It is an interruption or delay in the electrical conduction of the atria to the ventricles due to anomalies of the conduction system in the AV node or the tissue system.
The delay or blockage of conduction can be physiological if the atrial rate is abnormally fast or at average atrial rates. AV block is usually defined based on a regular atrial rhythm.
Signs and symptoms
First degree AV block
- Generally asymptomatic.
- Excessive delay causes dyspnea, weakness, or dizziness.
2nd degree AV block
- It can be asymptomatic.
- Palpitations, weakness, dizziness, or syncope.
- It manifests itself in the physical examination as bradycardia (especially Mobitz II) and irregular heart rate (during Mobitz I AV block).
Third-degree AV block
- Fatigue, dizziness, and dizziness are frequent, and, with concomitant structural heart disease, heart failure, weakness, chest pain, confusion, and syncope may occur.
- It is associated with profound bradycardia unless the block site is located in the AV node.
- It can cause asystole and cause cardiac arrest or death.
- Electrolyte levels (hyperkalemia) and drug levels (e.g., Digitálicos) if a high level of potassium or drug toxicity is suspected.
- Troponin levels: if AV block is suspected due to ischemia/infarction.
- Infection (Lyme titers), myxedema (levels of stimulating thyroid hormone [TSH]), or studies of connective tissue diseases (ANA) in cases of systemic disease.
- 12-lead ECG recordings and rhythm/monitor bands.
- Supervision of 24 hours and prolonged.
- Implantable loop recorder.
- Electrophysiological test: to determine the location of the block and other arrhythmias.
- Echocardiography: evaluate ventricular function (especially if an implantable device is required).
- Stress test: to assess if AV blockage worsens or improves with exercise.
See Clinical Presentation and Treatment for more specific information on the diagnosis of AV block.
A pacemaker is indicated when there is irreversible and long-term asymptomatic bradycardia due to AV block.
Considerations regarding the administration of anticholinergic agents include the following:
- Infusion of atropine and isoproterenol can improve AV conduction in emergencies where an AV block causes bradycardia in the AV node.
- Dopamine and dobutamine may occasionally be helpful.
See Medication for more specific information on the treatment of atrioventricular block.
Atrioventricular (AV) block is an interruption or delay in the electrical conduction of the atria to the ventricles due to abnormalities of the conduction system in the AV node or the Purkinje fiber system Purkinje tissue.
The delay or blockage of conduction can be physiological if the atrial rate is abnormally fast or pathological at normal atrial rates. AV block is usually defined based on a regular atrial rhythm.
AV block is classified as first, second, and third-degree AV block. First-degree AV block is defined as the decrease in AV conduction; in the electrocardiogram (ECG), the PR interval exceeds 0.20 seconds.
In second-degree AV block, some P waves lead while others do not. This type is subdivided into Mobitz I (Wenckebach), Mobitz II, 2: 1, paroxysmal and high-grade AV block.
During third-degree AV block (“full”), AV conduction does not occur when it should be able to happen.
The symptoms of AV block vary from absence of symptoms to weakness, fatigue, shortness of breath, intolerance to exercise, or syncope.
The low cardiac output can cause hypotension and hypoperfusion of the final organ. Death due to asystole is possible incomplete heart block.
Treatment involves correction or resolution of the underlying causes, and if the AV block is progressive, symptomatic and persistent, a pacemaker is needed.
First Degree AV Blocking
First degree AV block consists of the following:
- PR prolongation (PR interval> 0.20 seconds in adults and> 0.160 seconds in young children) with AV conduction 1: 1.
- A narrow QRS is associated with a decrease in conduction in the AV node, but it can be in the atria, and with a wide QRS, the reduction in conduction occurs in the Purkinje tissue system.
- The marked prolongation of the PR interval (> 0.30 sec) is generally associated with the deceleration in the AV node.
Second Degree AV Block
Second-degree AV block is characterized by the following:
- Some, but not all, atrial impulses (which usually occur with a regular frequency) do not lead to the ventricles.
- They are categorized as Mobitz I, II, 2: 1, paroxysmal or high-grade AV block.
- AV block of second degree Mobitz I.
