Premature Ventricular Contractions (PVCs), Animation

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 a few symptoms when present
symptoms may include skipped heartbeats 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 three 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 you

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