Premature ventricular contractions, PVCs, are premature heartbeats originating in one of the
lower chambers of the heart, the ventricles. PVCs can result from a variety of factors
and conditions, including: - consumption of alcohol,
caffeine, tobacco or drugs - certain medications
- physical exercise, stress, or anything that increases adrenaline levels
- certain electrolyte deficiencies - and damaged cardiac tissue caused
by other heart diseases. PVCs are very common,
but often produce no or few symptoms. When present, symptoms may include skipped heart
beats, palpitations, and lightheadedness. In normal conduction, electrical impulses start
in the SA node, depolarize the atria, then pass through the AV node to activate the ventricles.
A PVC happens when the ventricles are activated prematurely, by an abnormal firing site, called
ectopic site, located in one of the ventricles. Because ventricular depolarization does not come
from the atria, PVC complexes are not preceded by P waves. Unlike the normal conduction carried out
by specialized cells of the conduction pathway, the signal in PVC is conducted through the
myocytes of the heart muscle, and propagates more slowly, producing a broader QRS complex. Depending
on the location of the ectopic site, the resulting QRS complex may also be taller, or deeper than
usual. PVCs typically do not conduct back to the atria, so SA node firing is usually not affected,
and PP intervals remain unchanged. But the P wave that follows a PVC may not result in a beat
if it happens too closely to the PVC, when the ventricles are still in their refractory period.
This is referred to as a compensatory pause. There are 3 mechanisms by which a PVC may occur:
- Increased automaticity. The ability of ventricular myocytes to spontaneously
depolarize and generate action potentials is associated with intracellular calcium overload
caused by excess catecholamines, drugs such as digoxin, and certain electrolyte deficiencies,
such as hypokalemia or hypomagnesemia. - Re-entry circuit in a ventricle. This
usually happens when there is a damage to the heart muscle, such as a scar from a previous
heart attack, or other heart conditions. The damaged tissue conducts at a slower speed,
causing the electrical signal to go around it, creating a self-perpetuating loop.
- Triggered beats. These are extra-beats produced by early or delayed after-depolarizations. Early
after-depolarization may occur when the duration of action potentials is abnormally long,
in conditions such as long QT syndrome, hypokalemia, or as an effect of drugs such
as potassium channel blockers. Delayed after-depolarizations can be caused by
digoxin toxicity or excess catecholamines. Most people do not need treatment for PVC itself,
but the underlying factor or condition must be addressed.