First-degree atrioventricular block
First-degree AV block occurs when there is a delay, but not disruption, as the electrical signal moves between the atrium and the ventricles through the AV node. On ECG, this is defined by a PR interval greater than 200 msec. Additionally, there are no dropped, or skipped, beats. Second-degree atrioventricular block
Second-degree AV block occurs when the electrical signal between the atria and ventricles is even more impaired than in a first-degree AV block. In a second-degree AV block, the impairment results in a failure to conduct an impulse, which causes a skipped beat. Mobitz I
Mobitz I is characterized by a progressive yet reversible block of the AV node. On ECG, this is defined by progressive prolongation of the PR interval, with a resulting dropped beat (the PR interval gets longer and longer until a beat is finally dropped, or skipped).Some patients are asymptomatic; those who have symptoms respond to treatment effectively. There is a low risk of a Mobitz I AV block leading to heart attack and complete heart block. Mobitz II
Mobitz II is caused by a sudden, unexpected failure of the His-Purkinje cells to conduct the electrical impulse. On ECG, the PR interval is unchanged from beat to beat, but there is a sudden failure to conduct the signal to the ventricles, and resulting in random skipped beat.The risks and possible effects of Mobitz II are much more severe than Mobitz I in that it can lead to severe heart attack.
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).