Overview.
Delayed or interrupted conduction of atrial impulses to the ventricles
Some patients may be asymptomatic, while others may experience fatigue, palpitations, weakness, dizziness, and syncope.
Causes include physiologic factors and pathologic factors such as myocardial lesions, conduction system defects, and conduction system lesions.
Some patients do not require treatment, while others require pharmacologic and pacing therapy.
Definition
Second-degree atrioventricular block is a disease of the cardiac conduction system caused by a delay or interruption in the conduction of the atrial impulse to the ventricles, and is a type of atrioventricular block.
Heart block is a permanent or temporary impulse conduction disorder due to an anatomical or functional malfunction that can occur at any level of the heart’s conduction system, and occurs between the atria and the ventricles, called atrioventricular block.
According to the severity of the conduction block, it is usually categorized into three degrees.
一度房室传导阻滞的房室传导时间延长，但全部冲动仍能传导。
二度房室传导阻滞分为两型即Ⅰ型和Ⅱ型。Ⅰ型阻滞表现为传导时间进行性延长，直至一次冲动不能传导；Ⅱ型阻滞表现为间歇出现的传导阻滞。
三度房室传导阻滞又称完全性房室传导阻滞，此时全部冲动不能被传导。
Staging
Depending on where the conduction block occurs, second-degree AV block is categorized as type I and type II.
Second-degree type I atrioventricular block
Mohs’ type I, also known as Mann’s block, has the site of the block located in the sinus node and usually rarely progresses to third-degree atrioventricular block.
Second-degree type II atrioventricular block
Mohs’ type II, with the site of block located in the Hirschsprung’s bundle-Purkinje system, can progress to a high degree of atrioventricular block.
Incidence
In China, there is no exact epidemiologic survey data on second-degree AV block.
The prevalence of second-degree AV block among young people reported in the United States is approximately 0.003%.
Approximately 3% of patients with underlying structural heart disease will develop some type of second-degree AV block.
The male-to-female ratio of patients with second-degree AV block is 1:1.
Causes
Causes
There are many causes that can lead to second-degree AV block, and any cause that can have an effect on one part of the atrioventricular conduction process may lead to second-degree AV block.
Physiologic Factors
Higher vagal tone may be present in young people or resting athletes, causing second-degree type I AV block.
Inflammatory lesions of the myocardium
Inflammatory lesions of the myocardium are the most common cause of second-degree AV block, such as bacterial myocarditis, viral myocarditis, rheumatic myocarditis, and endocarditis.
Myocardial ischemia or necrosis
such as acute myocardial infarction, can damage the heart’s conduction system, leading to second-degree AV block.
Degenerative changes in the conduction system or myocardium
For example, coronary heart disease, rheumatic heart disease, fibrosis of the conduction system, valve calcification, myocardial fibrous degeneration, tumor compression and other lesions can cause degenerative changes of the cardiac conduction system or myocardium, and the conduction function is reduced, thus inducing the second-degree atrioventricular block.
Injurious lesions
Common causes include damage to the conduction system or edema of the surrounding tissues after cardiac surgery, resulting in hypoconduction.
Congenital defects of the cardiac conduction system
Individual or combined congenital lesions such as large vessel malposition, septal or endocardial cushion defects may be accompanied by deficits in the cardiac conduction system, resulting in second-degree atrioventricular block.
Functional lesions of the conduction system
Vagal hyperfunction, hypoxia, electrolyte disorders such as hyperkalemia, drug effects (e.g., digitalis and beta-blockers, amiodarone, and other medications), and hyperthyroidism may cause alterations in cardiac conduction function, which may precipitate second-degree AV block.
Symptoms
Main Symptoms
Second-degree type I atrioventricular block
Most patients have no symptoms.
Patients with comorbid underlying cardiovascular disease may experience weakness, fatigue, and palpitations.
A very small number of patients may suffer from ischemia of brain tissue due to insufficient cardiac output, resulting in dizziness or even fainting.
Second-degree type II AV block
Patients are more likely to experience weakness, fatigue, palpitations, dizziness and even fainting.
Patients may experience hypotension, dyspnea, and other signs of inadequate perfusion.
