Electrical conduction system of the heart
The heart consists of four chambers – the upper two chambers (atria) and the lower two chambers (ventricles). The rhythm of the heart is usually controlled by a pacing point (sinus node) located intrinsically in the right atrium. The sinus node generates electrical impulses that initiate each heartbeat. The electrical impulses are conducted from the sinoatrial node to the atria, causing the atria to contract and pump blood into the ventricles. The electrical impulses then travel to a group of cells called the AV node.
The AV node conducts the signal to a group of specialized cells called the bundle of Hirschsprung. These cells transmit the signal down the left bundle branch to the left ventricle and down the right bundle branch to the right ventricle. As the electrical impulse travels down these branches, the ventricles contract and pump blood out – the right ventricle pumps low-oxygen blood into the lungs and the left ventricle pumps oxygen-rich blood throughout the body. Bradycardia occurs when the conduction of electrical signals is slowed or blocked.
Sinus node disorders
Bradycardia usually begins in the sinoatrial node. Bradycardia occurs when the sinus node
1. sends electrical impulses at a frequency lower than the normal heart rate.
2. a pause or failure to send electrical impulses in normal heart rhythm
3. The electrical impulses are blocked before they reach the atria.
In some populations, sinus node disorders may result in alternating increases and decreases in heart rate (bradycardia-tachycardia syndrome); blocked cardiac conduction (atrioventricular block); and bradycardia when electrical signals fail to travel from the atria to the ventricles (blocked cardiac conduction or atrioventricular block).
The site of electrical block may be in the AV node, the bundle of Hirschsprung, or any of the left and right bundle branches that conduct electrical signals to the ventricles. Cardiac block can be classified according to the degree of electrical signal transmission from the atria to the main pumping chambers of the heart (ventricles).
1. First degree heart block. In the mildest form of block, all electrical signals are transmitted from the atria to the ventricles, but the rate of signal transmission is mildly slowed. First-degree heart block rarely causes symptoms and usually does not require treatment if there are no other electrical conduction abnormalities.
Second degree heart block. When second-degree heart block occurs, not all electrical signals can be transmitted to the ventricles. Some heartbeat electrical signals are “missed” and can cause a decrease in heart rate, sometimes resulting in an irregular heart rate.
In third-degree (complete) heart block, all electrical signals are not transmitted from the atria to the ventricles. When this occurs, the bundle of Hitchcock and other ventricular tissues function as alternative pacing points. This substitution results in slow and sometimes unreliable electrical impulses to control the ventricular beat.
Bundle branch conduction block. If the electrical signal is blocked at the end of the conduction pathway, either in the left or right bundle branch, it is called a bundle branch block. The severity of a bundle branch block depends on whether both bundle branches are involved, whether other types of heart block are present, and the extent of myocardial tissue damage.
Complications.
Different complications may occur with untreated bradycardia, depending on the degree of slowing of the heartbeat, the site of electrical conduction block, and the type of damage to cardiac tissue present.
If the bradycardia has become severe enough to cause clinical symptoms, possible complications of a slow heart rhythm include
1. frequent fainting.
2. Inability of the heart to pump enough blood (heart failure).
3. sudden cardiac arrest or sudden death.
Risk factors: age
The main risk factor for bradycardia is age. Heart disease is more common in older adults and is mostly associated with bradycardia. Risk factors associated with heart disease, bradycardia is often associated with certain heart diseases that damage the heart muscle tissue.
Therefore, factors that contribute to an increased risk of heart disease also increase the risk of bradycardia. Lifestyle modification or medication can reduce the risk of heart disease associated with: high blood pressure; smoking; alcohol abuse; drug use; and psychological stress or anxiety.
Pre-visit preparation.
Whether you see your family doctor first or receive emergency care, you may be referred to a cardiologist (heart doctor) for a complete diagnostic evaluation after one or more visits. If possible, bring a family member or friend along to provide some moral support and to help document new information. Since there are many issues to discuss, it is helpful to be as well prepared as possible.