The term "arrhythmia" refers to any change from the normal sequence of electrical impulses. The electrical impulses may happen too fast, too slowly, or erratically – causing the heart to beat too fast, too slowly, or erratically. When the heart doesn't beat properly, it can't pump blood effectively. When the heart doesn't pump blood effectively, the lungs, brain and all other organs can't work properly and may shut down or be damaged.
Structure of the heart: four chambers, four valves
The heart has four chambers, two on the right and two on the left:
Two upper chambers are called atria (one is called an atrium).
Two lower chambers are called ventricles.
The heart also has four valves that open and close to let blood flow in only one direction when the heart contracts (beats). The four heart valves are:
Tricuspid valve, located between the right atrium and right ventricle
Pulmonary or pulmonic valve, between the right ventricle and the pulmonary artery
Mitral valve, between the left atrium and left ventricle
Aortic valve, between the left ventricle and the aorta
Each valve has a set of flaps (also called leaflets or cusps). The mitral valve has two flaps; the others have three. Blood flow occurs only when there's a difference in pressure across the valves, which causes them to open. Under normal conditions, the valves permit blood to flow in only one direction.
The heart pumps blood to the lungs and to all the body's tissues by a sequence of highly organized contractions of the four chambers. For the heart to function properly, the four chambers must beat in an organized way.
Electrical system of the heart
Electrical signals control the pump
The heart beat (contraction) begins when an electrical impulse from the sinoatrial node (also called the SA node or sinus node) moves through it. The SA node is sometimes referred to as the heart's "natural pacemaker" because it initiates impulses for the heartbeat.
The normal electrical sequence begins in the right atrium and spreads throughout the atria to the atrioventricular (AV) node. From the AV node, electrical impulses travel down a group of specialized fibers called the His-Purkinje system to all parts of the ventricles.
This exact route must be followed for the heart to pump properly. As long as the electrical impulse is transmitted normally, the heart pumps and beats at a regular pace. In an adult, a normal heart beats 60 to 100 times a minute.
Arrhythmias are abnormal beats. The term "arrhythmia" refers to any change from the normal sequence of electrical impulses, causing abnormal heart rhythms. Arrhythmias may be completely harmless or life-threatening.
Some arrhythmias are so brief (for example, a temporary pause or premature beat) that the overall heart rate or rhythm isn't greatly affected. But if arrhythmias last longer, they may cause the heart rate to be too slow or too fast or the heart rhythm to be erratic – so the heart pumps less effectively.
A fast heart rate (in adults, more than 100 beats per minute) is called tachycardia.
A slow heart rate (less than 60 beats per minute) is referred to as bradycardia.
Normally, the heart's most rapidly firing cells are in the sinus (or sinoatrial or SA) node, making that area a natural pacemaker.
Under some conditions almost all heart tissue can start an impulse of the type that can generate a heartbeat. Cells in the heart's conduction system can fire automatically and start electrical activity. This activity can interrupt the normal order of the heart's pumping activity.
Secondary pacemakers elsewhere in the heart provide a "back-up" rhythm when the sinus node doesn't work properly or when impulses are blocked somewhere in the conduction system.
An arrhythmia occurs when:
The heart's natural pacemaker develops an abnormal rate or rhythm.
The normal conduction pathway is interrupted.
Another part of the heart takes over as pacemaker.