Cardiac Conduction Disorders

Definition and Diagnostic Criteria

• The American College of Cardiology defines complete Left Bundle Branch Block (LBBB) as requiring thorough investigation for underlying myocardial disease, including echocardiography and cardiac MRI with perfusion study, while incomplete LBBB generally requires less aggressive evaluation unless accompanied by concerning clinical features 1
• Traditional criteria for LBBB include QRS duration >120 ms, slurred predominant R waves in left precordial leads, and slurred predominant S waves in right precordial leads, as defined by the American Heart Association 2

Prevalence and Association with Cardiomyopathy

• Complete LBBB is found in less than 1 in 1,000 athletes but is common in patients with cardiomyopathy and ischemic heart disease, according to the American College of Cardiology 1
• The prevalence of isolated left anterior fascicular block (LAFB) is 0.5-1.0%, with a generally benign prognosis, but may be associated with underlying cardiovascular disorders 3, 4, 5
• The prevalence of isolated right bundle branch block (RBBB) is 6.2% in acute myocardial infarction (AMI) patients, with a good prognosis if asymptomatic, but worse prognosis with anterior myocardial infarction 3, 4, 5

Diagnostic Challenges

• The distinction between complete LBBB and left ventricular hypertrophy with intraventricular conduction delay can be difficult, as "complete" LBBB may not always be truly complete, as noted by the American Heart Association 2
• For patients with intermittent angina and LBBB, the American College of Physicians recommends dipyridamole or adenosine myocardial perfusion imaging as the initial workup, regardless of the patient's ability to exercise, for risk stratification (Class I recommendation) 6
• Cardiac biomarkers, such as troponin I/T and CK-MB, should be measured to rule out acute myocardial injury, as recommended by the American Heart Association 7

Diagnostic Evaluation

• A transthoracic echocardiogram is strongly recommended to exclude structural heart disease in patients with newly detected LBBB, as the essential first step in management, according to the American College of Cardiology (Class I recommendation) 8
• Advanced cardiac imaging, such as cardiac MRI, CT, or nuclear studies, may be considered (Class IIa recommendation) for selected patients with suspected structural heart disease if echocardiogram is unrevealing, as recommended by the American College of Cardiology 8
• Ambulatory electrocardiographic monitoring (Class I recommendation) is recommended for symptomatic patients with suspected atrioventricular block, according to the American College of Cardiology 8
• Electrophysiologic study is recommended (Class IIa recommendation) for patients with symptoms suggestive of intermittent bradycardia, such as lightheadedness or syncope, as per the American College of Cardiology 8

Treatment and Management

• The American College of Physicians recommends dipyridamole or adenosine myocardial perfusion imaging for patients with LBBB, regardless of exercise capacity (Class I recommendation) 6
• Exercise or dobutamine echocardiography and exercise myocardial perfusion imaging are not recommended in LBBB, as they have a Class III recommendation from the American College of Physicians 6
• The European Society of Cardiology recommends catheter ablation by experienced operators as first-line treatment for left posterior fascicular VT 9
• Verapamil may be effective for acute management of left posterior fascicular VT, but is not effective for long-term treatment 9
• Permanent pacing is indicated for patients with documented intermittent second or third-degree AV block, severe conduction disturbance below AV node, or syncope with documented high-degree AV block, according to the European Society of Cardiology 10 and the American College of Cardiology 8
• Cardiac resynchronization therapy (CRT) may be considered for patients with LBBB and mildly to moderately reduced left ventricular ejection fraction (36-50%) (Class IIb, Level C-LD), according to the American College of Cardiology 11
• CRT is indicated for patients with left ventricular ejection fraction (LVEF) ≤35% and LBBB (QRS ≥150 ms) after optimization of medical therapy, as stated by the American College of Cardiology 8

Prognosis and Follow-up

• The prognosis for patients with RBBB and normal myocardial perfusion scan (MPS) is good, with an annual cardiac death rate <1%, while those with perfusion defects have a significantly worse prognosis, with an annual cardiac death rate ~6.4% 3, 4, 5
• Regular follow-up with clinical evaluation every 3-6 months and annual ECG is recommended for patients with bifascicular block and first-degree AV block, as suggested by the American College of Cardiology 8
• Periodic ambulatory monitoring is recommended to detect asymptomatic conduction abnormalities in patients with bifascicular block and first-degree AV block, according to the American College of Cardiology 8
• Regular exercise (at least 150 minutes per week) is recommended for patients with RBBB, according to the American Cancer Society 12
• Healthy dietary habits (high intake of fruits/vegetables and whole grains) are suggested for patients with RBBB, by the European Society of Medical Oncology 12
• Maintaining normal weight is recommended for patients with RBBB, by the American Society of Clinical Oncology 12
• Patients with history of cardiotoxic therapy should consider periodic screening with cardiac biomarkers and imaging at 6-12 months, at 2 years post-treatment, and possibly periodically thereafter, according to the American Heart Association 12

3, 4, 5