Heart block

Refers to electrophysiological abnormalities of atrioventricular conduction.

The heart’s pacemaker is called the sinoatrial (SA) node or sinus node.

The AV node lies at the apex of Koch’s triangle.

May be a congental or acquired process.

Acquired heart block is more common than congenital heart block.

Damage to the heart muscle and its electrical system by disease processes, surgery or medications causes acquired heart block.

The sinoatrial node is a small mass of specialized cells in the heart’s right atrium.

The SA node produces electrical impulses that make the heart beat and its signal must travel from the SA node down a specific path to reach the ventricles passing through specialized conducting tissue called the atrioventricular (AV) node.

On an electrocardiogram the P wave shows the electrical impulse passing through the atria, while the QRS wave, shows the impulse passing through the ventricles.

Impairment of the electricl signal from the SA nde to the AV node indicates heart block.

Heart block is classified according to the level of impairment — first-degree heart block, second-degree heart block or third-degree, or complete heart block.

First-degree heart block, or first-degree AV block, occurs when the electrical impulse moves through the AV node more slowly than normal.

PR interval of greater than 0.2 seconds is first-degree heart block.

With first degree heart block the heart rate and rhythm are normal, and the heart may be normal.

Digitalis can slow conduction of the impulse from the atria to the ventricles and cause first-degree AV block.

First degree heart block may be seen in well-trained athletes.

No treatment is necessary for first-degree heart block.

Second-degree heart block refers to the situation when some signals from the atria do not reach the ventricles causing dropped beats on an ECG, where the P wave is not followed by the QRS wave, because the ventricles were not activated.

Two types of second degree heart block are recognized: Type I second-degree heart block, or Mobitz Type I, or Wenckebach’s AV block, and Type II second-degree heart block, or Mobitz Type II.

In Type I second-degree heart block electrical impulses are delayed more and more with each heartbeat until a beat is skipped.

Wenckebach type block is defined by grouped beating, a prolonging PR interval with successive beats and shorter RR intervals prior to the dropped QRS complex.

AV Wenckebach, a reversible vagally mediated phenomenon, is typically asymptomatic though at bradycardic heart rates pacemaker indicated only in the presence of attributable symptoms.

Type I second-degree heart block is not generally serious but may cause dizziness and/or other symptoms.

Type II second-degree heart block, or Mobitz Type II is less common than Type I but generally more serious.

Type II second-degree heart block, or Mobitz Type may result in an an abnormally slow heartbeat, for which a pacemaker may be required..

In third-degree heart block or complete heart block (complete AV block) the heart’s electrical signal does not go from the atrium to the ventricles.

In third-degree heart block or complete heart block (complete AV block), there is no normal relationship between the P and the QRS waves on the ECG.

With third-degree heart block ventricles can contract and pump blood, but at a slower, and cannot generate the signals needed to maintain full functioning of the heart muscle.

Complete heart block is most often caused by heart disease and by drug toxicity.

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