Approach Considerations
A review of the patient's comorbidities and medications is essential, as oftentimes these individuals have underlying structural heart disease or coronary artery disease, or they may be taking atrioventricular (AV) node–blocking medications that may be causing or exacerbating the block.
Inspection of the rhythm strip or electrocardiogram (ECG) can help clinicians identify the type and level of block.
Laboratory Studies
Laboratory studies may be helpful in patients with second- or third-degree atrioventricular (AV) block. Blood tests may indicate hyperkalemia, acidosis, or drug toxicity as causes for AV block. In cases when AV block is suspected due to an acute myocardial infarction, obtain measures of cardiac enzymes.
If clinical evaluation suggests systemic illness, laboratory studies for infection, Lyme disease, myxedema, or connective tissue disease should be performed.
Electrocardiography
Routine electrocardiographic (ECG) recording and cardiac monitoring with careful evaluation of the relationship between the P waves and QRS complexes will lead to the diagnosis of the type of atrioventricular (AV) block.
Identifying transient AV block with sudden pauses and/or low heart rate causing syncope requires 24-hour Holter monitoring, multiple ECG recordings, event (loop) ECG recordings or, in selected cases, monitoring with implantable loop recorders.
See Background for more information regarding the ECG characteristics of each type of AV block.
See the images below.
Atrioventricular Block. This rhythm strip shows first-degree atrioventricular block with a PR interval of 0.360 sec. Note the fixed prolonged PR interval.
Atrioventricular Block. Second-degree Mobitz type I atrioventricular block. Note the prolongation of the PR interval preceding the dropped beat and the shortened PR interval following the dropped beat.
Atrioventricular Block. Second-degree Mobitz II atrioventricular block. Note the fixed PR interval, but after the third beat, an atrial impulse fails to conduct to the ventricle. Courtesy of Wikimedia Commons (https://commons.wikimedia.org/wiki/File:Second_degree_heart_block.png).
Atrioventricular Block. A constant PP interval and normal PR interval in conducted beats is present. This progresses to 2:1 atrioventricular (AV) block. A 2:1 AV block can be present with conduction delay in the AV node or His-Purkinje system, but it is more likely to be in the AV node for all patients (with a greater chance of AV block in the His-Purkinje system if there is a bundle branch block). Review extended monitoring strips because Mobitz I or Mobitz II may be present at other times, and this might help to determine the level of the block.
Atrioventricular Block. High-degree atrioventricular block is demonstrated with a 4:1 atrial-to-ventricular conduction ratio. Note the P wave prior to the QRS conducts whereas the others do not. Courtesy of Life in the Fast Lane (https://lifeinthefastlane.com/ecg-library/basics/high-grade-block/), Edward J Burns, MD, Sydney, Australia.
Atrioventricular Block. This rhythm strip shows third-degree atrioventricular block (complete heart block). The atrial rate is faster than the ventricular rate, and no association exists between the atrial and ventricular activity. Courtesy of Life in the Fast Lane (https://lifeinthefastlane.com/ecg-library/basics/complete-heart-block/), Edward J Burns, MD, Sydney, Australia.
Electrophysiologic Testing
An electrophysiology study (EPS) is indicated when atrioventricular (AV) block is the suspected cause of syncope and the diagnosis cannot be made by electrocardiography alone to identify the level of the block.
For patients with AV block regardless of syncope, EPS may have value in the management of the following conditions, as it will determine the level of AV block, predict progression of the disease, and identify the need for pacemaker therapy [17, 18] :
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Asymptomatic second-degree Mobitz I AV block with bundle branch block, class II
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Questionable second-degree Mobitz II AV block with a narrow QRS complex, class II
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Transient second-degree AV block, bundle branch block, and an inferior myocardial infarction where the site of the block is suspected to be in the His-Purkinje system rather than in the AV node, class I
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Third-degree AV block with an accelerated ventricular rate, class II
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Progressive conduction disease due to neuromuscular disorders or suspected SCN5A mutations [19]
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A prolonged HV interval (conduction time from the His bundle to the ventricular myocardium) (>0.100 sec) is a highly predictive but insensitive marker of high-degree AV block
Imaging Studies
Imaging studies are not generally indicated in diagnosing atrioventricular (AV) block. However, echocardiography can assess for aortic valve stenosis with calcification, wall-motion abnormalities in acute ischemia, cardiomyopathy, and congenital heart disease (eg, congenitally corrected transposition of the great vessels).
