science

Premature ventricular contraction

Premature ventricular contraction is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Premature ventricular contraction rather than just read about it. In short: A premature ventricular contraction (PVC) is a common event where the heartbeat is initiated by Purkinje fibers in the ventricles rather than by the sinoatrial node. PVCs may cause no symptoms or may be perceived as a "skipped beat" or felt as palpitations in the chest.

Key takeaways

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Reference excerpt

A premature ventricular contraction (PVC) is a common event where the heartbeat is initiated by Purkinje fibers in the ventricles rather than by the sinoatrial node. PVCs may cause no symptoms or may be perceived as a "skipped beat" or felt as palpitations in the chest. PVCs do not usually pose any danger. The electrical events of the heart detected by the electrocardiogram (ECG) allow a PVC to be easily distinguished from a normal heart beat. However, very frequent PVCs can be symptomatic of an underlying heart condition (such as arrhythmogenic right ventricular cardiomyopathy). Furthermore, very frequent (over 20% of all heartbeats) PVCs are considered a risk factor for arrhythmia-induced cardiomyopathy, in which the heart muscle becomes less effective and symptoms of heart failure may develop. Ultrasound of the heart is therefore recommended in people with frequent PVCs. If PVCs are frequent or troublesome, medication (beta blockers or certain calcium channel blockers) may be used. Very frequent PVCs in people with dilated cardiomyopathy may be treated with radiofrequency ablation.

Signs and symptoms Although there are many possible symptoms associated with PVCs, PVCs may also have no symptoms at all. PVCs may be perceived as a skipped heart beat, a strong beat, palpitations, or lightheadedness. They may also cause chest pain, a faint feeling, fatigue, or hyperventilation after exercise. Symptoms may be more pronounced at times of stress. 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. 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 tract's area of slow conduction activates the other tract's 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). This ectopy of the ventricles when associated with a structurally normal heart most commonly occurs from the right ventricular outflow tract (RVOT) under the pulmonic valve. The mechanism behind this is thought to be enhanced automaticity versus triggered activity.

Molecular basis There are a number of different molecular explanations for PVCs.

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Illustrations

Worked examples

Example 1 — a first encounter with Premature ventricular contraction

Start with the simplest possible case. Write down what Premature ventricular contraction claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Premature ventricular contraction before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Premature ventricular contraction ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Premature ventricular contraction

In research: Premature ventricular contraction appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Premature ventricular contraction in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Premature ventricular contraction is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cardiac arrhythmia, so understanding it makes those chapters shorter.
In everyday life: Look for Premature ventricular contraction outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Premature ventricular contraction in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Premature ventricular contraction means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Premature ventricular contraction out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Premature ventricular contraction in simple terms?

Why does Premature ventricular contraction matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Premature ventricular contraction?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Premature ventricular contraction.