Tachycardia Tachycardia is a heart rate that exceeds the
normal range.
In general, a resting heart rate over 100 beats per minute is accepted as tachycardia.
Tachycardia can be caused by various factors that often are benign.
However, tachycardia can be dangerous, depending on the speed and type of rhythm.
Note that, if it is pathological, a tachycardia is more correctly defined as a tachyarrhythmia.
Etymology Tachycardia comes from the Greek ταχύς
tachys, "quick, rapid" and καρδία, kardia, "heart".
Definition The upper threshold of a normal human resting
heart rate is based upon age.
Tachycardia for different age groups is as listed below:
1–2 days: greater than (greater than) 159 beats per minute (bpm)
3–6 days: greater than 166 bpm 1–3 weeks: greater than 182 bpm
1–2 months: greater than 179 bpm 3–5 months: greater than 186 bpm
6–11 months: greater than 169 bpm 1–2 years: greater than 151 bpm
3–4 years: greater than 137 bpm 5–7 years: greater than 133 bpm
8–11 years: greater than 130 bpm 12–15 years: greater than 119 bpm
greater than 15 years – adult: greater than 100* bpm
Heart rate is often correlated with clinical picture, in sepsis greater than 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.
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.
Presented in the 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
Junctional tachycardia 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.
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 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 (VT or V-tach) 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.
Both of these rhythms normally last for only a few seconds to minutes (paroxysmal tachycardia),
but if VT persists it is extremely dangerous, often leading to ventricular fibrillation.
Supraventricular This is a type tachycardia that originates
from above the ventricles, such as the atria.
It is sometimes known as paroxysmal atrial tachycardia (PAT).
Several types of supraventricular tachycardia are known to exist.
Atrial fibrillation 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
(or even higher if an accessory pathway is present).
However, new onset atrial fibrillation tends to present with rates between 100 and 150
beats per minute.
AV nodal reentrant tachycardia AV nodal reentrant tachycardia (AVNRT) 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 AV reentrant tachycardia (AVRT) requires an
accessory pathway for its maintenance.
AVRT may involve orthodromic conduction (where the impulse travels down the AV node to the
ventricles and back up to the atria through the accessory pathway) or antidromic conduction
(which the impulse travels down the accessory pathway and back up to the atria through the
AV node).
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 (wide complex versus narrow complex), 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 In those that are stable, treatment is determined
by the exact electrocardiogram findings: wide versus narrow complex, regular versus irregular
heart rate, and whether the QRS is monomorphic or polymorphic.
Unstable In those that are unstable with a narrow complex
tachycardia, intravenous adenosine may be attempted.
In all others immediate cardioversion is recommended.