Depending on the duration and severity of the heart block, patients with AV block may be asymptomatic or complain of syncope, near syncope, sudden cardiac death, palpitations, angina, or transient ischemic attacks.
Secondary to conduction abnormalities in the AV node, bundle of His, or bundle branches impairing transmission from atria to the ventricles.
The disturbance may be intermittent or permanent.
Classification (Table 31-5)
In first-degree AV block, all of the sinus impulses (P waves) are conducted but the PR interval is prolonged.
In second-degree block, some of the impulses are conducted. There are 2 subtypes: Mobitz type I and Mobitz type II (Table 31-5).
In third-degree AV block, none of the P waves are conducted (Figure 31-8).
In second- or third-degree AV block, the ventricular rate slows and may depend on lower intrinsic pacemakers residing within the ventricle. The bradycardia can cause dyspnea, angina, hypotension, syncope, or death.
AV nodal disease should also be suspected in patients with atrial fibrillation who have a slow ventricular response and are not taking medications that slow AV conduction (eg, digoxin, beta-blockers, verapamil, or diltiazem).
Fibrosis of the conduction system
Ischemic heart disease
Medications (eg, beta-blockers, verapamil, diltiazem, digoxin, adenosine, amiodarone)
Most patients with AV block attributed to verapamil, diltiazem, or beta-blockers also have conduction disease and are likely (> 80%) to experience AV block even off medications.
Sarcoidosis is a common cause of unexplained second- or third-degree AV block in patients younger than 60 years (34%) and should be considered even if there is no prior diagnosis of sarcoidosis. 27% of such patients subsequently suffer from VT of HF.
Valvular heart disease (due to extension of calcification into the conduction system)
Increased vagal tone
Miscellaneous other causes (hypothyroidism, Lyme disease, amyloidosis, etc)
Cardiac procedures (ie, transcathetic aortic valve implantation)
Table 31-5.Classification of heart block. ||Download (.pdf) Table 31-5. Classification of heart block.
|Classification ||AV Conduction ||ECG Findings ||Clinical Findings ||Treatment |
|First-degree ||1:1 || |
PR interval > 0.2 seconds
QRS width usually within normal limits
|None ||None |
|Second-degree Mobitz I ||Intermittent || |
PR interval increases progressively until P wave is not conducted and QRS absent.
PR interval after dropped QRS shorter than PR prior to dropped QRS
QRS width < 0.12 sec
Associated with inferior MI
Rarely progresses to third-degree AV block
|Observation or atropine |
|Second-degree Mobitz II ||Intermittent || |
Intermittent nonconduction of P waves
QRS may be widened, BBB may be seen (due to more severe infranodal damage)
Associated with anterior MI
Often progresses to third-degree AV block
|Third-degree ||ϕ || |
P waves not conducted
Complete AV disassociation
Ventricular rate depends on escape pacemakers
|Associated with CAD, drugs, degeneration, abnormal electrolytes, bradycardia, hypotension ||Pacemaker |
Third-degree atrioventricular block.