AV Conduction Blocks
First-Degree AV Block
First-degree AV block is a delay in conduction, not a true block. First-degree block represents conduction delay somewhere between the sinus node and ventricles. It most often occurs at the site of the AV node when a sinus impulse encounters a part of the AV node with a prolonged refractory period. The delay may also be located within the atria or bundle of His.
Causes
Increased Parasympathetic Tone
First-degree block can be a normal finding in patients with increased parasympathetic (vagal) tone. Causes of increased vagal tone include sleeping, vomiting, coughing, and severe pain. Sinus bradycardia coexistent with first-degree block is a clue that the cause of the first-degree block is increased parasympathetic tone.
AV Nodal Blocking Agents
Beta-blockers, calcium channel blockers, and digitalis.
Ischemia
Can especially be seen with inferior wall MI.
Infiltrative Diseases
Myocarditis
Congenital Structural Heart Disease
Ebstein anomaly of the tricuspid valve, endocardial cushion defects.
ECG Features
All P waves are conducted in first-degree block (each P wave is followed by a QRS complex). The PR interval is prolonged, lasting more than 200 msec.
Pitfalls in Diagnosis
Extreme First Degree Block
The duration of the PR interval can be as long as 1 second.1 In extremely prolonged PR intervals, the P wave may be mistaken entirely for or become obscured by a T wave. When P waves are completely obscured by T waves, the rhythm may be mistakenly identified as a junctional bradycardia (Fig. 4.2).
 FIGURE 4.1 A. Diagram of firstdegree AV block. B. ECG appearance of first-degree block. C. PR interval compared with PR segment. |
This could result in simultaneous contraction of both atria and ventricles and decreased cardiac output.
 FIGURE 4.2 A. P wave appears in place of T wave. B. P wave fused with T wave. |
If the P wave were mistaken for a T wave, P waves would appear to be completely absent from the rhythm.
Second-Degree AV Block: Mobitz I
Mobitz I block (also called Wenckebach block) usually occurs in the AV node. It rarely progresses to complete heart block. This type of block is often reversible.
Mechanism
In Mobitz I block, each successive atrial impulse finds the AV node in an earlier phase of the refractory period and takes longer to reach the ventricles.2 When an impulse finds the AV node in its absolute refractory period, it becomes blocked (Fig. 4.3A, B).
 FIGURE 4.3 A. Schematic of refractory periods. The earlier in the refractory period that a sinus impulse arrives, the longer the conduction time. B. Diagram of Mobitz I block at the AV node. C. ECG appearance of Mobitz I block |
Causes
Mobitz I block may be a normal physiologic finding in healthy individuals with enhanced vagal tone. Beta-blockers, calcium channel blockers, and digitalis are extrinsic reversible causes. The acute phase of an inferior MI may cause block at the level of the AV node. Inflammation and degenerative disease of the conduction system are other common causes.
ECG Features
Lengthening of PR Intervals
Mobitz I is characterized by progression of PR interval length followed by nonconducted P waves. The greatest increase in conduction time occurs between the first and second beats (Fig. 4.3C). The absolute increase in conduction time decreases with each subsequent beat in a Wenckebach cycle.1
A PR interval may shorten and lengthen before the nonconducted beat. A few beats preceding the nonconducted P wave may show no change in PR interval duration.3
Variable R-R Intervals
The R-R interval immediately following a pause is longest and progressively shortens with each subsequent cardiac cycle within a Wenckebach group.
PR Shortest after the Block
The PR interval is predictably the shortest after the block. Sometimes, this is the only way to tell that the second-degree block is Mobitz I. Progressive prolongation of the PR interval can be very subtle.
 FIGURE 4.4 ECG appearance of Mobitz I second-degree block with only slight lengthening of PR intervals preceding a blocked impulse. |
Grouped Beating
Blocked P waves can separate QRS complexes into groups. QRS complexes tend to appear in pairs, as the most common atrial-to-ventricular conduction ratio is 3:2.
Location
Mobitz classification is based on ECG patterns, not the anatomical location of the block.
AV Nodal
Type I block is almost always at the level of the AV node. QRS complexes are typically narrow.
Infranodal
Not all Mobitz I blocks occur at the AV node. They can also occur at the level of the His-Purkinje system. Wider QRS complexes typically accompany infranodal Mobitz I block.
Pitfalls
A common pitfall is to mistake blocked premature atrial complexes (PACs) for seconddegree AV block. PACs occur earlier than would be suspected of a sinus beat, and their associated P waves often have a different morphology than that of the sinus P wave.
Second-Degree AV Block: Mobitz II
This is an unstable form of conduction that most often occurs in the bundle branches. This progresses to complete heart block.
Mechanism
In Mobitz II block, one bundle branch is continuously blocked while the other is only intermittently blocked. Intermittent block results in QRS complexes that are dropped (Figs. 4.5A and 4.6A).
