Third degree heart block
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Definition and Pathophysiology of Third Degree Heart Block
Third-degree heart block, also known as complete atrioventricular (AV) block, is a serious cardiac condition where electrical signals from the atria do not reach the ventricles, resulting in complete dissociation between atrial and ventricular activity. This means the atria and ventricles beat independently, and the heart’s normal rhythm is lost. The heart rate and rhythm in this condition depend on secondary pacemakers within the conduction system, and if these fail, it can lead to asystole and cardiac arrest 123489.
Causes and Risk Factors for Complete Heart Block
The most common causes of third-degree heart block in adults are ischemic heart disease, especially following acute myocardial infarction (MI), and age-related degeneration of the conduction system. Up to 8% of patients post-MI and 25-30% of those with acute MI may develop some form of conduction block, with a higher association in inferior MI compared to anterior MI 13. Other causes include myocarditis, infectious endocarditis, infiltrative cardiac diseases, congenital AV blocks, non-ischemic cardiomyopathy, electrolyte disturbances, drug side effects, and rare exposures such as to certain insecticides (e.g., pyrethroids) 1410. In children, congenital heart block can result from maternal autoimmune diseases or as a complication of surgery for congenital heart disease .
Lyme carditis is another notable cause, particularly in younger adults, where third-degree heart block can be a severe and potentially fatal manifestation if not promptly treated .
Clinical Presentation and Symptoms
Patients with third-degree heart block may present with a range of symptoms, including fatigue, dizziness, syncope (fainting), and bradycardia (slow heart rate). The severity of symptoms often depends on the rate and adequacy of the escape rhythm. Deep bradycardia or ventricular arrhythmias increase the risk of sudden death, especially if no effective escape rhythm is present 2478.
Diagnosis: ECG and Other Investigations
Diagnosis is primarily made using a 12-lead electrocardiogram (ECG), which shows complete asynchrony between P waves (atrial activity) and QRS complexes (ventricular activity). The QRS complex width and ventricular rate can help localize the site of the block. Additional tests may include Holter monitoring, stress ECG, blood tests, and imaging to rule out other causes and assess for underlying structural heart disease 1234.
Management and Treatment Strategies
Third-degree heart block is a cardiovascular emergency that requires prompt intervention. Initial management focuses on stabilizing the patient, often with temporary pacing (transcutaneous or transvenous) and medications such as intravenous atropine if indicated. Most patients will ultimately require permanent pacemaker implantation to restore effective heart rhythm and prevent life-threatening complications 14578.
In cases related to reversible causes, such as Lyme carditis, appropriate antimicrobial therapy and temporary pacing may be sufficient, but close monitoring is essential . For congenital cases in children, permanent pacemaker implantation is often required if symptoms are present or if there is significant bradycardia .
Prognosis and Long-Term Considerations
The prognosis of third-degree heart block depends on the underlying cause, the presence of comorbidities, and the timeliness of intervention. Permanent pacemaker implantation generally leads to good outcomes, but untreated or unrecognized cases can be fatal 14810. In rare cases, such as those related to occupational exposures, the block may be permanent and associated with other long-term health issues .
Conclusion
Third-degree heart block is a life-threatening condition characterized by complete dissociation between atrial and ventricular activity. It can result from a variety of causes, including ischemic heart disease, congenital factors, infections, and toxins. Prompt recognition, diagnosis via ECG, and timely intervention with pacing are critical to prevent serious complications and improve patient outcomes 123478910.
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