Atrioventricular block (AV block) is a type of heart block that occurs when the electrical signal traveling from the atria, or the upper chambers of the heart, to ventricles, or the lower chambers of the heart, is impaired. Normally, the sinoatrial node (SA node) produces an electrical signal to control the heart rate. The signal travels from the SA node to the ventricles through the atrioventricular node (AV node). In an AV block, this electrical signal is either delayed or completely blocked. When the signal is completely blocked, the ventricles produce their own electrical signal to control the heart rate. The heart rate produced by the ventricles is much slower than that produced by the SA node.
Some AV blocks are benign, or normal, in certain people, such as in athletes or children. Other blocks are pathologic, or abnormal, and have several causes, including ischemia, infarction, fibrosis, and drugs.
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Classification
There are three types, or degrees, of AV block: (1) first-degree, (2) second-degree, and (3) third-degree, with third-degree being the most severe. An ECG is used to differentiate between the different types of AV blocks. However, one important consideration when diagnosing AV blocks from ECGs is the possibility of pseudo- AV blocks which are due to concealed junctional extrasystoles. It is important to diagnose AV-blocks precisely because unnecessary pacemaker placement in patients with pseudo-AV blocks can worsen symptoms and create complications.
First-degree atrioventricular block
First-degree AV block occurs when there is a delay, but not disruption, as the electrical signal moves between the atrium and the ventricles through the AV node. On ECG, this is defined by a PR interval greater than 200 msec. Additionally, there are no dropped, or skipped, beats.
Mobitz I
Mobitz I is characterized by a progressive yet reversible block of the AV node. On ECG, this is defined by progressive prolongation of the PR interval, with a resulting dropped beat (the PR interval gets longer and longer until a beat is finally dropped, or skipped).
Third-degree atrioventricular block
Third-degree AV block occurs when the signal between the atria and ventricles is completely blocked, and there is no communication between the two. None of the signals from the upper chambers make it to the lower chambers. On ECG, there is no relationship between P waves and QRS complexes, meaning the P waves and QRS complexes are not in a 1:1 ratio.
Third-degree AV block is the most severe of the AV blocks. Persons with third-degree AV block need emergency treatment including but not limited to a pacemaker.
Etiology
There are many causes of AV block, ranging from a normal variant among people to the result of a heart attack.
First-degree AV block and Mobitz I second-degree block are often thought to be just normal, benign, conditions in people, and do not often result from a severe underlying condition.
Diagnosis
An electrocardiogram, or ECG, is used to differentiate between the different types of AV block. In AV block, there is a disruption between the signal traveling from the atria to the ventricles. This results in abnormalities in the PR interval, as well as the relationship between P waves and QRS complexes on the ECG tracing. If the patient is symptomatic from their suspected AV block, it is important that an ECG is also obtained while having symptoms. Physicians may also order a continuous ECG (i.e. Holter monitor or implanted cardiac monitor) to monitor the patient for symptoms and conduction abnormalities over a longer period of time, as AV blocks can be intermittent.
Because some types of AV block can be associated with underlying structural heart disease, patients may also undergo echocardiogram to look at the heart and assess the function. Based upon clinical suspicion, the physician may do lab tests to assess for reversible causes of AV block, such as hypothyroidism, rheumatologic disorders, and infections (such as Lyme disease).