Coronary angiography or myocardial perfusion imaging may be necessary in evaluating for ischemia.
Fluorine-18 fluoro-2-deoxyglucose positron emission computed tomography (18-FDG PET) scanning has the potential to be prognostic in patients with unexplained AV block; higher adverse cardiac events and worse outcomes appear to be more likely in patients with high-degree AV block and abnormal 18-FDG-PET scan findings. [20]
Cardiovascular magnetic resonance (CMR) may be useful in identifying the underlying cause of advanced AV block, particularly in young- and middle aged patients. [21, 22, 23] Baritussio et al investigated the diagnostic value of CMR in young and middle-aged patients (n=34) with high grade AV block. The investigators identified a pathologic substrate in 44% of patients; combined with the standard clinical assessment, CMR had an additional diagnostic role in 65%. [23]
Exercise
Exercise may shorten the PR interval in first-degree atrioventricular (AV) block if it is related to an increased vagal tone, but the interval will not change if the block is in the His-Purkinje system. A severely prolonged PR interval may cause exercise intolerance and pacemaker syndrome.
Exercise may be used to evaluate 2:1 heart block and to differentiate Mobitz I second-degree AV block (where the conducted rate increases) from Mobitz type II AV block (where the block becomes more significant and, often, symptomatic).
Exercise is not recommended in patients with third-degree AV block.
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Atrioventricular Block. This rhythm strip shows first-degree atrioventricular block with a PR interval of 0.360 sec. Note the fixed prolonged PR interval.
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Atrioventricular Block. First-degree atrioventricular block. PR interval is constant and is 0.280 sec.
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Atrioventricular Block. Second-degree Mobitz type I atrioventricular block. Note the prolongation of the PR interval preceding the dropped beat and the shortened PR interval following the dropped beat.
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Atrioventricular Block. Second-degree atrioventricular block, Mobitz type I (Wenckebach). Note the prolongation of the PR interval preceding the dropped beat and the shortened PR interval following the dropped beat.
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Atrioventricular Block. Second-degree Mobitz II atrioventricular block. Note the fixed PR interval, but after the third beat, an atrial impulse fails to conduct to the ventricle. Courtesy of Wikimedia Commons (https://commons.wikimedia.org/wiki/File:Second_degree_heart_block.png).
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Atrioventricular Block. Second-degree atrioventricular block, Mobitz type II. A constant PR interval in conducted beats is present. Intraventricular conduction delay is also present.
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Atrioventricular Block. A constant PP interval and normal PR interval in conducted beats is present. This progresses to 2:1 atrioventricular (AV) block. A 2:1 AV block can be present with conduction delay in the AV node or His-Purkinje system, but it is more likely to be in the AV node for all patients (with a greater chance of AV block in the His-Purkinje system if there is a bundle branch block). Review extended monitoring strips because Mobitz I or Mobitz II may be present at other times, and this might help to determine the level of the block.
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Atrioventricular Block. High-degree atrioventricular block is demonstrated with a 4:1 atrial-to-ventricular conduction ratio. Note the P wave prior to the QRS conducts whereas the others do not. Courtesy of Life in the Fast Lane (https://lifeinthefastlane.com/ecg-library/basics/high-grade-block/), Edward J Burns, MD, Sydney, Australia.
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Atrioventricular Block. This rhythm strip shows third-degree atrioventricular block (complete heart block). The atrial rate is faster than the ventricular rate, and no association exists between the atrial and ventricular activity. Courtesy of Life in the Fast Lane (https://lifeinthefastlane.com/ecg-library/basics/complete-heart-block/), Edward J Burns, MD, Sydney, Australia.
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Atrioventricular Block. Third-degree atrioventricular block (complete heart block). The atrial rate is faster than the ventricular rate, and no association exists between the atrial and ventricular activity.
