Approach Considerations
First-degree atrioventricular (AV) block is frequently noted as an incidental finding on electrocardiography (ECG).
Routine laboratory studies are usually not indicated in the evaluation of first-degree AV block. Electrolyte and drug screen can be obtained if a metabolic derangement or drug toxicity is suspected. Routine imaging studies are not indicated for first-degree AV block.
Electrocardiography
On a surface ECG, the PR interval exceeds 200 msec, and all P waves conduct to the ventricle with constant but prolonged PR interval (see the images below).
First-Degree Atrioventricular Block. The PR interval is 0.24 seconds (240 ms) in a patient with asymptomatic first-degree atrioventricular block.
First-Degree Atrioventricular Block. 12-Lead electrocardiogram from a patient with first-degree heart block.
First-Degree Atrioventricular Block. 12-Lead electrocardiogram from a patient with first-degree heart block.
His bundle ECG is necessary only in patients with symptomatic (ie, presyncope and syncope) first-degree AV block and a wide QRS complex, indicative of bundle-branch block. The study is used to locate the site of the block in these patients. As many as 50% of patients show an infranodal conduction delay.
Follow-up ECGs may be indicated in patients who are treated with AV nodal agents while in the emergency department (ED), as well as for patients with a concomitant myocardial infarction (MI).
Ultrasonography
In patients with first-degree AV block and left ventricular systolic dysfunction, Doppler ultrasonography may be used to document an improvement in cardiac output during dual-chamber pacing at short AV delay. This may provide evidence for the appropriateness of implanting a permanent pacemaker for hemodynamic support in such patients.
More recently, cardiac resynchronization therapy (ie, biventricular pacing) has been applied in patients with cardiomyopathy, congestive heart failure, or intraventricular conduction delay (IVCD). [28] First-degree AV block is frequently present in these patients as well.
Histologic Findings
Under light microscopy, an atrioventricular node (AVN) is seen to be composed of a thick mesh of tiny pale cells, which anastomose with one another via short pluridirectional cytoplasmic projections. Under electron microscopy, four types of cells are observed in the AVN: transitional cells, P cells, common myocardial cells, and Purkinje cells.
Three functional regions have been described in the AVN on the basis of their differing conductive properties: atrionodal (AV), nodal (N), and nodal-His (NH). Cells in the N region have slower conduction times than the other regions and have no automaticity properties. Cells of the AN and NH regions have faster conduction times and display spontaneous diastolic repolarization activity.
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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.
