Third-degree heart block, also known as complete heart block, is a serious cardiac condition that occurs when electrical impulses from the atria are not conducted to the ventricles. This results in the atria and ventricles beating independently of each other. The condition is often diagnosed by an electrocardiogram (ECG), which is a test that records the electrical activity of the heart.
Fundamentally, third-degree heart block is characterised by a complete absence of atrioventricular (AV) conduction. This results in the ventricles generating an escape rhythm that is usually slower than the normal heart rate. The condition can be life-threatening, and patients may present with symptoms such as dizziness, syncope, or shortness of breath.
Management and treatment of third-degree heart block vary depending on the underlying cause and the severity of the condition. Patients may require immediate treatment with medications, or in some cases, a pacemaker may be implanted to regulate the heart rate. In this article, we will discuss the clinical manifestations, diagnosis, and management of third-degree heart block, as well as its aetiology and prognosis.
Key Takeaways
- Third-degree heart block is characterised by a complete absence of atrioventricular conduction, resulting in the atria and ventricles beating independently of each other.
- Patients with third-degree heart block may present with symptoms such as dizziness, syncope, or shortness of breath.
- Management and treatment of third-degree heart block vary depending on the underlying cause and the severity of the condition, and may include medications or a pacemaker implantation.
Fundamentals of Third Degree Heart Block
Third-degree atrioventricular (AV) block, also known as complete heart block, is a serious condition that affects the conduction system of the heart. It occurs when the electrical impulses originating from the sinoatrial (SA) node, the natural pacemaker of the heart, fail to reach the ventricles due to a complete block at the AV node or below.
In third-degree AV block, the atria and ventricles beat independently of each other, resulting in a slow and irregular heart rate. The ventricles are usually driven by a subsidiary pacemaker in the lower part of the conduction system, such as the bundle of His or the Purkinje fibres. This leads to a wide QRS complex on the ECG, which can be helpful in diagnosing the condition.
The ECG findings in third-degree AV block include:
- Absent or dissociated P waves, due to AV dissociation
- Regular or irregular ventricular rhythm, depending on the underlying pacemaker
- Wide QRS complexes (>0.12 seconds), due to abnormal ventricular depolarisation
- Slow ventricular rate (<40 bpm), unless there is an escape rhythm from a subsidiary pacemaker
Third-degree AV block can be caused by various conditions that affect the conduction system of the heart, such as:
- Ischaemic heart disease
- Cardiomyopathy
- Congenital heart disease
- Myocarditis
- Drug toxicity
- Idiopathic fibrosis
It can also be a complication of cardiac surgery or catheter ablation. Third-degree AV block is a medical emergency that requires prompt intervention, such as temporary pacing or permanent pacemaker implantation, depending on the patient’s clinical condition.
Other types of AV block include first-degree AV block, which is characterised by a prolonged PR interval (>0.20 seconds), and second-degree AV block, which is characterised by intermittent AV conduction and can be further classified into Mobitz type I (Wenckebach) or Mobitz type II. However, third-degree AV block is the most severe form of AV block and requires urgent management.
It is important to note that the diagnosis and management of third-degree AV block should be made by a qualified healthcare professional, such as a cardiologist or electrophysiologist, based on the patient’s individual clinical presentation and ECG findings. This article is for informational purposes only and should not be used as a substitute for medical advice from a licensed healthcare provider.
Clinical Manifestations and Diagnosis
Symptoms
Third-degree atrioventricular (AV) block, also known as complete heart block, can present with a range of symptoms. Patients may experience fatigue, syncope, or bradycardia. However, some patients may be asymptomatic and the condition may be discovered incidentally. Chest pain and shortness of breath are not typically associated with third-degree AV block.
Electrocardiogram Findings
The diagnosis of third-degree AV block is made on electrocardiogram (ECG). On ECG, there is complete dissociation between the atrial and ventricular rhythms. The atrial rate is typically faster than the ventricular rate, with no relationship between the two. The ventricular rate is usually slow, and there may be escape rhythms present. P waves may be absent or dissociated from the QRS complexes.
Differential Diagnosis
Third-degree AV block must be differentiated from other arrhythmias and conduction abnormalities. First-degree AV block and second-degree AV block may have similar ECG findings but differ in the degree of conduction delay. Other causes of bradycardia, such as sick sinus syndrome or medication-induced bradycardia, should also be considered. A thorough evaluation is necessary to determine the underlying cause of third-degree AV block.
Clinical interpretation of the ECG findings is crucial in the diagnosis of third-degree AV block. Further evaluation, including a detailed history and physical examination, is necessary to determine the underlying cause and appropriate management. It is important to note that third-degree AV block can be a life-threatening condition and requires urgent evaluation and treatment.
Disclaimer: This article is for informational purposes only and is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified healthcare provider with any questions you may have regarding a medical condition.
Management and Treatment
Immediate Management
Third-degree atrioventricular block (AVB) is a serious condition that requires immediate attention. The patient should be monitored closely in a hospital setting, and the underlying cause of the block should be identified and treated. In cases where the patient is symptomatic, immediate intervention is necessary.
Atropine is the first-line treatment for symptomatic patients with third-degree AVB. It should be administered intravenously in incremental doses until the desired effect is achieved. If atropine is ineffective, temporary pacing should be initiated to maintain an adequate heart rate.
