Palpitations are feelings or sensations that your heart is pounding or racing. They can be felt in your chest, throat, or neck.

A palpitation is an abnormality of heartbeat that ranges from often unnoticed skipped beats
or accelerated heartrate to very noticeable changes accompanied by dizziness or difficulty breathing.
Palpitations are common and occur in most individuals with healthy hearts.

Palpitations without underlying heart disease are generally considered benign.
However, heart palpitations can be symptoms of illnesses such as coronary
heart disease, asthma, or emphysema.

Patients can notice palpitations through abnormal or normal awareness.
The difference between an abnormal awareness and a normal awareness
is that the former interrupts other thoughts, whereas the latter is almost
always caused by a concentration on the beating of one’s heart. In general,
a palpitation can be one of two types of beats: premature atrial contraction,
or a premature ventricular contraction.[citation needed] Palpitations may be
brought on by overexertion, stress, anxiety, panic, adrenaline, alcohol, nicotine,
caffeine, cocaine, amphetamines, and other drugs, disease (such as hyperthyroidism
and pheochromocytoma) or as a symptom of panic disorder and many other factors.
It can also happen in mitral stenosis.

Nearly everyone experiences an occasional awareness of their heart beating or
weak chest, but when it occurs frequently, (more than 6 per minute as a rule of thumb)
it can indicate a problem. Palpitations may be associated with heart problems, but also with
kidney disease, electrolyte imbalances for magnesium and calcium, anemias, asthma, and thyroid malfunction.

Attacks can last for a few seconds or hours, and may occur very infrequently, or more than daily.
Palpitations alongside other symptoms, including sweating, faintness, frequent headaches, chest pain
or dizziness, indicate irregular or poor heart function and should be investigated by a medical professional.
Palpitations may also be associated with anxiety and panic attacks, in which case psychological assessment
is recommended. This is a common disorder associated with many common medications such as anti-depressants.

You may:

Have an unpleasant awareness of your own heartbeat
Feel like your heart skipped or stopped beats
The heart’s rhythm may be normal or abnormal when you have palpitations.

Considerations
Normally the heart beats 60 – 100 times per minute. In people who exercise routinely or take medications that slow the heart, the rate may drop below 55 beats per minute.

If your heart rate is fast (over 100 beats per minute), this is called tachycardia. A slow heart rate is called bradycardia. An occasional extra heartbeat is known as extrasystole.

Palpitations are usually not serious. However, it depends on whether or not the sensations represent an abnormal heart rhythm ( arrhythmia). The following conditions make you more likely to have an abnormal heart rhythm:

Known heart disease at the time the palpitations begin
Significant risk factors for heart disease
An abnormal heart valve
An electrolyte abnormality in your blood — for example, a low potassium level
Causes
Heart palpitations are most often not serious. They can be due to:

Anxiety, stress, panic attacks, or fear
Caffeine and nicotine use
Cocaine or other illegal drugs
Diet pills
Exercise
Fever
However, some palpitations are due to an abnormal heart rhythm, which may be caused by:

Heart disease
Abnormal heart valve, such as mitral valve prolapse
Abnormal blood levels of potassium
Certain medications, including those used to treat asthma, high blood pressure, or heart problems
Overactive thyroid
Low levels of oxygen in your blood
Home Care
Reduce your caffeine and nicotine intake. This will often reduce heart palpitations.
Learn to reduce stress and anxiety. This can help prevent palpitations and help you better manage them when they occur.
Try breathing exercises or deep relaxation (a step-by-step process of tensing and then relaxing every muscle group in your body)
Practice yoga, meditation, or tai chi.
Get regular exercise.
Do not smoke.
Once a serious cause has been ruled out by your doctor, try not to pay close attention to heart palpitations. This may cause stress. However, contact your doctor if you notice a sudden increase or a change in them.

When to Contact a Medical Professional
If you have never had heart palpitations before, see your health care provider.

The following symptoms require immediate attention. Call 911 or your local emergency number:

Lose of alertness (consciousness)
Chest pain
Shortness of breath
Unusual sweating,
Dizziness lightheadedness
Call your doctor right away if:

You feel frequent extra heartbeats (more than 6 per minute or coming in groups of 3 or more).
You have risk factors for heart disease, such as high cholesterol, diabetes, or high blood pressure.
You have new or different heart palpitations.
Your pulse is more than 100 beats per minute (without exercise, anxiety, or fever).
What to Expect at Your Office Visit
Your doctor or nurse will examine you and ask questions about your medical history and symptoms.

You may be asked:

Do you feel skipped or stopped beats?
Does your heart rate feel slow or fast when you have the palpitations?
Do you feel a racing, pounding, or fluttering?
Is there a regular or irregular pattern to the unusual heartbeat sensations?
Did the palpitations begin or end suddenly?
When do the palpitations occur? In response to reminders of a traumatic event? When you are lying down and resting? When you change your body position? When you feel emotional?
Do you have any other symptoms?
An electrocardiogram will be done.

In the emergency room, you will be connected to a heart monitor.

If your doctor finds you have an abnormal heart rhythm, other tests may be done. This may include:

Holter monitor for 24 hours, or another heart monitor for two weeks or longer
Echocardiogram
Electrophysiology study (EPS)
Coronary angiography

Sources:
http://www.nlm.nih.gov/medlineplus/ency/article/003081.htm

http://en.wikipedia.org/wiki/Palpitation