Tachycardia

Key takeaways

Tachycardia belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Tachycardia to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Tachycardia from memory before moving on to harder problems.

Reference excerpt

Tachycardia, also called tachyarrhythmia, is a heart rate that exceeds the normal resting rate. In general, a resting heart rate over 100 beats per minute is accepted as tachycardia in adults. Heart rates above the resting rate may be normal (such as with exercise) or abnormal (such as with electrical problems within the heart).

Complications Tachycardia can lead to fainting. When the rate of blood flow becomes too rapid, or fast blood flow passes on damaged endothelium, it increases the friction within vessels resulting in turbulence and other disturbances. According to the Virchow's triad, this is one of the three conditions (along with hypercoagulability and endothelial injury/dysfunction) that can lead to thrombosis (i.e., blood clots within vessels).

Causes Some causes of tachycardia include:

Diagnosis The upper threshold of a normal human resting heart rate is based on age. Cutoff values for tachycardia in different age groups are fairly well standardized; typical cutoffs are listed below:

1–2 days: >159 beats per minute (bpm) 3–6 days: >166 bpm 1–3 weeks: >182 bpm 1–2 months: >179 bpm 3–5 months: >186 bpm 6–11 months: >169 bpm 1–2 years: >151 bpm 3–4 years: >137 bpm 5–7 years: >133 bpm 8–11 years: >130 bpm 12–15 years: >119 bpm 15 years–adult: Tachycardia >100 bpm Heart rate is considered in the context of the prevailing clinical picture. When the heart beats excessively or rapidly, the heart pumps less efficiently and provides less blood flow to the rest of the body, including the heart itself. The increased heart rate also leads to increased work and oxygen demand by the heart, which can lead to rate related ischemia.

Differential diagnosis

An electrocardiogram (ECG) is used to classify the type of tachycardia. They may be classified into narrow and wide complex based on the QRS complex. Equal or less than 0.1s for narrow complex. Presented in order of most to least common, they are:

Narrow complex Sinus tachycardia, which originates from the sino-atrial (SA) node, near the base of the superior vena cava Atrial fibrillation Atrial flutter AV nodal reentrant tachycardia Accessory pathway mediated tachycardia Atrial tachycardia Multifocal atrial tachycardia Cardiac tamponade Junctional tachycardia (rare in adults) Wide complex Ventricular tachycardia, any tachycardia that originates in the ventricles Any narrow complex tachycardia combined with a problem with the conduction system of the heart, often termed "supraventricular tachycardia with aberrancy" A narrow complex tachycardia with an accessory conduction pathway, often termed "supraventricular tachycardia with pre-excitation" (e.g. Wolff–Parkinson–White syndrome) Pacemaker-tracked or pacemaker-mediated tachycardia Tachycardias may be classified as either narrow complex tachycardias (supraventricular tachycardias) or wide complex tachycardias. Narrow and wide refer to the width of the QRS complex on the ECG. Narrow complex tachycardias tend to originate in the atria, while wide complex tachycardias tend to originate in the ventricles. Tachycardias can be further classified as either regular or irregular.

Sinus

The body has several feedback mechanisms to maintain adequate blood flow and blood pressure. If blood pressure decreases, the heart beats faster in an attempt to raise it. This is called reflex tachycardia. This can happen in response to a decrease in blood volume (through dehydration or bleeding), or an unexpected change in blood flow. The most common cause of the latter is orthostatic hypotension (also called postural hypotension). Fever, hyperventilation, diarrhea and severe infections can also cause tachycardia, primarily due to increase in metabolic demands. Upon exertion, sinus tachycardia can also be seen in some inborn errors of metabolism that result in metabolic myopathies, such as McArdle's disease (GSD-V). Metabolic myopathies interfere with the muscle's ability to create energy. This energy shortage in muscle cells causes an inappropriate rapid heart rate in response to exercise. The heart tries to compensate for the energy shortage by increasing heart rate to maximize delivery of oxygen and other blood borne fuels to the muscle cells. "In McArdle's, our heart rate tends to increase in what is called an 'inappropriate' response. That is, after the start of exercise it increases much more quickly than would be expected in someone unaffected by McArdle's." As skeletal muscle relies predominantly on glycogenolysis for the first few minutes as it transitions from rest to activity, as well as throughout high-intensity aerobic activity and all anaerobic activity, individuals with GSD-V experience during exercise: sinus tachycardia, tachypnea, muscle fatigue and pain, during the aforementioned activities and time frames. Those with GSD-V also experience "second wind", after approximately 6–10 minutes of light-moderate aerobic activity, such as walking without an incline, where the heart rate drops and symptoms of exercise intolerance improve. An increase in sympathetic nervous system stimulation causes the heart rate to increase, both by the direct action of sympathetic nerve fibers on the heart and by causing the endocrine system to release hormones such as epinephrine (adrenaline), which have a similar effect. Increased sympathetic stimulation is usually due to physical or psychological stress. This is the basis for the so-called fight-or-flight response, but such stimulation can also be induced by stimulants such as ephedrine, amphetamines or cocaine. Certain endocrine disorders such as pheochromocytoma can also cause epinephrine release and can result in tachycardia independent of nervous system stimulation. Hyperthyroidism can also cause tachycardia. The upper limit of normal rate for sinus tachycardia is thought to be 220 bpm minus age.

Inappropriate sinus tachycardia

Inappropriate sinus tachycardia (IST) is a diagnosis of exclusion, a rare but benign type of cardiac arrhythmia that may be caused by a structural abnormality in the sinus node. It can occur in seemingly healthy individuals with no history of cardiovascular disease. Other causes may include autonomic nervous system deficits, autoimmune response, or drug interactions. Although symptoms might be distressing, treatment is not generally seen by clinicians as needed.

Ventricular

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Tachycardia

Start with the simplest possible case. Write down what Tachycardia 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 Tachycardia 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 Tachycardia 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 Tachycardia

In research: Tachycardia 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 Tachycardia 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: Tachycardia is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cardiac arrhythmia, Symptoms and signs: Cardiac, so understanding it makes those chapters shorter.
In everyday life: Look for Tachycardia 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.

How to study Tachycardia in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Tachycardia 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 Tachycardia out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Tachycardia in simple terms?

Tachycardia, also called tachyarrhythmia, is a heart rate that exceeds the normal resting rate. In general, a resting heart rate over 100 beats per minute is accepted as tachycardia in adults.

Why does Tachycardia 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 Tachycardia?

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 Tachycardia.