eMedicine Specialties > Cardiology > Arrhythmias
Second-Degree Atrioventricular Block: Differential Diagnoses & Workup
Updated: Sep 2, 2009
- Overview
- Differential Diagnoses & Workup
- Treatment & Medication
- Follow-up
- Multimedia
Differential Diagnoses
Third-Degree Atrioventricular Block
Other Problems to Be Considered
Sinoatrial exit block
Nonconducted premature atrial contractions
Workup
Laboratory Studies
- Serum electrolyte and magnesium levels
- Serum digoxin level
- Thyroid function studies
- Other drug levels (when clinically suspected)
Other Tests
- Type I AV block is the most common form of second-degree AV block. The typical electrocardiographic findings of type I block (Wenckebach periodicity; see Media file 1) include the following:
Typical Wenckebach atrioventricular block with progressive prolongation of the PR interval before the blocked P wave. The pauses are always less than the sum of the 2 preceding beats because the PR interval after the pause always shortens.
- Gradually progressive PR interval prolongation occurs prior to the blocked sinus impulse, with the greatest PR increment occurring typically between the first and second beat of a cycle, gradually decreasing in subsequent beats.
- Shortening of the PR interval occurs after the blocked sinus impulse, provided the P wave is conducted to the ventricle. A common situation after type I block is the occurrence of junctional escape beats along with nonconducted P waves.
- A pause occurs after the blocked P wave that is less than the sum of the 2 beats before the block.
- A type I block manifesting with atypical electrocardiographic findings that result in a misdiagnosis of type II block is not uncommon. During very long sequences (typically >6:5), PR-interval prolongation may be minimal until the last beat of the cycle, when it prolongs abruptly. Postblock PR interval shortening remains the cornerstone of the diagnosis of type I block, regardless of whether the periodicity has typical or atypical features.
- Type II block (see Media file 2) is characterized by the following:
Mobitz II atrioventricular (AV) block with intermittent periods of 2:1 AV block. If only 2:1 block was seen in the beginning of the strip, then the site of block could not be localized with certainty; however, the single dropped QRS complex at the end of the strip with a constant PR interval indicates that this block is localized in one of the bundle branches.
- Consecutively conducted beats with the same PR interval are followed by a blocked sinus P wave.
- A PR interval in the first beat occurs after the block, with the same PR interval as the previous beats.
- A pause encompassing the blocked P wave is equal to exactly twice the sinus cycle length.
- Evaluating for stability of the sinus rate is important because conditions associated with increases in vagal tone may cause simultaneous sinus slowing and AV block and, therefore, mimic a type II block.
- In addition, diagnosing type II block in the presence of a shortened postblock PR interval is impossible. This sequence can be secondary to enhanced conduction or a nonconducted P wave occurring with a junctional escape beat. Prolonged electrocardiographic recordings or intracardiac recordings may be needed to establish the correct site of block, ie, AV nodal versus infranodal.
- Type II block is typically associated with significant underlying conduction system disease. Therefore, the QRS complex is usually wide, and the PR interval is usually normal. However, a long PR interval and a narrow QRS complex does not exclude type II block because AV nodal conduction disease may coexist with an infranodal lesion. Another consideration in a type II block with narrow QRS is atypical type I block.
- Determining the site of the block
- A type I block with a narrow QRS complex is almost always located in the AV node (see Media file 3).
Variable-ratio type I atrioventricular (AV) block. Note the marked PR-interval prolongation in the first beat of each cycle. The maximum prolongation of the PR interval takes place in the second beat of the cycle, with much smaller increments in subsequent beats. Also, notice that the R-R interval actually shortens with each beat. This is the paradox of a shortening R-R interval when the PR interval increases by diminishing increments.
- An exception is the rare occurrence of an infranodal Wenckebach block. A normal PR interval with miniscule increments in AV conduction delay should raise the suggestion of an infranodal Wenckebach block but is not a diagnostic finding. Larger increments in AV conduction do not necessarily exclude infranodal Wenckebach block.
- In the presence of a wide QRS complex, a type I block is more often infranodal (see Media file 4). An invasive His bundle recording is required to make the diagnosis of an infranodal block.
Sinus rhythm with second-degree type I 3:2 infranodal atrioventricular (AV) block and bifascicular block. Note that the AH interval (indicative of AV nodal conduction) remains constant. The HV interval (indicative of His-Purkinje conduction) increases from 65 milliseconds (following the first P wave) to 185 milliseconds (following the second P wave). The third P wave is followed by a His bundle deflection (H) but no QRS complex. An AV block occurs in the His-Purkinje system below the site of recording of the His bundle potential. Note the shorter PR interval after the nonconducted P wave, a feature typical of type I AV block. HRA indicates high right atrial electrogram; A, atrial deflection; HB, His bundle electrogram, proximal and distal; H, His bundle deflection; RV, right ventricular electrogram; T, time line, 50 milliseconds.
