heart block

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Heart Block



Heart block refers to a delay in the normal flow of electrical impulses that cause the heart to beat. They are further classified as first-, second-, or third-degree block.


The muscles of the heart contract in a rhythmic order for each heart beat, because electrical impulses travel along a specific route called the conduction system. The main junction of this system is called the atrioventricular node (AV node). Just as on a highway, there are occasionally some delays getting the impulse from one point to another. These delays are classified according to their severity.
In first-degree heart block, the signal is just slowed down a little as it travels along the defective part of the conduction system so that it arrives late traveling from the atrium to the ventricle.
In second-degree heart block, not every impulse reaches its destination. The block may affect every other beat, every second or third beat, or be very rare. If the blockage is frequent, it results in an overall slowing of the heart called bradycardia.
Third-degree block, also called complete heart block, is the most serious. When no signals can travel through the AV node, the heart uses its backup impulse generator in the lower portion of the heart. Though this impulse usually keeps the heart from stopping entirely, it is too slow to be an effective pump.

Causes and symptoms

First-degree heart block is fairly common. It is seen in teenagers, young adults and in well-trained athletes. The condition may be caused by rheumatic fever, some types of heart disease and by some drugs. First-degree heart block produces no symptoms.
Some cases of second-degree heart block may benefit from an artificial pace-maker. Second-degree block can occasionally progress to third-degree.
Third-degree heart block is a serious condition that affects the heart's ability to pump blood effectively. Symptoms include fainting, dizziness and sudden heart failure. If the ventricles beat more than 40 times per minute, symptoms are not as severe, but include tiredness, low blood pressure on standing, and shortness of breath.
Young children who have received a forceful blunt chest injury, can experience first-, or second-degree heart block.


Diagnosis of first-, and second-degree heart block is made by observing it on an electrocardiograph (ECG).
Third-degree heart block usually results in symptoms such as fainting, dizziness and sudden heart failure, which require immediate medical care. A physical exam and ECG confirm the presence of heart block.


Some second- and almost all third-degree heart blocks require an artificial pacemaker. In an emergency, a temporary pacemaker can be used until an implanted device is advisable. Most people need the pacemaker for the rest of their lives.


Most people with first- and second-degree heart block don't even know they have it. For people with third-degree block, once the heart has been restored to its normal, dependable rhythm, most people live full and comfortable lives.



American Heart Association. 7320 Greenville Ave. Dallas, TX 75231. (214) 373-6300. http://www.americanheart.org.

Key terms

Atrioventricular node (AV node) — Highly specialized area of the heart muscle which transmits electrical impulses.
Bradycardia — A slow heart rate, usually under 60 beats per minute.

heart block

impairment of conduction in heart excitation; often applied specifically to atrioventricular block. See types listed at block. 

When isolated impulses from the atria fail to reach the ventricles, heartbeats are missed and the block is called incomplete. When no impulses reach the ventricles from the atria the heart block is complete, with the result that the atria and the ventricles beat at separate rates. In this case the beats of the atria and ventricles remain regular but the rate of the ventricular beats is much slower.

Heart block can occur with various forms of heart disease and as a result of excessive dosages of certain medications such as digitalis. A particularly severe form of heart block is stokes-adams disease, in which sudden unconsciousness results from the slowed heartbeat. It may be accompanied by convulsions.

The treatment for heart block caused by digitalis overdosage is to stop the medication. When there is hemodynamic instability, an antidote, digoxin immune Fab (ovine) (Digibind), can be administered intravenously for more rapid reduction of serum levels. If there is no response, temporary cardiac pacing is indicated.

a·tri·o·ven·tric·u·lar block

, AV block
partial or complete block of electrical impulses originating in the atrium or sinus node, preventing them from reaching the AV node and ventricles. In first-degree AV block, there is prolongation of AV conduction time (PR interval); in second-degree AV block, some but not all atrial impulses fail to reach the ventricles, thus some ventricular beats are dropped; in complete AV block (third degree), complete atrioventricular dissociation (2) occurs; atria and ventricles beat independently.
Synonym(s): block (3) , heart block

heart block

(hahrt blok) impairment of conduction of an impulse in heart excitation; often applied specifically to atrioventricular block. For specific types, see under block.

heart block

A condition in which faulty transmission of the impulses that control the heartbeat results in a lack of coordination in the contraction of the atria and ventricles of the heart.

heart block

an interference with the normal conduction of electrical impulses that control activity of the heart muscle. Heart block is usually specified by the location of the block and the type, such as a first-degree atrioventricular (AV) block, in which atrial impulses are delayed by a fraction of a second before being conducted to the ventricles. Heart block can occur in the sinus node, atria, AV node, AV bundle, fascicles, or a combination of these structures. See also atrioventricular block, bundle branch block, cardiac conduction defect, infranodal block, intraatrial block, intraventricular block, sinoatrial block.

heart block

Cardiology An arrhythmia in which an impulse is not conducted normally from the atria to the ventricles due a damaged cardiac conduction system Types 1st, 2nd, 3rd degree; various HBs can occur in the bundle of His or bundle branches
Heart block
First-degree Prolonged interval ≥ 200 ms/0.2 sec between atrial and ventricular depolarizations
Second-degree A block of some, but not all, impulses traveling from the atria to the ventricles. It can take 2 forms
• Mobitz Type I or Wenckebach phenomenon–successive prolongation of the P-R interval until one P-wave does not elicit a QRS response
• Mobitz Type II block–occurs when occasional P-waves are blocked from ventricles
Third-degree Complete Heart Block Condition in which all impulses from the atria are blocked; usually an ectopic focus in the ventricles takes over  

heart block

A functional dissociation between the contractions of the upper chambers of the heart and those of the lower, due to interruption of the specialized conducting muscle fibres in the heart that normally coordinate this relationship. Such damage is usually due to inadequate coronary blood supply to the heart muscle but may be due to other causes such as rheumatic fever, syphilis or tumour. In heart block the pulse is generally very slow, as the lower chambers can only beat at their intrinsically low rate. Heart block is one of the major indications for the implantation of an artificial PACEMAKER.

heart block

impairment of conduction in heart excitation; often applied specifically to atrioventricular heart block.
When isolated impulses from the atria fail to reach the ventricles, heartbeats are missed and the block is called incomplete. When no impulses reach the ventricles from the atria the heart block is complete, with the result that the atria and the ventricles beat at separate rates. In this case the beats remain regular but the rate of the ventricular beats is greatly slowed down.

atrioventricular (A-V) heart block
a form in which the blocking is at the atrioventricular junction. It is first degree when A-V conduction time is prolonged; second degree (partial heart block) when some but not all atrial impulses reach the ventricle; third degree (complete heart block) when no atrial impulses at all reach the ventricle, and the atria and ventricles act independently of each other.
bundle-branch heart block
a form in which one ventricle is excited before the other because of absence of conduction in one of the branches of the bundle of His.
complete heart block
see atrioventricular heart block (above).
fascicular heart block
one originating on one of the two divisions of the left bundle branch. See also hemiblock.
interventricular heart block
bundle-branch heart block.
Mobitz heart block b's
variations of second-degree heart blocks. See also wenckebach's phenomenon.
sinoatrial heart block
partial or complete impairment of conduction from the sinoatrial node to the atria, resulting in delay or absence of an atrial beat.
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