However, when a PVC occurs the impulse nearly always travels through only one bundle fiber, so there is no neutralization effect; this results in the high voltage QRS wave in the electrocardiograph. There are three main physiological explanations for premature ventricular contractions: enhanced ectopic nodal automaticity, re-entry signaling, and toxic/reperfusion triggered. Ectopic enhanced nodal automaticity suggests foci of sub-pulmonic valvular pacemaker cells that have a subthreshold potential for firing. The basic rhythm of the heart raises these cells to threshold, which precipitates an ectopic beat. This process is the underlying mechanism for arrhythmias due to excess catecholamines and some electrolyte deficiencies, particularly low blood potassium, known as hypokalemia. Reentry occurs when an area of 1-way block in the Purkinje fibers and a second area of slow conduction are present. This condition is frequently seen in patients with underlying heart disease that creates areas of differential conduction and recovery due to myocardial scarring or ischemia. During ventricular activation, one bundle tracts area of slow conduction activates the other tracts bundle fibers post block after the rest of the ventricle has recovered. This resulting in an extra beat. Reentry can produce single ectopic beats, or it can trigger paroxysmal tachycardia. Triggered beats are considered to be due to after-depolarizations triggered by the preceding action potential. These are often seen in patients with ventricular arrhythmias due to digoxin toxicity and reperfusion therapy after myocardial infarction (MI).
Women may be more aware of PVCs at the time of the menstrual period.Premature ventricular contractions may be associated with underlying heart disease, and certain characteristics are therefore elicited routinely: the presence of signs of heart disease or a known history of heart disease (e.g. previous myocardial infarction), as well as heart disease or sudden cardiac death in close relatives. PVCs and palpitation associated with syncope (transient loss of consciousness) or provoked by exertion are also concerning. Physical examination is focused on identifying evidence of underlying heart disease. Causes
Premature ventricular contractions occur in healthy persons of any age, but are more prevalent in the elderly and in men. In a very significant proportion of people they occur spontaneously with no known cause.Some possible underlying causes of PVCs include:

Non-cardiac causes
Adrenaline excess
Anemia
Catecholamine excess
Certain medicines such as tricyclic antidepressants, digoxin, sympathomimetics, aminophylline
Chemical (electrolyte) abnormalities in the blood (for example hypokalemia (low blood potassium), which can occur in those taking diuretics ("water pills") and hypomagnesaemia (magnesium deficiency)). Contact with the carina (trachea/bronchi) when performing medical suctioning stimulates vagus nerve
Drugs/substances such as:Amphetamines
Alcohol
Caffeine
Cocaine
Nicotine
Theobromine
Hypercapnia (CO2 poisoning)
Hypertension (high blood pressure)
Hyperthyroidism
High blood calcium
Hypoxia and/or hypercapnia
Lack of sleep/exhaustion
Sarcoidosis
Smoking
Stress
Anxiety

Cardiac causes
Any structural heart disease which alters electrical conduction pathways due to tissue alterations
Cardiomyopathy, hypertrophic or dilated
Myocardial infarction
Myocarditis
Myocardial contusion
Mitral valve prolapse

Pathophysiology
Normally, impulses pass through both ventricles almost at the same time and the depolarization waves of the two ventricles partially cancel each other out in the ECG.