Premature Ventricular Contractions
(PVCs)
Medical Author: Dennis Lee, MD
Medical Editor: William C. Shiel, Jr, MD, FACP, FACR
What are premature ventricular contractions?
Premature ventricular contractions (PVCs) are premature heartbeats
originating from the ventricles of the heart. PVCs are premature because they
occur before the regular heartbeat.
Normal function of the heart
The heart has four chambers. The upper two chambers are the atria, and the lower
two chambers are the ventricles. The atria deliver blood to the ventricles, and
the ventricles deliver blood to the lungs and to the rest of the
body. (The right ventricle delivers blood to the lungs while the left ventricle delivers
blood to the rest of the body). The heartbeat (pulse) that we feel is
caused by the contraction of the ventricles.
The heartbeat is normally
controlled by the electrical system of the heart. The electrical system of the
heart consists of the SA node (sino-atrial
node), the AV node (atrio-ventricular
node) and special tissues in the ventricles that conduct electricity.
The SA
node is the heart's electrical pacemaker. It is a small patch of cells located
in the wall of the right atrium; the
frequency with which the SA node discharges electricity determines the rate at
which the heart normally beats. The SA node keeps the heart beating in a regular
manner. At rest, the frequency of the electrical discharges originating from the
SA node is low, and the heart beats at the lower range of normal (60-80 beats/minute). During exercise or
excitement, the frequency of discharges from the SA node increases, increasing
the rate at which the heart beats.
The electrical discharges pass from the SA
node, through the special tissues of the atria into the AV node, and through the
AV node to the special conduction tissues of the ventricles and causing the
ventricles to contract.
What happens during a premature ventricular contraction?
During a premature ventricular contraction, the
ventricle
electrically discharges (and contracts) prematurely before the normal electrical
discharges arrive from the SA node. These premature discharges are due to
electrical "irritability" of the heart muscle of the ventricles, and can be
caused by heart attacks, electrolyte imbalances, lack of oxygen, or medications.
Immediately after a premature ventricular contraction, the electrical system of the heart resets. This
resetting cause a brief pause in heartbeat, and some patients report feeling the
heart briefly stopping after a premature ventricular contraction.
Next: How common are premature ventricular contractions? »
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