Other symptoms
Chest pain
Patients with second-degree AV block may experience chest pain if it is due to myocarditis or myocardial ischemia.
Complications
High degree AV block
In patients with second-degree AV block, a high degree of AV block is said to occur when ≥2 consecutive P waves fail to conduct to the ventricle.
The patient may have obvious symptoms such as palpitations, dyspnea, dizziness, blackouts, and syncope.
A high degree of AV block can easily induce As syndrome, and even lead to sudden death, endangering the patient’s life.
Asperger’s Syndrome
Second-degree AV block may lead to slowing down of the ventricular rate, causing a significant reduction in cardiac output and insufficient perfusion of brain tissue, resulting in ischemia and hypoxia of brain tissue, inducing Asch syndrome.
Patients may experience acute cerebral tissue ischemia symptoms such as pallor, blackout, and transient impairment of consciousness, and some patients may experience convulsions and urinary and fecal incontinence.
As syndrome can lead to acute cerebrovascular accidents, and even lead to sudden death.
Sudden death of cardiac origin
Second-degree atrioventricular block may lead to slowing down of ventricular rate, causing significant reduction of cardiac output, leading to insufficient myocardial perfusion, inducing malignant arrhythmia such as ventricular fibrillation, or even leading to direct cardiac arrest, causing sudden cardiac death.
Patients may experience severe chest pain, severe dyspnea, loss of consciousness, or even sudden death and other serious conditions.
Consultation
Department of Medicine
Cardiovascular Medicine
When patients experience symptoms such as fatigue, chest pain, dyspnea, dizziness, etc., it is recommended that they seek medical attention promptly.
When the patient’s physical examination electrocardiogram shows second-degree atrioventricular block, it is recommended to consult a doctor promptly.
Prompt medical attention is recommended when one is found to have lowered blood pressure, slowed pulse rate, or disturbed pulse rhythm.
Emergency Department
If the patient develops a critical condition such as dizziness, blackout, syncope, loss of consciousness, severe chest pain, severe respiratory distress, respiratory arrest, etc., he/she should immediately consult the Emergency Department or call 120 Emergency for emergency services.
Preparation for medical treatment
Consultation: Registration, Preparation of Information, Frequently Asked Questions
Tips for seeking medical treatment
Don’t be overly alarmed if you find second-degree atrioventricular (AV) block.
Some patients with second-degree AV block may have transient symptoms. When symptoms such as palpitations and dizziness occur, it is recommended to immediately seek the nearest medical institution to complete an electrocardiogram (ECG) examination in order to obtain an ECG during the attack.
Preparation Checklist for Medical Attendance
Symptom checklist
Particular attention needs to be paid to the time of symptom onset, special manifestations, etc.
Are there any symptoms such as dizziness, palpitations, fatigue, weakness, dyspnea, etc.?
Are there any symptoms such as darkness, fainting, loss of consciousness, drop in blood pressure?
How long have these symptoms been present?
List of medical history
Is there any history of cardiovascular disease such as hypertension, coronary heart disease, heart valve disease, etc.? Do you take medication regularly?
Do you have regular medical checkups on a regular basis? What is the history of electrocardiogram?
Is there any family history of related cardiovascular disease in the immediate family?
Any recent history of special medication such as digitalis, antiarrhythmic drugs, etc.?
Checklist
Test results from the last six months, which can be brought to the doctor’s appointment
Electrocardiogram or ambulatory electrocardiogram.
Laboratory tests: cardiac enzymology, troponin, BNP, blood lipids, blood sugar
Imaging tests: cardiac ultrasound, coronary angiography, myocardial nuclear imaging, etc.
Drug List
Medications used in the last 3 months, if available in boxes or packages, bring with you to the doctor’s office
Digitalis: e.g. Digoxin
Anti-arrhythmic drugs: e.g. amiodarone, verapamil
Beta-blockers: e.g. metoprolol, bisoprolol
Others: e.g. potassium chloride extended-release tablets
Diagnosis
Diagnosis is based on
Medical history
Most commonly seen in children, professional athletes, and those with underlying cardiovascular disease.
Clinical manifestations
Symptoms
Most patients may not have any symptoms.
