What are first, second, and third degree heart blocks and their types?

Heart Blocks: First, Second, and Third Degree and Their Types

Heart blocks are classified as first, second, and third degree based on the severity of conduction impairment, with specific subtypes determined by the anatomical location and pattern of the block. 1

First-Degree AV Block

• First-degree AV block is characterized by delayed conduction where every atrial impulse is conducted to the ventricles but with a prolonged PR interval >200 ms 1
• It's more accurately described as "first-degree AV delay" rather than a true block since all impulses are conducted 1
• The conduction delay may occur at the level of the AV node or His-Purkinje system 1
• When the QRS is narrow, the delay is usually in the AV node; with wide QRS, the delay could be in either the AV node or His-Purkinje system 1
• Profound first-degree AV block (PR interval >300 ms) can cause symptoms similar to pacemaker syndrome due to loss of AV synchrony, including fatigue and exercise intolerance 1

Second-Degree AV Block

Second-degree AV block is characterized by intermittent failure of atrial impulse conduction to the ventricles, with two main types:

Mobitz Type I (Wenckebach)

• Progressive lengthening of the PR interval until an atrial impulse fails to conduct to the ventricles 1
• The PR interval is shortest after the blocked P wave 1
• Usually occurs at the AV node level, especially with narrow QRS 1, 2
• Traditionally considered more benign than Type II, but some evidence suggests similar prognosis to Type II in chronic cases 3
• Rarely progresses to higher degrees of block when the site is the AV node 1

Mobitz Type II

• Sudden failure of conduction without prior PR prolongation 1
• PR interval remains constant before and after blocked P waves 1, 2
• Usually occurs below the AV node in the His-Purkinje system, especially with wide QRS 1, 2
• Higher risk of progression to complete heart block and Stokes-Adams arrest 2
• More likely to cause symptoms and compromise prognosis 1

2:1 AV Block

• Cannot be classified as Mobitz I or II based on ECG pattern alone 1
• Requires determination of the level of block for proper classification 1
• May be considered a form of high-grade AV block when part of a pattern with multiple consecutive blocked P waves 1

Advanced/High-Grade AV Block

• Defined as two or more consecutive P waves not conducted to the ventricles without complete loss of AV conduction 1
• Generally considered to be intra- or infra-Hisian in origin 1
• Requires pacing treatment in most cases 1

Third-Degree (Complete) AV Block

• No atrial impulses are conducted to the ventricles 1
• Ventricles are depolarized by an escape rhythm (junctional or ventricular) 1, 4
• The escape rhythm's origin determines many clinical features:
  • AV nodal escape: Typically narrow QRS, rate 40-60 bpm, more stable 1, 4
  • Ventricular escape: Wide QRS, slower rate (20-40 bpm), less stable 4
• Complete heart block is seen in 8% of post-MI patients 4
• Permanent pacing improves survival, especially in patients with syncope 1

Anatomical Classification of AV Blocks

AV blocks can also be classified based on the anatomical location of the conduction disturbance:

• Supra-Hisian (AV nodal): Usually more stable, responds to atropine, has faster and more reliable junctional escape rhythm 1
• Intra-Hisian: Within the His bundle, may progress rapidly 1
• Infra-Hisian: Below the His bundle in the Purkinje system, associated with slower and unpredictable ventricular escape mechanism, higher risk of sudden progression 1

Clinical Significance and Management

• First-degree AV block was historically considered benign, but marked prolongation (>300 ms) can cause symptoms and hemodynamic compromise 1, 5
• Type II second-degree AV block typically indicates diffuse conduction system disease and constitutes an indication for pacing even without symptoms 1
• Complete AV block requires permanent pacing, especially if symptomatic 1
• The site of block (nodal vs. infranodal) is more important for prognosis than the degree of block in some cases 1
• ECG interpretation must be careful to distinguish true AV block from other causes of pauses such as non-conducted premature atrial contractions 2

Heart blocks represent a spectrum of conduction abnormalities with varying clinical implications depending on the type, location, and associated symptoms, requiring individualized assessment for appropriate management decisions.