Long-Term Treatment Options
Once the patient is stable, long-term treatment options should be considered. The treatment of third-degree AVB depends on the underlying cause and the patient’s overall health status.
In cases where the block is due to medication, the offending drug should be discontinued. If the block is caused by a reversible factor, such as electrolyte imbalance or hypoxia, correction of the underlying cause may restore normal conduction.
In cases where the block is due to structural heart disease, permanent pacing is usually required. A pacemaker is a small device that is implanted under the skin and connected to the heart. It delivers electrical impulses to the heart to maintain a regular heartbeat.
Beta-blockers and amiodarone may be used to manage symptoms and prevent arrhythmias in patients with underlying heart disease. Digoxin should be used with caution in patients with third-degree AVB, as it can worsen conduction abnormalities.
It is important to note that third-degree AVB is associated with an increased risk of morbidity and mortality. Patients with this condition should be closely monitored and receive appropriate treatment to prevent complications.
Disclaimer: The information provided in this article is for educational purposes only and should not be used as a substitute for professional medical advice. Always consult a qualified healthcare provider for diagnosis and treatment of medical conditions.
Aetiology and Prognosis
Causes
Third-degree atrioventricular (AV) block, also known as complete heart block, is a serious condition that occurs when the electrical impulses from the atria fail to reach the ventricles. This can be caused by a number of factors, including:
- Myocardial infarction
- Ischemic heart disease
- Structural heart disease
- Congenital heart disease
- Cardiomyopathy
- Infection (e.g. endocarditis, myocarditis)
- Lupus
- Rheumatic fever
- Sarcoidosis
- Amyloidosis
- Idiopathic fibrosis
Inferior myocardial infarction is the most common cause of complete heart block. Other causes include fibrosis of the conduction system, degeneration of the conducting system, and the use of AV-nodal blocking drugs such as calcium-channel blockers, beta-blockers, and digoxin.
Complications and Prognosis
Third-degree AV block can lead to a number of complications, including arrhythmias, cardiac arrest, and sudden cardiac death. Patients with complete heart block are also at risk of becoming hemodynamically unstable, especially if they have underlying heart disease.
The long-term prognosis of third-degree AV block is not well studied, as it often requires treatment in acute settings. However, the prognosis likely depends on the patient’s underlying disease burden and the severity of the clinical presentation on arrival. Patients with third-degree AV block without pacemaker implantation have a poor prognosis, especially if they presented symptoms such as syncope. Their survival in the first year is less than 50% compared with the same age population.
It is important for patients with complete heart block to receive appropriate treatment and follow-up care. This may include the use of a pacemaker, medication, and lifestyle changes. Patients should also be monitored for complications and receive regular check-ups with their healthcare provider.
Disclaimer: The information provided is intended for educational purposes only and should not be used as a substitute for professional medical advice or treatment. Always seek the advice of your physician or other qualified healthcare provider with any questions you may have regarding a medical condition.
Frequently Asked Questions
What are the distinguishing characteristics of third-degree heart block on an electrocardiogram?
Third-degree heart block, also known as complete heart block, is characterized by a complete failure of electrical impulses to pass from the atria to the ventricles. On an electrocardiogram (ECG), this is seen as an independent atrial rhythm and a ventricular rhythm that is slower than the atrial rhythm. The ventricular rhythm is typically regular and slow, with a rate of less than 40 beats per minute.
What treatment options are available for third-degree heart block?
Treatment for third-degree heart block depends on the patient’s symptoms and overall health. In some cases, no treatment is necessary if the patient is asymptomatic. However, if the patient experiences symptoms such as dizziness, syncope, or chest pain, treatment may be required. The most common treatment for third-degree heart block is the placement of a pacemaker. In rare cases, medication or surgery may be used to treat the condition.
How does the QRS complex present in cases of third-degree heart block?
In cases of third-degree heart block, the QRS complex is typically wide and bizarre, with a duration of more than 0.12 seconds. This is due to the fact that the ventricles are depolarizing independently of the atria, leading to abnormal conduction patterns.
What differentiates first, second, and third-degree heart blocks on an ECG?
First-degree heart block is characterized by a prolonged PR interval, while second-degree heart block is characterized by intermittent failure of electrical impulses to pass from the atria to the ventricles. Third-degree heart block, as previously mentioned, is characterized by a complete failure of electrical impulses to pass from the atria to the ventricles.
What are the criteria for diagnosing heart block on an electrocardiogram?
The criteria for diagnosing heart block on an ECG depend on the degree of heart block. First-degree heart block is diagnosed when the PR interval is greater than 0.20 seconds. Second-degree heart block is diagnosed when there are intermittent dropped beats. Third-degree heart block is diagnosed when there is complete dissociation between the atria and ventricles.
How does a complete heart block manifest on an ECG compared to other degrees of heart block?
On an ECG, complete heart block manifests as an independent atrial rhythm and a ventricular rhythm that is slower than the atrial rhythm. The QRS complex is typically wide and bizarre, with a duration of more than 0.12 seconds. This is in contrast to first-degree heart block, which is characterized by a prolonged PR interval, and second-degree heart block, which is characterized by intermittent dropped beats.
