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How the Heart Works

[electrical conduction]

Monitoring Heart Electrical Activity (EKG)

The heart contains special tissue that produces and sends electrical impulses to the heart muscle. It is these impulses that trigger the heart to contract. Each time the heart beats, it sends out an electric-like signal. The heart's electrical signals can be measured with a special machine called an electrocardiogram (EKG or ECG).

To record the ECG, small patches or stickers called electrodes are placed on different parts of the body. One is put on each arm and leg and six across the chest. Each of these leads monitors distinct areas of the heart. Using combinations of these electrodes, different tracings of the heart's electrical activity can be made and permanently recorded on paper or in a computer. The tracing at the top is how a given lead of an EKG tracing might appear .

Three major waves of electric signals appear on the ECG. Each one shows a different part of the heartbeat. The first wave is called the P wave. It records the electrical activity of the heart's two upper chambers (atria).The second and largest wave, the QRS wave, records the electrical activity of the heart's two lower chambers (ventricles).The third wave is the T wave. It records the heart's return to the resting state.

By studying the shape and size of the waves, the time between waves, and the rate and regularity of beating, a doctor can learn a lot about the heart and its rhythm.

Heart Conduction System

The sinoatrial node (SAN), located within the wall of the right atrium (RA), normally generates electrical impulses that are carried by special conducting tissue to the atrioventricular node (AVN).

Upon reaching the AVN, located between the atria and ventricles, the electrical impulse is relayed down conducting tissue (Bundle of HIS) that branches into pathways that supply the right and left ventricles. These paths are called the right bundle branch (RBBB) and left bundle branch (LBBB) respectively. The left bundle branch further divides into two sub branches (called fascicles).

Electrical impulses generated in the SAN cause the right and left atria to contract first. Depolarization (heart muscle contraction caused by electrical stimulation) occurs nearly simultaneously in the right and left ventricles 1-2 tenths of a second after atrial depolarization. The entire sequence of depolarization, from beginning to end (for one heart beat), takes 2-3 tenths of a second.

All heart cells, muscle and conducting tissue, are capable of generating electrical impulses that can trigger the heart to beat. Under normal circumstances all parts of the heart conducting system can conduct over 140-200 signals (and corresponding heart beats) per minute.

The SAN is known as the "heart's pacemaker" because electrical impulses are normally generated here. At rest the SAN usually produces 60-70 signals a minute. It is the SAN that increases its' rate due to stimuli such as exercise, stimulant drugs, or fever.

Should the SAN fail to produce impulses the AVN can take over. The resting rate of the AVN is slower, generating 40-60 beats a minute. The AVN and remaining parts of the conducting system are less capable of increasing heart rate due to stimuli previously mentioned than the SAN.

The Bundle of HIS can generate 30-40 signals a minute. Ventricular muscle cells may generate 20-30 signals a minute.

Heart rates below 35-40 beats a minute for a prolonged period usually cause problems due to not enough blood flow to vital organs.

Problems with signal conduction, due to disease or abnormalities of the conducting system, can occur anyplace along the heart's conduction pathway. Abnormally conducted signals , resulting in alterations of the heart's normal beating, are called arrhythmias or dysrrythmia.

By analyzing an EKG a doctor is often able to tell if there are problems with specific parts of the conducting system or if certain areas of heart muscle may be injured.

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