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Atrioventricular Block

Atrioventricular Block an interruption of the electrical signal between the atria and the ventricles. Atrioventricular block of the electrical signal between the atria and ventricles; can vary in severity from first, second or third degree (complete heart block).. A conduction disturbance that consists of a delay (or complete inability) of a electrical impulse, generated in the atria, to reach the ventricles. Clinical types are divided into first (no serious), second and third degree (most serious).The heart block: recurrent sudden attacks of unconsciousness caused by impaired conduction of the impulse that regulates the heartbeat . Some drugs may precipitate atrioventricular block (for example clonidine, methyldopa, verapamil). A permanent pacemaker may be required for a third degree (complete) heart block.

Atrioventricular block is categorized as first-degree (PR interval > 0.21 second with all atrial impulses conducted), second-degree (intermittent blocked beats), or third-degree (complete heart block, in which no supraventricular impulses are conducted to the ventricles). Atrioventricular Block is an unusually slow heartbeat due to a slowing or blocking of electrical impulses in the heart's conduction system .That is a third degree heart block due to total cessation of impulse conduction through the atrioventricular junctional tissue; no correspondence exists between atrial and ventricular activity, and ventricular asystole and death occur unless a ventricular pacemaker is activated.

Second-degree block is subclassified. In Mobitz type I (Wenckebach) atrioventricular block, the atrioventricular conduction time (PR interval) progressively lengthens, with the RR interval shortening, before the blocked beat (see ECG); this phenomenon is almost always due to abnormal conduction within the atrioventricular node. In Mobitz type II atrioventricular block, there are intermittently nonconducted atrial beats not preceded by lengthening atrioventricular conduction (see ECG). It is usually due to block within the His bundle system. The classification as Mobitz type I or Mobitz type II is only partially reliable, because patients may appear to have both types on the surface ECG, and the site of origin of the 2:1 atrioventricular block cannot be predicted from the ECG. The width of the QRS complexes assists in determining whether the block is nodal or infranodal. When they are narrow, the block is usually nodal; when they are wide, the block is usually infranodal. Electrophysiologic studies may be necessary for accurate localization. Management of atrioventricular block in acute myocardial infarction has already been discussed. This section deals with patients in the nonischemic setting.

First-degree and Mobitz type I block may occur in normal individuals with heightened vagal tone (see ECG); (see ECG). They may also occur as a drug effect (especially digitalis, calcium channel blockers, -blockers, or other sympatholytic agents), often superimposed on organic disease. These disturbances also occur transiently or chronically due to ischemia, infarction, inflammatory processes, fibrosis, calcification, or infiltration. The prognosis is usually good, since reliable alternative pacemakers arise from the atrioventricular junction below the level of block if higher degrees of block occur.

Mobitz type II block is almost always due to organic disease involving the infranodal conduction system (see ECG). In the event of progression to complete heart block, alternative pacemakers are not reliable. Thus, prophylactic ventricular pacing is required.

Complete (third-degree) heart block is a more advanced form of block often due to a lesion distal to the His bundle and associated with bilateral bundle branch block. The QRS is wide and the ventricular rate is slower, usually less than 50 beats/min (see ECG). Transmission of atrial impulses through the atrioventricular node is completely blocked, and a ventricular pacemaker maintains a slow, regular ventricular rate, usually less than 45 beats/min. Exercise does not increase the rate. The first heart sound varies in intensity; wide pulse pressure, a changing systolic blood pressure level, and cannon venous pulsations in the neck are also present. Patients may be asymptomatic or may complain of weakness or dyspnea if the rate is less than 35 beats/min; symptoms may occur at higher rates if the left ventricle cannot increase its stroke output. During periods of transition from partial to complete heart block, some patients have ventricular asystole that lasts several seconds to minutes. Syncope occurs abruptly.

Cause of Atrioventricular Block

Some common causes of Atrioventricular Block:

  • First-degree and second-degree Mobitz I (Wenckebach) AV blocks may occur in healthy, well-conditioned people as a physiologic manifestation of high vagal tone.
  • History of heart attacks
  • Coronary artery disease
  • Infectious diseases of the heart, such as endocarditis
  • Hereditary defect of the heart, called congenital heart block
  • Atrioventricular Block block may be caused by acute myocardial ischemia or infarction. Inferior myocardial infarction may lead to third-degree block, usually at the atrioventricular block node level.
  • And by other mechanisms via the Bezold-Jarisch reflex.
  • Anterior myocardial infarction usually is associated with third-degree block due to ischemia or infarction of bundle branches.
  • Beta-adrenergic blockers Calcium antagonists Cholinesterase inhibitors (organophosphates, carbamates & nerve agents) Clonidine and other centrally-acting alpha-2 adrenergic agonists Digitalis and other cardiac glycosides

Symptones of Atrioventricular Block

The main symptones of Atrioventricular Block:

  • Symptoms depend on how severe the heart block is and in what part of the heart it occurs.
  • The person may feel dizzy, weak, confused, or may have less tolerance for exercise. At times, some people may even pass out.
  • Often, heart block causes no symptoms. If the heart block is severe enough, the heart rate will slow, and symptoms of low blood pressure or stroke will appear.
  • fainting
  • stroke
  • sudden death

Treatment of Atrioventricular Block

Some importent treatment methods of Atrioventricular Block:

  • The major interventions are atropine, transcutaneous pacing, catecholamine infusions (dopamine or epinephrine), and transvenous pacemaker. Isoproterenol is rarely indicated.
  • Administer specific antidotes if indicated (see below).
  • Consider continuous intravenous infusion of isoprenaline 1 to 10 mcg/min and/or insertion of a transvenous or transcutaneous pacemaker.
  • Many times heart blocks are not treated at all. Treatment of other heart problems reduces the risk of the block getting worse.
  • The patient with no evidence of end-organ hypoperfusion, simple observation, establishment of intravenous access, administration of supplemental oxygen and cardiac rhythm monitoring may be all that is required.

Patients with episodic or chronic infranodal complete heart block require permanent pacing, and temporary pacing is indicated if implantation of a permanent pacemaker is delayed.

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