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.
Types
of Arrhythmias
The normal heart is a strong, muscular pump a
little larger than a fist. It pumps blood continuously through the circulatory
system. - Each day the average heart
beats (expands and contracts) 100,000 times and pumps about 2,000 gallons
of blood.
- In a 70-year lifetime, an
average human heart beats more than 2.5 billion times.
To understand how the heart pumps, learn about:
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 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
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
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.
Causes
- 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.
"This content was last reviewed on 05/30/2012."