Mobitz I second-degree AV block is associated with constant PP intervals, progressive PR prolongation, followed by a non-driven P wave.
Often, there is a progressive shortening of the RR intervals during the Wenckebach cycle and a pause due to a blocked P wave that is less than the sum of two PP intervals.
This form of AV block occurs in less than 50% of all types of AV block and is usually due to a blockage in the AV node.
Rarely, it may occur with a blockage within or below the atrioventricular fasciculus; this is an intra or infravision block of Wenckebach, respectively.
The Mobitz I block rarely progresses to a third-degree block. Distinguishing a block in the AV node versus an infrahisian block can be challenging.
Carotid sinus massage will worsen conduction of the AV node during Wenckebach’s block but may improve conduction if a blockage occurs below the atrioventricular bundle.
Exercise will worsen a blockage in the Purkinje fiber system or Purkinje tissue but improve it if a blockage occurs in the AV node. The AV Mobitz I block generally consists of a ratio of undrived to conducted rhythms of 4: 3, 3: 2, etc.
Mobitz II second-grade AV block
Mobitz II second-degree AV block is characterized by constant PP and RR intervals.
There is a constant PR interval before the failure of an atrial impulse to lead to the ventricle (small changes in the PR interval may occur after the blocked heartbeat).
The pause, including the blocked P waves, is equivalent to two cycles of PP (unless there is an early binding escape beat).
Mobitz II block occurs in the Purkinje fiber system or Purkinje tissue. It may be associated with a narrow QRS, but as is often the case in Purkinje fiber or Purkinje tissue disease, a complex QRS ( block) is commonly observed. Of branch ) in the directed beats.
The AV Mobitz II block can progress to complete the AV block. Carotid sinus massage improves AV conduction in the Purkinje fiber system or Purkinje tissue since it causes the sinus frequency to decrease.
However, walking, exercising, or using atropine can worsen AV block since impulses can affect an already damaged driving system.
Second degree AV block 2: 1
A 2: 1 AV block is observed when all other P waves (with a regular PP interval) are conducted to the ventricle.
The block can be at the level of the AV node or below. The detailed analysis of a long-range strip can help to define the location of AV block 2: 1, especially during particular maneuvers.
For example, if the blockage is in the atrioventricular fasciculus or below, exercise, walking, or even stopping can improve conduction of the AV node; however, ventricular velocity will decrease, and the level of blockage will increase.
Carotid massage or the use of atropine can improve conduction if the blockage is in the Purkinje tissue system but will worsen the blockade if it is at the level of the AV node.
Look for the presence of a branch block. If present, 2: 1 AV block is more likely to occur in the Purkinje tissue system.
Paroxysmal AV block
A paroxysmal AV block is a second-degree AV block that is not persistent or repetitive since there is an abrupt block in AV conduction.
It can occur due to a paroxysmal increase in vagal activation.
High-grade AV blocking
A high-grade AV block consists of multiple constant P waves in a row that do not drive.
The driving ratio can be 3: 1 or higher; the PR interval of the directed beats may be constant (with Purkinje tissue disease) or variable (with AV nodal block).
This is different from the AV block because some P waves lead to the ventricle. Atrial fibrillation with pauses more significant than 5 seconds is due to a high-grade AV block. It can be associated with a narrow or wide QRS complex.
Third-degree AV block
Third-degree AV block consists of the following characteristics:
- No atrial impulses lead to the ventricle when they should lead (i.e., complete AV block).
- P waves, generally regular, reflect a rhythm of the sinus node independent of a (minor) or ventricular (wide) escape rhythm and may be normal or irregular.
- The union escape rate is 40-60 bpm, and the ventricular escape rate is 20 to 40 bpm.
- Non-sinus atrial rhythms such as atrial fibrillation, atrial flutter, or atrial tachycardia may be associated with third-degree AV block.
AV block imitators
Blocked premature atrial complexes can be a physiological process. They are not considered a form of AV block, but they are due to ectopic beats that occur when the AV node is refractory.
Paroxysmal vagal activation may occur during sleep and may be associated with sleep apnea. Other causes include coughing, vomiting, urinating, eating, and defecating.