Location
This is typically an infranodal block. Typically, it occurs at the level of the bundle branches. Only rarely does it occur more proximally at the His bundle.
Bundle Branch
Bundle of His
ECG Features
Constant PR Length
The PR interval is fixed and does not shorten or lengthen before a dropped beat.
Wide QRS Complexes
The QRS complexes of conducted beats typically appear similarly to those associated with right or left bundle-branch block (Figs. 4.5B and 4.6B).
Causes
Causes of type II block are less likely to be reversible. Common causes include calcification or fibrosis of the conduction system. Infiltrative diseases and ischemia are also notable causes.
Pitfalls
A common mistake is to classify 2 to 1 AV block as Mobitz II because the PR intervals across the ECG appear fixed. It is best to classify this rhythm as “2:1 AV block.”
It is important to be able to distinguish between the different types of second-degree block. The natural history and management differ.
 FIGURE 4.5 A. Mobitz II block with a fixed block in the left bundle. Conducted beats proceed down the right bundle. B. They appear as wide QRS complexes with an LBBB morphology on the ECG. A nonconducted beat occurs when conduction fails to pass through the right bundle branch (in addition to the fixed left bundle-branch block). |
 FIGURE 4.6 A. Mobitz II block with a fixed block in the right bundle. B. Conducted beats appear as wide QRS complexes with an RBBB morphology on the ECG. A nonconducted beat occurs when conduction fails to pass through the left bundle branch. |
TABLE 4.1 Comparison of Mobitz I and Mobitz II Second-Degree AV Block | ||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
Third-Degree AV Block
Third-degree AV block is also referred to as complete heart block. Complete heart block is one form of AV dissociation in which the atria and ventricles are controlled by independent pacemakers and contract irrespective of each other. In complete heart block, the atrial rate is faster than the ventricular rate. P waves “march out” through the ECG, unrelated to QRS complexes. The ventricular rhythm is an escape rhythm (from a junctional focus or more distal in the Purkinje system). Escape rhythms are notably regular as they are less influenced by autonomic control.2
Causes
Infiltrative
Amyloidosis, sarcoidosis, hemochromatosis, hypothyroidism
Drug Toxicity
Digitalis, beta-blocker, calcium channel blocker, antiarrhythmics
Ischemia
Inferior MI and anterior MI
Infection/Autoimmune
Lyme carditis, Chagas disease, diphtheria, acute rheumatic fever, infectious endocarditis, tuberculosis
Fibrosis
Lenegre disease, Lev disease
Congenital
Anti-Ro and anti-La antibodies (maternal Sjögren disease or lupus)
Electrolyte Disturbance
Severe hyperkalemia
Location
Complete heart block can occur at the AV node or anywhere more distal to it. Block at the level of the AV node is most often congenital. Acquired causes of AV block are usually distal to the His bifurcation and reflect trifascicular conduction disturbance.1
AV Node
If complete block is at the level of the AV node, a junctional escape rhythm will likely take over (Fig. 4.7).
His Bundle
Bundle Branches
The escape rhythm for infranodal block is ventricular. Prior ECGs demonstrating bundle-branch block favor the likelihood of infranodal block.
 FIGURE 4.7 Possible locations of third-degree AV block (highlighted in gray) include the AV node, bundle of His, and below the His bifurcation. The focus of escape typically occurs just below the site of the block. |
Clinical Significance
Cardiac conduction and contraction become dependent on an escape rhythm. If an escape rhythm fails to take over and to provide sufficient cardiac output, the patient will develop syncope or sudden death.
Pitfalls
A common pitfall is to assume that complete heart block is the cause of all forms of AV dissociation. Complete heart block is one of several causes of AV dissociation.
Complete absence of atrial activity is not synonymous with complete heart block. Junctional rhythms are often mistakenly referred to as complete heart block.
Another pitfall is failure to recognize atrial fibrillation with a regular ventricular response (also known as “regularized afib”) as complete heart block.
AV Dissociation
This is a semantically confusing topic. Complete AV dissociation can be thought of as a broad term to describe any rhythm in which the atria and ventricles conduct independently of each other. Complete heart block is only one form of AV dissociation. Other mechanisms can cause AV dissociation.
Decreased Sinus Automaticity
 FIGURE 4.8 A. Isorhythmic dissociation with a junctional escape. B. Mechanism. |
In isorhythmic dissociation, the atria and ventricles conduct independently, but at nearly the same rate. The spontaneous depolarization of the SA node slows enough for the rate to fall just below the lower normal limit of a normal sinus rhythm (60 bpm). A junctional focus then takes over the role as pacemaker, and a junctional escape rhythm occurs. This is not a pathologic rhythm. Conduction between the sinus and AV node is intact, but the two different pacemakers conduct to the atria and ventricles independently.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
