History
Patients with first-degree atrioventricular (AV) block are generally asymptomatic at rest. Markedly prolonged PR interval may reduce exercise tolerance in some patients with left ventricular systolic dysfunction. Syncope may result from transient high-degree AV block, especially in those with infranodal block and wide QRS complex.
Patients may have a history of past heart disease, including myocarditis or myocardial infarction (MI). Patients may be highly conditioned athletes with a high degree of vagal tone, or they may be on medications that slow conduction through the atrioventricular node (AVN).
A history of an infectious disease, such as Lyme disease, may be present. Asymptomatic first-degree heart block is part of the spectrum of presentation of Lyme carditis in children. Lyme carditis is most likely in children with Lyme disease who are older than 10 years of age, those with arthralgias, and those with cardiopulmonary symptoms. [16]
Borderline first-degree AV block in patients with long-standing systemic lupus erythematosus (SLE) may be a clue to more significant cardiac disease, resulting from the progression of SLE; these patients require careful screening for underlying myocardial disease. [25] Conduction disturbances may also be secondary to drugs used to treat SLE.
Patients who have undergone mitral valve replacement or mitral valve annuloplasty may have heart block postoperatively. [26]
Physical Examination
No findings on the physical examination are specifically associated with first-degree AV block; it is generally an incidental finding noted on an electrocardiogram (ECG).
The intensity of the first heart sound (S1) is decreased in patients with first-degree AV block. Patients with first-degree AV block may have a short, soft, blowing, diastolic murmur heard at the cardiac apex. This diastolic murmur is not caused by diastolic mitral regurgitation, because it reaches its peak before the onset of regurgitation. The diastolic murmur is thought to be related to antegrade flow through closing mitral valve leaflets that are stiffer than normal. [27] Administration of atropine may reduce the duration of this murmur by shortening the PR interval.
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First-Degree Atrioventricular Block. The PR interval is 0.24 seconds (240 ms) in a patient with asymptomatic first-degree atrioventricular block.
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First-Degree Atrioventricular Block. 12-Lead electrocardiogram from a patient with first-degree heart block.
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First-Degree Atrioventricular Block. 12-Lead electrocardiogram from a patient with first-degree heart block.