The PR interval slows down around the AV block time when the AV block is present. With a high vagal tone, the PR intervals will be lengthened, while in the AV block 2: 1, the PR intervals will be constant.
Atrial tachycardia / atrial flutter with AV 2: 1 conduction is not a form of AV block. It is due to the physiological compression of the refractoriness of the AV node.
During AV dissociation, the atria and ventricles beat independently of each other.
AV dissociation occurs when a secondary pacemaker at the AV node or ventricle reaches the sinus node for the onset of the impulse due to deceleration of the sinus node, or it can occur when a subsidiary site beats faster than the sinus node.
Complete AV block can occur with AV dissociation, but AV dissociation alone does not indicate AV block.
The atrioventricular (AV) block is due to the following conditions:
- Ischaemia .
- Degenerative changes
- Infection ( myocarditis ) and infiltrative processes such as Lyme disease and sarcoidosis.
- Drugs (beta-blockers, calcium channel blockers) slow down the AV node or block the conduction of Purkinje tissue (e.g., Procainamide, flecainide) or lengthen the refractoriness of the AV node.
- The reflex of Bezold-Jarisch: a lower myocardial infarction may cause an increase in the temporary vagal tone that leads to transient Mobitz I or complete AV block.
The delay or lack of conduction through the atrioventricular (AV) node and below has multiple causes.
Degenerative changes (e.g., fibrosis, calcification, or infiltration) are the most frequent cause of non-ischemic AV block.
Idiopathic fibrosis or calcification of the AV conduction system, which is seen frequently in the elderly, can cause complete AV block.
First degree AV block
The causes of first-degree AV block include the following:
- Delayed conduction within the AV node or Woven Purkinje system.
- The intrinsic disease of the AV node, high vagal tone, or double AV ganglionic pathways (two separate PR intervals).
- Medications that block the AV node (beta-blockers, calcium channel blockers, digitalis, and selected antiarrhythmic medications) may prolong the PR interval.
First-degree AV block is also associated with an increased risk of atrial fibrillation and a worse prognosis in the general population and patients with heart failure.
2nd degree AV block
The causes of Mobitz I second-degree AV block include the following:
- Increased vagal tone (functional block).
- Degenerative disease of the AV conduction system (commonly seen in older adults).
- It occurs physiologically at high heart rates (especially with stimulation) due to the increased refractoriness of the AV node.
- AV nodule blocking drugs.
- Lyme disease, myocarditis, and radiofrequency ablation of the slow pathway.
The causes of Mobitz II second-degree AV block include the following:
- Degenerative disease of the Purkinje tissue system.
- Damage to the conduction system due to coronary artery disease, valvular surgery, myocardial infarction, myocarditis, infiltrative cardiomyopathies (sarcoidosis, hemochromatosis ), myxedema, Lyme disease, neuromuscular disease, and ablation of the AV junction.
- Systemic diseases (e.g., ankylosing spondylitis, Reiter’s syndrome).
Third-degree AV block
The causes of third-degree AV block include the following:
Reversible Causes: AV and Ischemic Nodule Blockers.
Pathological causes: includes cardiomyopathy (infiltrative, idiopathic, and infarcted), myocarditis (Lyme disease), endocarditis with abscess formation, and hyperkalemia.
Anterior myocardial infarction: causing ischemia or infarction of the branches of the beam.
Cardiac surgery: catheter ablation of the AV node, septal ablation with alcohol or neuromuscular disease.
The prognosis in atrioventricular (AV) block is directly related to its degree and the patient’s underlying medical problems.
Those with advanced AV block who are not treated with a permanent pacemaker remain at risk for syncope and sudden cardiac death, especially people with underlying structural heart disease.
Complications include the following:
- Death due to asystole.
- Cardiovascular collapse with syncope, worsening ischemic heart disease, congestive heart failure, and exacerbation of kidney disease.
In general, patients who present AV block of Mobitz type 1 of the first or second degree do not require treatment.
Any provocative medication can be eliminated, and patients can be monitored on an outpatient basis. However, patients with a higher degree of AV block tend to have severe damage to the conduction system.