Drugs that slow the conduction of the electrical signal through AV node, such as beta-blockers, digoxin, calcium channel blockers, and amiodarone, can cause heart block if they are taken in excessive amounts, or the levels in the blood get too high. Anatomy
The synchronized contraction of the heart occurs through a well-coordinated electrical signal pathway. The initial electrical signal originates from the SA node located in the upper portion of the right atrium. The electrical signal then travels through both the right and left atrium and causes the two atria to contract at the same time. This simultaneous contraction results in the P wave seen in an ECG tracing.The electrical signal then travels to the AV node located on the lower portion of the interatrial septum. At the AV node there is a delay in the electrical signal, which allows the atria to contract and blood to flow from the atria to the ventricles. This delay accounts for the ECG period between the P wave and the QRS complex, and creates the PR interval. From the AV nodes, the electrical signal travels through Bundle of His and divides into the right bundle and left bundle, which are located within the interventricular septum. Finally, the electrical signal travels into the Purkinje fibers. The division of the signal into a right and left bundle and then into the Purkinje fibers allows for a simultaneous depolarization and contraction of the right and left ventricles. The contraction of the ventricles results in the QRS complex seen on an ECG tracing.
After contraction, the ventricles must repolarize, or reset themselves, in order to allow for a second depolarization and contraction. The repolarization creates the T wave in the ECG tracing. Diagnosis
An electrocardiogram, or ECG, is used to differentiate between the different types of AV block. In AV block, there is a disruption between the signal traveling from the atria to the ventricles. This results in abnormalities in the PR interval, as well as the relationship between P waves and QRS complexes on the ECG tracing. If the patient is symptomatic from their suspected AV block, it is important that an ECG is also obtained while having symptoms. Physicians may also order a continuous ECG (i.e. Holter monitor or implanted cardiac monitor) to monitor the patient for symptoms and conduction abnormalities over a longer period of time, as AV blocks can be intermittent.Because some types of AV block can be associated with underlying structural heart disease, patients may also undergo echocardiogram to look at the heart and assess the function.Laboratory diagnosis for AV blocks include electrolyte, drug level and cardiac enzyme level tests. Based upon clinical suspicion, the physician may do lab tests to assess for reversible causes of AV block, such as hypothyroidism, rheumatologic disorders, and infections (such as Lyme disease).