Heart Block

heart block
a condition in which conduction of the electrical impulses generated by the natural pacemaker of the heart (the sinoatrial node) is impaired.

In partial or incomplete heart block conduction between atria and ventricles is delayed (first degree heart block) or not all the impulses are conducted from the atria to the ventricles (second degree heart block). In third degree or complete heart block no impulses are conducted and the ventricles beat at their own slow intrinsic rate (20–40 per minute).
Heart block may be congenital or it may be due to heart disease, including myocardial infarction, myocarditis, cardiomyopathy, and disease of the valves. It is most frequently seen in the elderly as the result of chronic degenerative scarring around the conducting tissue. There may be no symptoms, but when very slow heart and pulse rates occur the patient may develop heart failure or syncope (see Stokes-Adams syndrome). Symptoms may be abolished by the use of an artificial pacemaker.

sinoatrial node
(SA node) the pacemaker of the heart: a microscopic area of specialized cardiac muscle located in the upper wall of the right atrium near the entry of the vena cava. Fibres of the SA node are self-excitatory, contracting rhythmically at around 70 times per minute. Following each contraction, the impulse spreads throughout the atrial muscle and into fibres connecting the SA node with the atrioventricular node. The SA node is supplied by fibres of the autonomic nervous system; impulses arriving at the node accelerate or decrease the heart rate.