Background

A third-degree atrioventricular block, also known as a complete heart block, is a severe cardiac condition characterized by a complete interruption of the electrical conduction between the atria and ventricles of the heart.

In a normal heart, electrical signals originating in the sinoatrial (SA) node in the atria travel through the atrioventricular (AV) node, which then conducts these signals to the ventricles, causing them to contract and pump blood.

The atria continue to generate their own electrical signals, and the ventricles generate their own, usually slower, signals. As a result, the coordination between atrial and ventricular contractions is lost. 

Epidemiology

Third-degree AV block is relatively uncommon compared to other cardiac conduction disorders. Incidence and prevalence rates may increase with age, as degenerative changes in the conduction system are more likely in older individuals.

The condition is more commonly seen in older adults, especially those above the age of 65. It can affect both genders, but there might be a slightly higher prevalence in men. Geographic and cultural factors may also contribute to variations in the prevalence of specific causes. 

Anatomy

Pathophysiology

Third-degree atrioventricular (AV) block, also known as complete heart block, involves a disruption in the normal electrical conduction system of the heart, leading to a complete dissociation between atrial and ventricular contractions. In a healthy heart, electrical signals generated in the sinoatrial (SA) node travel through the atrioventricular (AV) node, facilitating the coordinated contraction of the atria and ventricles.

In third-degree AV block, this conduction is severely impaired, resulting in the atria and ventricles functioning independently. The atria continue to generate their electrical impulses, maintaining their rhythm, while the ventricles generate their own, typically slower signals. As a consequence, there is a lack of synchronization between the upper and lower chambers of the heart. The atria and ventricles contract at their rates, leading to a significant reduction in cardiac output.

The condition can be caused by various factors, including aging, structural heart diseases, myocardial infarction, inflammatory conditions affecting the heart, and certain medications. Clinically, third-degree AV block can manifest with symptoms such as fatigue, dizziness, shortness of breath, and, in severe cases, fainting. Prompt diagnosis and appropriate intervention, often involving the placement of a pacemaker, are crucial in managing this condition and preventing potential complications, including cardiac arrest. 

Etiology

• Age-Related Degeneration: Aging is a significant risk factor for the development of third-degree AV block. Degenerative changes in the electrical conduction system of the heart can lead to impaired signal transmission between the atria and ventricles. 

• Ischemic Heart Disease: Coronary artery disease, atherosclerosis, and myocardial infarction can damage the heart tissue, affecting the conduction pathways and increasing the risk of AV block. 

• Structural Heart Diseases: Conditions such as cardiomyopathy, hypertrophic cardiomyopathy, and other structural abnormalities of the heart can disrupt the normal conduction system, leading to AV block. 

• Inflammatory Conditions: Myocarditis, an inflammation of the heart muscle often caused by viral infections, can disrupt the conduction system and contribute to the development of third-degree AV block. 

• Medications: Certain medications, especially those that affect the electrical conduction of the heart, can contribute to the development of AV block. This includes drugs like beta-blockers, calcium channel blockers, and antiarrhythmic medications. 

• Congenital Factors: In some cases, third-degree AV block can be congenital, meaning it is present from birth. Congenital heart defects or abnormalities in the conduction system may lead to complete heart block. 

• Autoimmune Disorders: Some autoimmune diseases, such as rheumatoid arthritis and systemic lupus erythematosus, can affect the heart and contribute to the development of conduction abnormalities. 

• Electrolyte Imbalances: Abnormal levels of electrolytes, particularly potassium, can disrupt the electrical signaling in the heart and potentially lead to AV block. 

• Idiopathic Causes: In some instances, third-degree AV block may occur without an apparent underlying cause, and it is classified as idiopathic. 

Genetics

Prognostic Factors

The underlying cause of third-degree AV block often influences the prognosis. The prognosis is generally better for individuals who receive appropriate and timely treatment. The age and overall health of the individual also contribute to the prognosis.

Younger individuals and those without significant comorbidities may generally have a better prognosis. The presence of other medical conditions, such as hypertension, diabetes, or other cardiovascular diseases, can impact the overall prognosis and complicate the management of third-degree AV block. 

