In first-degree atrioventricular block, the time for electrical impulse conduction from the sinoatrial node to the ventricles is prolonged. On the electrocardiogram (ECG), this is revealed as lengthening of the PR interval. In the past, first-degree atrioventricular block was regarded a benign condition, but more recent studies show that it can be associated with increased risk of adverse events.
For most patients, the diagnosis of first-degree atrioventricular block (AVB) is made by chance on an electrocardiogram (ECG or EKG) . This is because in general, the condition is largely asymptomatic.
Nevertheless, a distinction has to be made based on the extent of PR lengthening. The normal values for the PR interval are between 120 and 200 msec. The definition of first-degree AVB is the lengthening of this period to values of longer than 200 msec . The prolongation is characterized as “marked” if it is greater than 300 msec . Marked first-degree AVB can cause lowered tolerance to exercise, and can have effects similar to those caused by the pacemaker syndrome   . Patients with symptomatic marked first-degree AVB should be treated with biventricular pacing .
The electrical signal from the sinoatrial node to the ventricles travels through a conduction system consisting of the atrioventricular (AV) node, the bundle of His, the bundle branches, and the Purkinje fibers. There are differences in symptoms depending upon where in this system the defect in conduction is located. Of the above structures, the AV node is the most frequent site of first-degree AVB, but more than one defect in the conduction system is often encountered  . An additional defect in the His-Purkinje system is indicated on the ECG by a wide QRS complex.
Early studies on otherwise healthy individuals with first-degree AVB found no negative effects on health prognosis associated with the condition, and first-degree AVB was regarded as benign  . Recent works on more diverse populations, including the Framingham Study   , revealed that first-degree AVB is not a benign condition, but is associated with elevated risks of atrial fibrillation and mortality.
First-degree AVB is often observed by chance on an ECG as a lengthening of the PR interval. Imaging or laboratory tests are not usually indicated. For patients with episodes of syncope, or with ECG showing a wide QRS complex, His bundle ECG could be performed .
There are a number of possible underlying causes for first-degree AVB. These include heart disease, myocardial infarction, myocarditis, and AV node disease. Hypokalemia and hypomagnesemia can also cause first-degree AVB. Other conditions associated with first-degree AVB are infectious diseases, rheumatoid arthritis, systemic lupus erythematosus and scleroderma.
Drugs that tend to cause first-degree AVB are antiarrhythmics and cardiac glycosides, such as digoxin. Thus, administration of antiarrhythmics (calcium channel blockers, beta-blockers, and others) and digoxin to people with first-degree AVB requires careful consideration.
First-degree AVB may also occur during adenosine stress testing. This happens in about one tenth of patients tested, when those who already have first-degree AVB may temporarily develop a higher degree of block. These episodes are usually harmless .