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Second-degree heart block, also referred to as second-degree atrioventricular (AV) block, is a heart condition in which there are certain irregularities in the electrical conduction system of the heart. This condition basically refers to the irregular or a slower-than-normal heartbeat. The heart pumps the flow of blood via the circulatory system to ensure smooth functioning of all the organs in the body. The flow of the blood is regulated from the beginning of one heartbeat to the beginning of the next. This cycle is known as the cardiac cycle, and its frequency is known as the heart rate. Any disruption in this frequency can lead to a type of heart block.

Types
The heart is made up of four chambers―the top two are called the atria, and the bottom two the ventricles. The heart contracts to facilitate blood flow. It does so due to the electrical signals that travel between the heart's atria and ventricles via specialized conducting tissue known as the atrioventricular (AV) node. A healthy heart beats 60 - 80 times a minute, but if these electrical impulses are delayed, stopped or are not transmitted efficiently, the heart will not beat regularly. This phenomenon is known as a heart block or AV block. In this condition, some signals from the atria don't reach the ventricles, resulting in 'dropped beats'. It is classified in two types:
  • Type 1: Type 1 is also known as Mobitz Type I or Wenckebach's AV block. In this condition, electrical impulses are delayed, until a heartbeat is skipped.
  • Type 2: In this condition, the impairment is more, as the electrical impulses can't reach the ventricles, resulting in an abnormally slow heartbeat. This type is called Mobitz Type II, and is more serious in nature.
Causes
Causes of a second-degree AV block more or less depend upon the type. Type 1 is caused by a high vagal tone, and is seen more in athletes, especially long-distance runners and active young children. Cardioactive drugs like beta-blockers, amiodarone, calcium channel blockers, etc., can also cause a heart block of type 1. Individuals who have undergone valvular surgery, or those with structural heart disease, either congenial or a later development, is also seen as a cause for the condition.

Various inflammatory diseases like Endocarditis, Lyme disease, Acute rheumatic fever, etc., collagen vascular diseases such as rheumatoid arthritis, Lupus erythematosus, Reiter's syndrome, etc., infiltrative diseases like Amyloidosis, Sarcoidosis, etc., or metabolic and endocrine disorders such as hyperkalemia, Addison's disease, hyperthyroidism, etc., are also seen as causes of this condition. Other causes include Acute myocardial infarction, cardiac tumors, trauma, myocardial bridging, muscular dystrophies, etc.

Symptoms
In most cases, the symptoms do not exhibit themselves. But a few that do, include tightness in the chest, fatigue, light headiness, dizziness, and fainting spells.

Treatment
Treatment depends upon the type of block and the severity of the condition. Medication like Atropine is prescribed for treating immediate symptomatic second-degree AV block in the AV node. However, this medication may not help improve electrical conduction of the heart in certain conditions. The implantation of a permanent pacemaker is also a recommended option treatment. Related symptoms should not be ignored, else, the condition can prove to be life-threatening.