Clinical History

Patients with third-degree atrioventricular (AV) block commonly exhibit diverse clinical manifestations, with few presenting without symptoms. Typically, individuals may complain of generalized fatigue, chest pain, shortness of breath, feelings of near-fainting (presyncope), or actual syncope. Some patients may experience significant hemodynamic instability, and in severe cases, they can become obtunded.

The clinical presentation varies based on factors such as concurrent diseases and the rate of the escape rhythm. In instances where complete AV block coincides with an acute myocardial infarction, patients may exhibit ischemic symptoms such as dyspnea or chest pain. Notably, their medical history often reveals a background of cardiovascular disease or risk factors, including but not limited to diabetes mellitus, dyslipidemia, hypertension, and a history of smoking. 

Physical Examination

The physical examination typically reveals bradycardia as a prominent finding. Examination of the jugular venous pressure often reveals cannon A-waves, a result of simultaneous atrial and ventricular contractions creating a substantial pressure wave against the vein. Particularly in cases where heart rates fall below 40/min, patients may exhibit signs indicative of decompensated heart failure, including hypoperfusion, respiratory distress, diaphoresis, altered mental status, retractions, tachypnea, clammy skin, and decreased capillary refill.

The presence of new murmurs is noteworthy, as there is a significant association between complete AV block and conditions such as mitral calcification, cardiomyopathies, aortic calcification, or endocarditis. In instances where coexistent heart failure is evident through findings like an S gallop, peripheral edema, or hepatomegaly, immediate pacing becomes a critical aspect of management. Attention should also be given to signs of infection or skin rashes, considering conditions such as rheumatic fever, Lyme disease, and endocarditis, all of which can contribute to the development of heart blocks

Age group

Associated comorbidity

Associated activity

Acuity of presentation

Differential Diagnoses

Laboratory Studies

Imaging Studies

Procedures

Histologic Findings

Staging

Treatment Paradigm

Treatment for third-degree AV block typically involves interventions to manage symptoms and restore adequate heart rate and rhythm. Temporary measures may include the use of external pacemakers to provide external electrical stimulation and maintain a stable heart rate. In the long term, permanent pacemaker implantation is often considered to regulate the heart’s rhythm by delivering electrical impulses as needed.

Regular monitoring and follow-up care are crucial to assess the pacemaker function and adjust settings as necessary. Additionally, addressing underlying causes, such as myocardial infarction or certain medications, is essential to optimize overall cardiovascular health. 

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Chemotherapy

Radiation Therapy

Surgical Interventions

Hormone Therapy

Immunotherapy

Hyperthermia

Photodynamic Therapy

Stem Cell Transplant

Targeted Therapy

Palliative Care

Administration of a pharmaceutical agent

The initial approach to managing symptomatic bradycardic patients involves the administration of intravenous atropine, in accordance with advanced cardiac life support recommendations.

However, atropine primarily affects the AV node and is generally ineffective in elevating the heart rate for individuals with complete heart block. Consequently, alternative medical interventions for addressing symptomatic bradycardia include the use of dopamine and epinephrine.

It is crucial to recognize that these options may offer only temporary support and could prove ineffective in augmenting the heart rate of patients with third-degree AV block. In such cases, pacing often becomes necessary. While transcutaneous pacing is a swifter option, ensuring both electrical and mechanical capture is essential. 

intervention-with-a-procedure

If transcutaneous pacing proves ineffective, the consideration of a transvenous pacemaker may be necessary on an individualized basis. The success of pacing is contingent upon addressing underlying diseases contributing to heart block, particularly in cases involving drug toxicity.

While pacing might be attempted in such patients, the primary focus should be on treating the root cause. Individuals with high-degree AV block, Mobitz II block, and third-degree AV block require prompt admission for cardiac monitoring, thorough evaluation, potential consideration of backup temporary cardiac pacing as needed, and eventual insertion of a permanent pacemaker.

Existing evidence, coupled with robust clinical consensus, underscores the imperative for individuals with persistent second or third-degree AV block to undergo permanent cardiac pacing therapy. 

Medication

Future Trends

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