Some patients may have dizziness, palpitations, dyspnea, fatigue, weakness, etc. In severe cases, syncope and blackout may occur.
Physical signs
Decrease in blood pressure.
Heart rhythm slows down and regular/irregular pulse rate may be present.
Auscultation of the heart may show a weakened first heart sound or a missed heart beat.
Ancillary tests
Electrocardiogram
Important in the diagnosis of second degree AV block.
The type of second-degree AV block can be determined.
The electrocardiogram in second-degree AV block shows partial P waves followed by no QRS waves.
The electrocardiogram of second-degree type I AV block shows a gradual prolongation of the PR interval and a gradual shortening of the RR interval until the P wave fails to transmit downward and the ventricles are decoupled, and the ratio of the P wave to the QRS wave is mostly irregular.
The electrocardiogram of second-degree type II atrioventricular block shows a more fixed PR interval before ventricular leakage, which may be normal or prolonged, and QRS wave clusters may show intermittent leakage.
Ambulatory electrocardiogram
An ambulatory electrocardiogram provides insight into changes in the patient’s electrocardiographic activity over a 24-hour period.
It is important for the diagnosis of patients with transient second-degree AV block.
Blood biochemistry
Electrolyte tests may be performed to see if electrolyte disturbances are present and to assist in the search for the cause of second-degree AV block.
Serum troponin and cardiac enzymes are used to assess myocardial metabolism and to determine the cause of second-degree AV block, which is often elevated in patients with acute myocardial infarction and acute myocarditis.
Blood B-type natriuretic peptide (BNP) or N-terminal B-type natriuretic peptide proteins (NT-ProBNP) are used to assess cardiac function and to determine the presence of heart failure.
Serum digitalis levels should be tested in patients with a history of digitalis use.
Imaging
Routine imaging is usually not necessary.
If myocarditis is considered to be the cause, a complete cardiac ultrasound is needed.
Cardiac and thoracic CT scans combined with echocardiography can improve the sensitivity of detection of organic heart disease.
Cardiac electrophysiology
Electrophysiologic examination of the heart can determine the extent of second-degree AV block and can identify patients who potentially need a permanent pacemaker.
Cardiac electrophysiology should be performed in patients in whom the site of block is suspected to be in the bundle-branch-Purkinje system but has not been identified.
Diagnostic Criteria
The diagnosis of second-degree AV block is made primarily by characteristic ECG findings.
Second-degree type I atrioventricular block
Atrioventricular conduction is progressively blocked.
P waves appear regularly.
The PR interval appears to be progressively prolonged until the P wave cannot be transmitted downward.
The RR interval is progressively shortened until ventricular deafferentation occurs.
The QSR wave cluster is mostly normal, and the ratio of P waves to QRS waves is mostly irregular.
The RR interval including the blocked P wave is less than 2 PP intervals.
Second-degree type II atrioventricular block
Atrioventricular conduction is abruptly blocked.
PR intervals are constant before ventricular deregulation.
The QRS wave cluster may be intermittently deregulated, and some P waves are not followed by ORS wave clusters.
The QRS wave cluster is widened when the block occurs in the AV node.
When the block occurs in the Hitchcock’s bundle-Purkinje system, the QRS wave cluster morphology is abnormal.
Treatment
Aims of treatment: to remove the cause of the disease, to relieve symptoms, to maintain hemodynamic stability, and to improve the prognosis.
Treatment principle: The treatment of second-degree atrioventricular block needs to be based on the type, severity, etiology and other factors, and adopt different treatment strategies.
For asymptomatic patients with second-degree type I AV block, no special treatment is usually needed and regular follow-up is sufficient.
For patients with second-degree AV block with a clear etiology, etiologic treatment will be provided.
For patients with symptomatic refractory second-degree AV block, medications or pacemakers are given as appropriate.
General treatment
Rest
Bed rest and reduced activity are recommended for patients with significant symptoms.
Removal of the cause
If the block is caused by drugs such as digitalis, antiarrhythmic drugs, etc., reducing the dose of these drugs or stopping them can gradually restore normal AV conduction.
If it is caused by hypoxia, oxygen inhalation or mechanical ventilation can be given to correct the hypoxic state of the body.
