Tachycardia

• Dizziness
• Shortness of breath
• Lightheadedness
• Rapid pulse rate
• Heart palpitations — a racing, uncomfortable or irregular heartbeat or a sensation of "flopping" in the chest
• Chest pain
• Fainting (syncope)

Some people with tachycardia have no symptoms, and the condition is only discovered during a physical examination or with a heart-monitoring test called an electrocardiogram.

Causes

Tachycardia is caused by something that disrupts the normal electrical impulses that control the rhythm of your heart's pumping action. Many things can cause or contribute to problems with the heart's electrical system. These factors include:

• Damage to heart tissues from heart disease
• Abnormal electrical pathways in the heart present at birth (congenital)
• Disease or congenital abnormality of the heart
• High blood pressure
• Smoking
• Heavy alcohol consumption
• Heavy caffeine consumption
• Adverse reaction to medications
• Abuse of recreational drugs, such as cocaine
• Imbalance of electrolytes, mineral-related substances necessary for conducting electrical impulses
• Overactive thyroid (hyperthyroidism)

In some cases, the exact cause of tachycardia can't be determined.

Electrical circuitry of the heart

Your heart is made up of four chambers — two upper chambers (atria) and two lower chambers (ventricles). The rhythm of your heart is normally controlled by a natural pacemaker — the sinus node — located in the right atrium. The sinus node produces electrical impulses that initiate each heartbeat.

From the sinus node, electrical impulses travel across the atria, causing the atria muscles to contract and pump blood into the ventricles. The electrical impulses then arrive at a cluster of cells called the atrioventricular node (AV node) — usually the only pathway for signals to travel from the atria to the ventricles.

The AV node slows down the electrical signal before sending it to the ventricles. This slight delay allows the ventricles to fill with blood. When electrical impulses reach the muscles of the ventricles, they contract, causing them to pump blood either to the lungs or to the rest of the body.

Types of tachycardias

Tachycardia occurs when a problem in electrical signals produces a heartbeat that is faster than normal. Common types of tachycardia include the following:

• Atrial fibrillation is a rapid heart rate caused by chaotic electrical impulses in the atria. These signals result in rapid, uncoordinated, weak contractions of the atria. The chaotic electrical signals bombard the AV node, usually resulting in an irregular, rapid rhythm of the ventricles. Episodes of atrial fibrillation may last a few hours or several days. Some episodes won't end unless treated. Most people with atrial fibrillation have some structural abnormalities of the heart related to such conditions as heart disease or high blood pressure. Other factors that may contribute to atrial fibrillation include heart valve disorder, hyperthyroidism or heavy alcohol consumption.
• Atrial flutter is a very fast, but regular rate of the atria caused by irregular circuitry within the atria. The fast rate results in weak contractions of the atria. The rapid signals entering the AV node cause a rapid and sometimes irregular ventricular rate. Episodes of atrial flutter may also last a few hours or several days, or the condition may persist unless treated. Various forms of heart disease can contribute to the onset of atrial flutter, and it is sometimes a complication of heart surgery. People who experience atrial flutters often experience atrial fibrillation as well.
• Supraventricular tachycardias (SVTs), which originate somewhere above the ventricles, are caused by abnormal circuitry in the heart, usually present at birth, that creates a loop of overlapping signals. Episodes may last only a few seconds or several hours. In one form of SVT, an abnormality in the AV node may "split" an electrical signal into two, sending one signal to the ventricles and another back to the atria. Another common abnormality is the presence of an extra electrical pathway from the atria to the ventricles that bypasses the AV node. This may result in a signal going down one pathway and up the other. Wolff-Parkinson-White syndrome is the most common disorder featuring an extra pathway.
• Ventricular tachycardia is a rapid rate that originates with abnormal electrical signals in the ventricles. The rapid beat doesn't allow the ventricles to fill and contract efficiently to pump enough blood to the body. Ventricular tachycardia is often a life-threatening medical emergency. Ventricular tachycardia is usually related to damage to the heart muscle from a previous heart attack or disease of the heart muscle (cardiomyopathy).
• Ventricular fibrillation occurs when rapid, chaotic electrical impulses cause the ventricles to quiver ineffectively instead of pumping necessary blood to the body. This serious malfunction results in death if the heart isn't restored to a normal rhythm within minutes. Most people who experience ventricular fibrillation have an underlying heart disease or have experienced serious trauma, such as being struck by lightning. 

Risk factors

Any condition that puts a strain on the heart or damages heart tissue can increase your risk of tachycardia. Lifestyle changes or medical treatment may decrease the risk associated with the following factors:

• Heart disease
• High blood pressure
• Smoking
• Heavy alcohol consumption
• Heavy caffeine consumption
• Use of recreational drugs
• Psychological stress or anxiety

Other risk factors

Other factors that may increase the risk of tachycardia include:

• Older age. Aging-related wear on the heart makes you more likely to develop tachycardia.
• Family. If you have a family history of tachycardia or other heart rhythm disorders, you may have an increased risk of tachycardia.

Complications

Complications of tachycardias vary in severity depending on such factors as the type of tachycardia, the rate and duration of a rapid heart rate, and the existence of other heart conditions. Possible complications include:

• Blood clots that can cause a stroke or heart attack
• Inability of the heart to pump enough blood (heart failure)
• Frequent fainting spells
• Sudden death, usually only associated with ventricular tachycardia or ventricular fibrillation

Diagnosis

Your doctor can make a diagnosis of a specific tachycardia based on your answers to questions about symptoms, a physical exam and heart tests. Common tests include the following.

Electrocardiogram (ECG)

An electrocardiogram — also called an ECG or EKG — is a primary tool for diagnosing tachycardia. An ECG uses small sensors (electrodes) attached to your chest and arms to record electrical signals as they travel through your heart. Your doctor can look for patterns among these signals to determine what kind of tachycardia you have and how abnormalities in the heart may be contributing to a fast heart rate.

Your doctor may also ask you to use portable ECG devices at home to provide more information about your heart rate. These devices include:

• Holter monitor. This portable ECG device is carried in your pocket or worn on a belt or shoulder strap. It records your heart's activity for an entire 24-hour period, which provides your doctor with a prolonged look at your heart rhythms. Your doctor will likely ask you to keep a diary during the same 24 hours. You'll describe any symptoms you experience and record the time they occur.
• Event recorder. This portable ECG device is intended to monitor your heart activity over a few weeks to a few months. You activate it only when you experience symptoms of a fast heart rate. When you feel symptoms, you push a button, and an ECG strip of the preceding few minutes and following few minutes is recorded. This permits your doctor to determine your heart rhythm at the time of your symptoms.

Electrophysiological testing

Your doctor may recommend an electrophysiological test to confirm the diagnosis or to pinpoint the location of problems in your heart's circuitry. During this test, thin, flexible tubes (catheters) tipped with electrodes are threaded through your blood vessels to various spots in your heart. Once in place, the electrodes can precisely map the spread of electrical impulses during each beat and identify abnormalities in your circuitry.

Tilt table test

This test helps your doctor better understand how your tachycardia contributes to fainting spells. Under careful monitoring, you'll receive a medication that induces a tachycardia episode. You lie flat on a special table, and then the table is tilted as if you were standing up. Your doctor observes how your heart and nervous system respond to these changes in position.

Additional tests

Your cardiologist may order additional tests to diagnose an underlying condition that is contributing to tachycardia and judge the condition of your heart. 

References

http://www.mayoclinic.org/diseases-conditions/tachycardia/basics/definition/con-20043012