Right Bundle Branch Block (RBBB) and Left Bundle Branch Block (LBBB). These blocks disrupt the normal electrical pathway in the heart, leading to characteristic changes on the ECG. Recognizing them is crucial for accurate diagnosis and patient management.

Learning Objectives 🎯

• 💡 Define Right Bundle Branch Block (RBBB) and Left Bundle Branch Block (LBBB).
• 📝 Identify the key ECG criteria for diagnosing RBBB and LBBB.
• 🩺 Understand the clinical significance of RBBB and LBBB.
• 🔍 Differentiate between RBBB and LBBB on an ECG strip.
• 🧠 Apply your knowledge to analyze ECGs and identify BBBs.

Step-by-Step Content 📝

What are Bundle Branch Blocks? A Bundle Branch Block (BBB) occurs when there’s a delay or blockage in the electrical impulse traveling down either the right or left bundle branch. This leads to asynchronous ventricular depolarization.

Right Bundle Branch Block (RBBB) ➡️

In RBBB, the right ventricle is depolarized later than the left ventricle because the electrical impulse has to travel across the septum from the left ventricle. The key ECG features are:

• QRS duration > 0.12 seconds (120 ms)
• RSR’ pattern (rabbit ears) in V1-V3. This is a classic finding. Look for the ‘R’ wave followed by a wider ‘S’ wave, and then another ‘R’ wave (‘R’ prime).
• Wide, slurred S wave in leads I, aVL, and V6.
• ST-segment depression and T-wave inversion in leads V1-V3 (secondary repolarization abnormalities).

Remember: RBBB doesn’t usually change the axis.

Left Bundle Branch Block (LBBB) ⬅️

In LBBB, the left ventricle is depolarized late. The electrical impulse has to travel across from the right ventricle. The key ECG features are:

• QRS duration > 0.12 seconds (120 ms)
• Broad, notched R wave in leads I, aVL, V5 and V6.
• Absent Q waves in leads I, V5 and V6.
• ST-segment depression and T-wave inversion in leads I, aVL, V5 and V6 (secondary repolarization abnormalities).
• Discordant ST-T wave changes (opposite direction of the QRS).

LBBB may cause left axis deviation.

Clinical Significance 🏥

BBBs can be caused by various factors, including:

• Heart disease (ischemic heart disease, cardiomyopathy)
• Hypertension
• Pulmonary embolism (RBBB)
• Medications
• Sometimes they can be normal variants.

It’s important to correlate ECG findings with the patient’s clinical context. New onset LBBB can be associated with acute myocardial infarction.

ECG Practice 🧪

Look at example ECG strips. Pay attention to the QRS duration and the morphology in the precordial leads (V1-V6) and limb leads (I, aVL). Try to identify RSR’ pattern or broad notched R waves.

Video Resources 🎬

Watch videos on YouTube that illustrate RBBB and LBBB ECG features. Visual aids can significantly enhance your understanding.

Summary 📝

• Bundle Branch Blocks occur when the electrical impulse is blocked or delayed in the bundle branches.
• RBBB is characterized by a QRS duration > 0.12 seconds and an RSR’ pattern in V1-V3.
• LBBB is characterized by a QRS duration > 0.12 seconds, broad notched R waves in I, aVL, V5, V6, and absent Q waves in these leads.
• BBBs can have significant clinical implications and should be interpreted in the context of the patient’s history.

Review and Practice 📚

Review the ECG criteria for RBBB and LBBB. Practice interpreting ECG strips to identify these blocks. Look for examples online or in textbooks.

Next Day Preview 🚀

Tomorrow, we will explore Hypertrophy: Left Ventricular Hypertrophy (LVH) and Right Ventricular Hypertrophy (RVH). Prepare to learn about voltage criteria and other ECG signs of ventricular enlargement.

Quiz Time ❓

1. Which of the following is a key ECG feature of Right Bundle Branch Block (RBBB)?
   A. Narrow QRS complex
   B. RSR’ pattern in V1-V3 (Correct)
   C. Absent Q waves in I, aVL, V5, V6
2. What is the minimum QRS duration that suggests a Bundle Branch Block?
   A. > 0.08 seconds
   B. > 0.10 seconds
   C. > 0.12 seconds (Correct)
3. Which of the following leads is MOST useful in identifying the “rabbit ears” pattern for RBBB?
   A. Lead I
   B. Lead V6
   C. Lead V1 (Correct)
4. In Left Bundle Branch Block (LBBB), what happens to the Q waves in leads I, V5 and V6?
   A. They are widened
   B. They are inverted
   C. They are absent (Correct)
5. A patient presents with a wide QRS complex, broad notched R waves in I, aVL, V5, and V6. Which of the following is the MOST likely diagnosis?
   A. Right Bundle Branch Block
   B. Left Bundle Branch Block (Correct)
   C. Normal ECG

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