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From a cardiac perspective: Do you know the three types of atrioventricular block?
Atrioventricular block (AV block) is a heart conduction abnormality in which electrical signals from the atria to the ventricles are impaired. Under normal circumstances, the sinoatrial node (SA node) generates electrical signals that control the heartbeat, and this signal is transmitted to the ventricles through the atrioventricular node (AV node). In AV block, this electrical signal may be delayed or completely blocked. In severe cases, the ventricles generate their own electrical signals to control the heartbeat, often at a rate much lower than that produced by the sinoatrial node. Some forms of AV block are benign in athletes or children, but pathological conditions may also be present, possibly caused by ischemia, myocardial infarction, fibrosis, or drug influences.
Category
There are three main types of atrioventricular block: grade 1, grade 2, and grade 3, with grade 3 being the most severe. An electrocardiogram (ECG) can be used to differentiate between different types of atrioventricular block. However, when diagnosing AV block, doctors need to pay special attention to the condition of pseudo-AV block, which is caused by occult junctional abnormalities. If not accurately diagnosed, unnecessary pacemaker implantation in patients with pseudo-AV block may worsen the condition and lead to complications.
First degree atrioventricular block
First-degree AV block occurs when electrical signals are delayed but not interrupted in their passage through the AV node. On an electrocardiogram, this condition is characterized by a PR interval of more than 200 milliseconds (msec) and no missed beats (or skipped beats).
Second degree atrioventricular block
Second-degree atrioventricular block is more severe than first-degree atrioventricular block, and the pathways of electrical signals are damaged to a greater extent. In second-degree atrioventricular block, certain electrical signals lose conduction, resulting in dropped beats.
Mobitz I
Mobitz I is characterized by gradual and reversible blockage of the AV node. On the electrocardiogram, this condition manifests itself as a gradual prolongation of the PR interval, eventually leading to missed beats. Some patients may be asymptomatic, while symptomatic patients may easily experience relief with treatment. Mobitz I AV block has a lower risk of causing complete heart block or asystole.
Mobitz II
Mobitz II is caused by the sudden inability of His-Purkinje cells to effectively conduct electrical signals. Unlike Mobitz I, the PR interval remains unchanged in the ECG, but unexpected loss of conduction occurs, followed by random dropped beats. Mobitz II has a higher risk of progression, which may lead to complete heart block or asystole.
Third degree atrioventricular block
Third-degree atrioventricular block is the most severe condition, in which the signal between the atria and ventricles is completely blocked and no communication can occur between the two. At this time, there is no proportional relationship between P waves and QRS waves in the electrocardiogram, which is a characteristic of third-degree atrioventricular block. Such patients require urgent treatment, including the installation of a pacemaker.
Cause
The causes of AV block vary from normal variations to the result of a heart attack. First-level AV blocks and Mobitz I second-level blocks are generally considered to be benign conditions that do not often lead to serious underlying conditions. In contrast, Mobitz II second-degree block and third-degree AV block are not normal variations and are usually related to some underlying health problem. Common causes include ischemia or progressive fibrosis of the heart.
Diagnosis
The preferred method of diagnosing AV block is an electrocardiogram, which can help identify different types of AV block. On the electrocardiogram, the relationship between P waves and QRS complexes and abnormalities in the PR interval can reveal the possibility of AV block. If a patient is significantly symptomatic, an ECG should be performed while symptomatic. Additionally, the physician may order serial electrocardiograms to monitor the patient's symptoms and conduction abnormalities over a longer period of time, since AV block may be intermittent.
Management
Management of AV block depends on the severity of the block, consistency of symptoms, and cause. Patients with first- and second-degree Mobitz I AV block usually do not require treatment because they do not develop life-threatening symptoms. However, patients with Mobitz II and III AV block are more likely to require a temporary pacemaker because they may experience a more threatening and unstable condition. If the heart block is caused by a reversible condition, these underlying conditions should be addressed first, which often results in resolution of the heart block and associated symptoms.
The characteristics of atrioventricular block and its management remain a topic of interest in cardiology. Can we develop a deeper understanding of these complex cardiac conduction systems to prevent potential heart disease?