Electrolyte disorders should be corrected promptly if they are caused by electrolyte disorders.
If it is caused by acute coronary syndrome, blood flow reconstruction therapy should be given immediately to restore myocardial blood supply.
If it is caused by coronary heart disease, it should actively improve myocardial ischemia through drugs, interventional therapy or surgery.
If it is caused by rheumatic heart disease or congenital heart disease, surgery should be carried out in time.
If it is caused by infection, anti-infection treatment should be actively carried out.
Drug therapy
Drug therapy is usually used only as a temporary first aid.
Atropine
Atropine reduces vagal tone and improves atrioventricular block by inducing a receptor blocking effect.
It is useful only in patients whose block is located in the sinus node.
It tends to be ineffective after prolonged use and is prone to adverse effects, so it is only indicated for short-term emergency treatment.
Isoprenaline
Elevates the patient’s heart rate and is used for short-term emergency treatment when the ventricular rate is severely slowed and cardiac pacing is not available.
It carries a risk of causing severe ventricular arrhythmias and should not be used long-term.
Contraindicated in the presence of angina pectoris, myocardial infarction, and atrial fibrillation.
Pacemaker implantation therapy
Indications
Patients with second-degree atrioventricular block with significant symptoms, severe slowing of the heart rhythm or cardiac arrest.
Patients with second-degree type II AV block who are asymptomatic but have a widened or narrowed QRS on the ECG.
Patients with second-degree type II atrioventricular block in which the site of block is clearly below the Hitchcock’s bundle after electrophysiologic examination.
Contraindications
Patients with skin infection in the operative area that has not been effectively controlled.
Patients with severe coagulation disorders or bleeding tendency.
Other conditions that make the surgery intolerable.
Prognosis
Cure
Overall prognosis
The type and degree of second-degree AV block determines the prognosis.
Some second-degree type I AV block cures spontaneously after removal of the cause.
Some patients can be cured with treatment, while others cannot be cured and require a permanent pacemaker.
Prognosis of each type
The prognosis for second-degree type I AV block is mostly favorable and is not associated with an increased risk of death in the absence of organic heart disease, but there is an increased mortality rate when second-degree type I AV block is present during an acute myocardial infarction.
Second-degree type II AV block carries a risk of progression to complete heart block and can therefore lead to an increased risk of death.
Second-degree AV block localized in the Hirschsprung’s bundle-Purkinje system has a very high risk of progressing to complete heart block and has a poor prognosis.
Daily
Daily Management
Dietary management
Low-salt and low-fat diet, with daily sodium intake not exceeding 5g. Try to avoid high-fat and high-cholesterol foods such as animal oil, fatty meat, animal offal and fried foods.
It is advisable to eat fresh fruits and vegetables rich in vitamins and fiber, such as oatmeal, celery, onion, fungus and kiwi.
Quit smoking and drinking
Daily care
Regular work and rest, avoid overwork and late nights.
Exercise moderately when the condition is stabilized, but avoid strenuous exercise.
Patients with permanent pacemakers should pay attention to avoiding pressure on the pacemakers, and should stay away from places with high electric power and strong magnetic fields, such as avoiding contact with microwave ovens and magnetic resonance examinations, etc. Meanwhile, the upper limbs on the side of the installed pacemakers should avoid too much exertion and too large movements.
Disease monitoring
Pay attention to any sudden palpitations, darkness, dizziness, drop in blood pressure, weakening of pulse rate, and so on.
If you have high blood pressure or diabetes, you should monitor and record the changes of blood pressure and blood sugar every day.
Pacemakers should be monitored for redness, swelling, heat, pain and bleeding at the site of installation.
Follow-up review
Pay attention to regular monitoring of ECG changes.
Patients with pacemakers should follow the doctor’s instructions for regular follow-up.
Prevention
Avoid high risk factors for cardiovascular disease
Control body weight within a reasonable range.
Eat a low-salt, low-fat diet and stop smoking and drinking.
Avoid overwork and late nights.
Emphasize the management of cardiovascular diseases
After suffering from hypertension, coronary heart disease and other cardiovascular diseases, we should standardize the treatment as early as possible, strengthen the daily management, control the condition, and slow down and curb the progress of the disease.