Atrioventricular (AV) conduction is evaluated by assessing the relationship between the P waves and QRS complexes. Normally, there is a P wave that precedes each QRS complex by a fixed PR interval of 120 to 200 milliseconds. AV block represents a delay or disturbance in the transmission of an impulse from the atria to the ventricles. This can be due to an anatomical or functional impairment in the heart’s conduction system. This disruption in normal electrical activity can be transient or permanent, and then further characterized as delayed, intermittent, or absent. In general, there are three degrees of AV nodal blocks: first degree, second degree (Mobitz type 1 or 2), and third-degree.
Higher degrees of AV block than those seen from increased vagal tone often suggest some underlying pathology. This is known as a pathophysiologic AV block. About half of such cases are a result of chronic idiopathic fibrosis and sclerosis of the conduction system as seen in Lenegre’s disease and Lev's disease 
Another common source is ischemic heart disease which is responsible for around 40 percent of cases of AV block .
AV block is also associated with cardiomyopathies, including hypertrophic obstructive cardiomyopathy and infiltrative conditions such as sarcoidosis and amyloidosis. Infectious causes such as Lyme disease, rheumatic fever, endocarditis, viruses as well as autoimmune disease such as systemic lupus erythematosus should also be explored .
Other potential triggers include cardiac surgery, medications, and inherited conditions .
There have not been large population-based studies on the prevalence of AV blocks. One study suggested that First-degree AVblock was more prevalent in African-American patients compared with Caucasian patients in all age groups except in the eighth decade of life. However, at this time, there is no well-characterized large study about the correlation between different types of AV block with age, racial, or gender. AV block is sometimes seen in athletes and in patients with congenital heart disorders.
First degree AV block can originate from various locations within the conduction system. The levels of conduction delay include the atrium, AV node (most common in first-degree heart block), Bundle of His, bundle branches, fascicles, Purkinje system. Mobitz type I second degree AV block usually occurs within the AV block while Mobitz type II second degree AV block mainly originates from conduction system disease below the level of the AV node (in the bundle of His and in the bundle branches). In third-degree AV block, no atrial impulses could reach the ventricle- it can occur in the AV node or in the infranodal specialized conduction system. 
The following medications can affect different levels of conduction delay:
1) Increased parasympathetic tone, digoxin (which upgrades vagotonic action), calcium channel blockers (which obstructs the inward calcium current responsible for depolarization) and beta-blockers can affect the AV node
2) Medications such as procainamide, quinidine, and disopyramide can block sodium channels and delay conduction in the bundle of His
3) Similarly though rarely, medications such as procainamide, quinidine, and disopyramide can also delay infra-Hisian conduction system
History taking for patients with concerns for AV block should include:
The following symptoms should raise concerns:
First degree. In first-degree AV block, the P waves always precede the QRS complexes, but there is a prolongation of the PR interval. That is, the PR interval will be greater than 200 milliseconds in duration without any dropped beats. There is a delay, without interruption, in conduction from the atrium to the ventricle. In other words, while the impulse is slowed, it is still able to get through to the ventricles. All atrial activation is eventually transmitted to the ventricles. The delay is typically due to a minor AV conduction defect occurring at or below the AV node. If the PR interval is more than 300 milliseconds, it is considered “marked” first-degree AV block and the P waves may be buried in the preceding T wave.
Second degree (incomplete). Second-degree or incomplete AV block occurs when there is intermittent atrial to ventricle conduction. That is, the P waves are sometimes related to the QRS complexes. It often occurs in a regular P:QRS pattern with ratios of 2:1. 3:2, 4:3, 5:4, and so forth. Second-degree AV blocks can be further classified into Mobitz type 1 (Wenckebach) or Mobitz type 2, which can be distinguished by examining the PR interval.
Second degree, Mobitz type 1 (Wenckebach). In second-degree Mobitz type 1 AV block, there is a progressive prolongation of the PR interval, which eventually culminates in a non-conducted P wave. It is often evident by clustering of QRS complexes in groups that are separated by non-conducted P waves. The greatest increase in PR interval prolongation is often between the first two beats of the cycle. While the PR interval continues to prolong with each beat of the cycle, the subsequent PR lengthening is progressively shorter. Even though the PR interval is progressively increasing in duration, the PP interval remains relatively unchanged. One way to confirm the presence of this is by noticing that the PR interval after the dropped beat is shorter than the PR interval that came before the dropped beat. In other words, the PR interval before the dropped beat is the longest of the cycle, and the PR interval after the dropped beat is the shortest as the cycle starts over.