- The incremental pattern of AV block may be helpful in determining the correct site of block. For example, an increment in PR interval of longer than 100 milliseconds favors a block site in the AV node.
- A type I block with a narrow QRS complex is almost always located in the AV node (see Media file 3).
- A type II block is always infranodal.
- An infranodal block is associated with a wide QRS complex and accounts for the majority of type II blocks.
- Less commonly, the block is intranodal and, therefore, is associated with a narrow QRS complex.
- Sinus slowing with AV block is characteristic of vagal activation and effectively excludes a type II block.
- Among the conditions that may mimic type II block are atypical type I block, junctional parasystole, and concealed extrasystoles arising from the His-Purkinje system.
- A 2:1 block can be either in the AV node or in the His-Purkinje system (see Media file 5).
Representative 12-lead ECG in an asymptomatic 78-year-old woman during recent noncardiac surgery. The patient was referred for implantation of a permanent pacemaker with a diagnosis of sinus tachycardia with 2:1 atrioventricular (AV) block and a narrow QRS complex. As the sinus rate slowed, 1:1 AV conduction resumed. Intracardiac recordings confirmed the diagnosis of an infra-Hisian 2:1 AV block.
- If the QRS complex is narrow, the block is more likely located in the AV node.
- If a wide QRS complex is present, the block may be located either in the His-Purkinje system (80-85%) or, less commonly, in the AV node (15-20%).
- Observing for a narrow QRS, type II–like block and a type I block in close temporal proximity can sometimes help determine the correct site of block. A true type II block almost never coexists with an intra-Hisian type I block.
- Autonomic manipulation (eg, carotid sinus massage, exercise) may help distinguish between an AV nodal and a His-Purkinje (infranodal) block.
- Improvement in the degree of block with exercise strongly favors an AV nodal location.
- Conversely, an increase in block with exercise or atropine more strongly favors a His-Purkinje (infranodal) block.
- Compared with exercise, vagal maneuvers have opposite effects on AV blocks, exacerbating the AV nodal blocks and improving infranodal blocks.
- Acute myocardial infarction
- AV block localized to the infranodal specialized conduction system occurs in as many as 5% of patients with acute anterior MI, giving rise to a wide QRS type II block.
- A transient AV block of any degree in acute inferior MI almost always is AV nodal and not an indication for permanent pacing.
- Electrophysiology study may be indicated to establish the level of the block. The indications for electrophysiology study may be as follows:
- In patients in whom His-Purkinje (infranodal) block is suspected but has not been confirmed, such as patients with the following:
- Type I AV block associated with a wide QRS complex in the absence of symptoms
- Second-degree AV block of the 2:1 form with a wide QRS complex in the absence of symptoms
- Type I block with history of unexplained syncope
- In patients with pseudo AV block and those with premature, concealed junctional depolarization, which may be the cause of second- or third-degree AV block
- In patients with second- or third-degree AV block in whom another arrhythmia is suspected as the etiology of the symptoms (eg, those who remain symptomatic after pacemaker placement).
- In patients in whom His-Purkinje (infranodal) block is suspected but has not been confirmed, such as patients with the following:
Procedures
- Diagnostic electrophysiologic studies can help determine the site of block and the potential need for a permanent pacemaker.
More on Second-Degree Atrioventricular Block |
| Overview: Second-Degree Atrioventricular Block |
 Differential Diagnoses & Workup: Second-Degree Atrioventricular Block |
| Treatment & Medication: Second-Degree Atrioventricular Block |
| Follow-up: Second-Degree Atrioventricular Block |
| Multimedia: Second-Degree Atrioventricular Block |
| References |
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Further Reading
Keywords
second-degree atrioventricular block, heart block, 2nd degree heart block, second-degree AV block, 2nd degree AV block, AV block, Mobitz AV block, Mobitz heart block, Mobitz type I, Mobitz type II, Wenckebach phenomenon, Wenckebach heart block, high-grade AV block, complete heart block, third-degree AV block, Stokes-Adams syncopal attack, heart failure, angina, acute myocardial infarction, sinus slowing, cardioactive drugs, endocarditis, myocarditis, Lyme disease, acute rheumatic fever, amyloidosis, hemochromatosis, sarcoidosis, hyperkalemia, hypermagnesemia, Addison disease, ankylosing spondylitis, dermatomyositis, rheumatoid arthritis, scleroderma, lupus erythematosus, Reiter syndrome, progressive idiopathic fibrosis of the cardiac skeleton, aortic stenosis, aortic valve replacement surgery, muscular dystrophies, corrective congenital heart surgery
