(USMLE topics, cardiology) Causes, Symptoms, Pathology of PVCs.

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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.

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