Second degree, Mobitz type 2. In second-degree Mobitz type 2 AV block, there are intermittent non-conducted P waves without warning. Unlike Mobitz type 1 (Wenckebach), there is no progressive prolongation of the PR interval; instead, the PR interval remains constant, and the P waves occur at a constant rate with unchanged P-P intervals. Because the P waves continue to occur at normal intervals, the R-R interval surrounding the dropped beat is simply a multiple of the preceding R-R interval and remains unchanged.
Note in cases in which every other QRS complex is dropped, there are never two consecutive PR intervals. Therefore, there is not enough information to evaluate the PR interval to further classify it as either second-degree Mobitz type 1 (Wenckebach) or Mobitz type 2 AV block. The site of block is also indeterminate.
Second degree, high-grade. High-grade AV block is a form of second-degree (incomplete) heart block that can commonly be confused with third-degree (complete) heart block. It occurs when there are two or more consecutively blocked P waves. This conduction disturbance can be particularly dangerous as it can progress to complete heart block. The anatomic region involved is almost always below the AV node as in Mobitz type 2. The P:QRS is 3:1 or higher and the ventricular rate is typically very slow. What differentiates high-grade AV block from the third-degree (complete) heart block is that there remains some relationship between the P waves and QRS complexes. In other words, there is still some AV conduction taking place.
Third-degree (complete). In third-degree, or complete, heart block there is an absence of AV nodal conduction, and the P waves are never related to the QRS complexes. In other words, the supraventricular impulses generated do not conduct to the ventricles. Instead, if ventricular conduction occurs, it is maintained by a junctional or ventricular escape rhythm. There is a complete dissociation between the atria and ventricles. The atria and ventricles conduct independent of each other. The P waves (atrial activity) are said to “march through” the QRS complexes at their regular, faster rate. The QRS complexes (ventricular activity) also occur at a regular, but slower rate. There are two independent rhythms occurring simultaneously.
AV dissociation. AV dissociation occurs when there is no relationship between the P waves and QRS complexes; however, the QRS complexes occur at a faster rate than the P rate. Unlike AV block, in which failure of an intrinsically more rapid atrial rhythm to conduct antegrade and supersede a slower ventricular rhythm is abnormal, failure of a rapid ventricular rhythm to conduct retrograde and supersede a slower atrial rhythm does not necessarily imply damage to the conducting system. In fact, AV dissociation with more rapid ventricular rates is typically due to unusual ventricular irritability.
In general, patients that present with first-degree or second-degree Mobitz type 1 AV block do not require treatment. Any provoking medications can be removed, and patients can be monitored on an outpatient basis. However, patients with higher degrees of AV block (Mobitz type 2 AV block, 3rd degree) tend to have severe damage to the conduction system. They are at a much greater risk of progressing into asystole, ventricular tachycardia, or sudden cardiac death. Hence, they require urgent admission for cardiac monitoring, backup temporary cardiac pacing, and insertion of a permanent pacemaker.
Once diagnosed, underlying causes should be evaluated that include ischemic workup, autoimmune diseases in young patients that can cause fibrosis of the conducting system, offending medications and electrolyte disturbances such as hyperkalemia.
Prognosis depends on the various factors that include age and other chronic medical conditions such as diabetes mellitus, chronic kidney disease, underlying heart disease, and underlying types of AV block.
Pacemaker infection is common in the elderly, especially with underlying medical conditions. Also, sometimes it can be challenging for pacemaker patients who need other studies like MRI for diagnosing other medical conditions such as stroke.
Patients with first-degree and asymptomatic Mobitz type 1 AV block usually can continue their usual activities but should be advised to avoid medications that can prolong PR interval Patients with Mobitz type 2 and third-degree AV block should discuss with their cardiologists about the need for pacemakers. All patients should be educated on alarming symptoms of hypoperfusion such as fatigue, lightheadedness, syncope, presyncope, or angina and seek timely medical treatment
The management of heart block is best done with an interprofessional team because if the diagnosis is missed (esp higher degrees of heart block), the condition can have significant morbidity and mortality.
Patients with heart block may be encountered by the nurse practitioner, primary care provider, internist or the emergency department physician. Except for a first-degree heart block, the rest of the patients should be referred to a cardiologist for more definitive workup. Some of these patients may require a pacemaker which can be life-saving. Following treatment, the cardiology nurse should follow up on the patients to ensure that the heart rate has normalized and the patients have no symptoms.
Anytime patients with a pacemaker undergo surgery, the cardiologist should be consulted first. In some cases, the pacemaker may have to be deactivated with a magnet to prevent interference from electrocautery. After surgery, the pacemaker has to be reprogrammed. Today, most centers have a pacemaker nurse who monitors these patients for complications. Only through a combined team approach can the morbidity of heart block be decreased.
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