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.
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:
Heart rate is considered in the context of the prevailing clinical picture. For example, in sepsis >90 bpm is considered tachycardia. 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. Relative tachycardia involves a greater increase in rate than would be expected in a given illness state.
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 order of most to least common, they are:
* A narrow complex tachycardia with an accessory conduction pathway, often termed "supraventricular tachycardia with pre-excitation"
* Pacemaker-tracked or pacemaker-mediated tachycardia
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. 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.
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.
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.
Unstable
In those that are unstable with a narrow complex tachycardia, intravenous adenosine may be attempted. In all others immediate cardioversion is recommended.
Terminology
The word tachycardia came to English from New Latin as a neoclassical compound built from the combining forms ' + ', which are from the Greek ταχύς tachys, "quick, rapid" and καρδία, kardia, "heart". As a matter both of usage choices in the medical literature and of idiom in natural language, the words tachycardia and tachyarrhythmia are usually used interchangeably, or loosely enough that precise differentiation is not explicit. Some careful writers have tried to maintain a logical differentiation between them, which is reflected in major medical dictionaries and major general dictionaries. The distinction is that tachycardia be reserved for the rapid heart rate itself, regardless of cause, physiologic or pathologic, and that tachyarrhythmia be reserved for the pathologic form. This is why five of the previously referenced dictionaries do not enter cross-references indicating synonymy between their entries for the two words, and it is why one of them explicitly specifies that the two words not be confused. But the prescription will probably never be successfully imposed on general usage, not only because much of the existing medical literature ignores it even when the words stand alone but also because the terms for specific types of arrhythmia are deeply established idiomatically with the tachycardia version as the more commonly used version. Thus SVT is called supraventricular tachycardia more than twice as often as it is called supraventricular tachyarrhythmia; moreover, those two terms are always completely synonymous—in natural language there is no such term as "healthy/physiologic supraventricular tachycardia". The same themes are also true of AVRT and AVNRT. Thus this pair is an example of when a particular prescription can no longer be invariably enforced without violating idiom. But the power to differentiate in an idiomatic way is not lost, regardless, because when the specification of physiologic tachycardia is needed, that phrase aptly conveys it.
