ECG Heart Blocks Overview

Overview

This lecture covers the identification, causes, and characteristics of heart blocks seen on ECG/EKG, including bundle branch and AV blocks.

Bundle Branch Blocks

• Bundle branch blocks are delays in electrical impulse along either the right or left bundle branch.
• Right bundle branch block (RBBB) shows a wide QRS with a terminal positive deflection in lead V1.
• Common RBBB causes: pulmonary hypertension, valvular disease, pulmonary embolism, conduction degeneration, coronary artery disease, drugs, electrolyte issues.
• Left bundle branch block (LBBB) shows a wide QRS with a terminal negative deflection in lead V1.
• Common LBBB causes: cardiomyopathy, hypertensive/valvular disease, coronary artery disease, conduction degeneration.
• Use lead V1 for identifying bundle branch blocks; wide QRS complexes are key features.

First-Degree AV Block

• Characterized by prolonged PR interval (>0.20 seconds) with regular P waves, regular R waves, and narrow QRS.
• Indicates delayed impulse transmission through the AV node, but all impulses are conducted.

Second-Degree AV Block: Mobitz Type I (Wenckebach)

• PR interval progressively lengthens until a QRS is dropped (P wave with no QRS follows).
• Causes: increased vagal tone, myocardial ischemia, drugs (beta blockers, calcium channel blockers, digoxin).
• Irregular P waves and irregular QRS; narrow QRS complexes.

Second-Degree AV Block: Mobitz Type II

• Fixed PR interval with occasional dropped QRS complexes.
• Regular P waves, irregular R waves, narrow QRS.
• Block occurs below the AV node, often requires close monitoring due to risk of progression to complete heart block.

Third-Degree (Complete) AV Block

• Complete dissociation of atrial (P waves) and ventricular (QRS) activity.
• P waves and R waves are regular but unrelated, PR intervals are chaotic, QRS complexes are wide.
• Results in decreased cardiac output, often requires immediate pacing intervention.

Key Terms & Definitions

• Bundle Branch Block (BBB) — Blockage in right or left bundle branch leading to wide QRS.
• Right Bundle Branch Block (RBBB) — Wide QRS with terminal positive deflection in V1.
• Left Bundle Branch Block (LBBB) — Wide QRS with terminal negative deflection in V1.
• AV Block — Delay or block of impulse conduction through the AV node.
• First-Degree AV Block — Prolonged PR interval, all impulses conducted.
• Mobitz Type I (Wenckebach) — Gradually increasing PR interval until a QRS is dropped.
• Mobitz Type II — Intermittent dropped QRS with fixed PR intervals.
• Third-Degree AV Block — Complete dissociation between atrial and ventricular rhythms.

Action Items / Next Steps

• Practice identifying heart blocks on ECG strips.
• Review criteria for each heart block type.
• Explore the "six-second ECG" resource for additional practice.
• Prepare for immediate intervention protocols for complete heart block.