Tachycardia


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 or abnormal.

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 that can lead to thrombosis.

Causes

Some causes of tachycardia include:
Drug related:
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
  • 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 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:
Tachycardias may be classified as either narrow complex 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, or an unexpected change in blood flow. The most common cause of the latter is orthostatic 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. 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, 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
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

Ventricular tachycardia is a potentially life-threatening cardiac arrhythmia that originates in the ventricles. It is usually a regular, wide complex tachycardia with a rate between 120 and 250 beats per minute. A medically significant subvariant of ventricular tachycardia is called torsades de pointes, which tends to result from a long QT interval.
Both of these rhythms normally last for only a few seconds to minutes , but if VT persists it is extremely dangerous, often leading to ventricular fibrillation.

Supraventricular

This is a type of tachycardia that originates from above the ventricles, such as the atria. It is sometimes known as paroxysmal atrial tachycardia. Several types of supraventricular tachycardia are known to exist.
Atrial fibrillation
is one of the most common cardiac arrhythmias. In general, it is an irregular, narrow complex rhythm. However, it may show wide QRS complexes on the ECG if a bundle branch block is present. At high rates, the QRS complex may also become wide due to the Ashman phenomenon. It may be difficult to determine the rhythm's regularity when the rate exceeds 150 beats per minute. Depending on the patient's health and other variables such as medications taken for rate control, atrial fibrillation may cause heart rates that span from 50 to 250 beats per minute. However, new-onset atrial fibrillation tends to present with rates between 100 and 150 beats per minute.
AV nodal reentrant tachycardia
is the most common reentrant tachycardia. It is a regular narrow complex tachycardia that usually responds well to the Valsalva maneuver or the drug adenosine. However, unstable patients sometimes require synchronized cardioversion. Definitive care may include catheter ablation.
AV reentrant tachycardia
requires an accessory pathway for its maintenance. AVRT may involve orthodromic conduction or antidromic conduction. Orthodromic conduction usually results in a narrow complex tachycardia, and antidromic conduction usually results in a wide complex tachycardia that often mimics ventricular tachycardia. Most antiarrhythmics are contraindicated in the emergency treatment of AVRT, because they may paradoxically increase conduction across the accessory pathway.
Junctional tachycardia
Junctional tachycardia is an automatic tachycardia originating in the AV junction. It tends to be a regular, narrow complex tachycardia and may be a sign of digitalis toxicity.

Management

The management of tachycardia depends on its type, whether or not the person is stable or unstable, and whether the instability is due to the tachycardia. Unstable means that either important organ functions are affected or cardiac arrest is about to occur. Stable means that there is a tachycardia, but it does not seem an immediate threat for the patient's health, but only a symptom of an unknown disease, or a reaction that is not very dangerous in that moment.