Merck Manual

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Some Causes and Features of Fainting

Some Causes and Features of Fainting

Cause

Common Features*

Tests†

Serious causes

Heart valve disorders, such as aortic stenosis, mitral stenosis, or failure of an artificial heart valve

Cardiomyopathy (disorders affecting heart muscle), particularly hypertrophic cardiomyopathy

Fainting during or after exercise, followed by a prompt recovery

In young or old people

Often in people who are known to have had a heart murmur

Echocardiography (ultrasonography of the heart)

A very slow heart rate (typically less than 35 beats per minute), more common in older people

A very rapid heart rate (typically over 150 beats per minute)

Fainting without warning, followed by recovery immediately after awakening

Fainting that may occur in any position

Sometimes in people taking certain drugs, especially drugs used to treat heart disorders such as abnormal heart rhythms (antiarrhythmic drugs)

ECG, sometimes continuous ambulatory ECG (using a Holter monitor worn for 24 hours or, rarely, a recording device implanted under the skin)

Sometimes blood tests to measure electrolytes such as sodium and potassium

Pulmonary embolism (blockage of an artery to the lungs by a blood clot)

Often sharp pain when breathing in, shortness of breath, rapid breathing, and a rapid heart rate

Sometimes mild fever, coughing up blood, or shock

More likely in people with risk factors for pulmonary embolism (such as previous blood clots, recent surgery especially surgery on the legs, prolonged bed rest, a cast or splint on a leg, older age, smoking, or cancer)

CT or nuclear scanning of the lungs

A blood test to detect blood clots (D-dimer test)

Heart attack (myocardial infarction)

Usually in older people

Sometimes chest discomfort, feeling of indigestion, shortness of breath, or nausea

ECG

Blood tests to measure substances that indicate heart damage (cardiac markers)

A severe allergic reaction (anaphylaxis) causing very low blood pressure

Fainting during or shortly after being exposed to a trigger for an allergic reaction, such as a drug or an insect bite

Excessive sweating and pale skin color

In people who may or may not have a history of allergies

Usually hives, wheezing, or swelling of part of the body (called angioneurotic edema)

Allergy testing

A low blood sugar level (hypoglycemia)

Fainting after a period of other symptoms, including confusion, shakiness, and sweating

Unresponsiveness or confusion that remains until people are treated

Almost always in people with diabetes

Fingerstick glucose measurement

Immediate recovery following glucose infusion

Less serious causes

Increased pressure in the chest (for example, due to coughing or straining during urination or a bowel movement)

Fainting during an activity that increases pressure in the chest

Warning symptoms (for example, dizziness, nausea, or sweating)

Recovery that is prompt but not immediate (within 5 to 15 minutes, but the person may feel unwell for several hours)

Only a doctor's examination

Strong emotion (such as pain, fear, or distress at the sight of blood)

Fainting when experiencing strong emotion

Warning symptoms (for example, dizziness, nausea, or sweating)

Recovery that is prompt but not immediate (within 5 to 15 minutes)

A cause that is usually apparent

Only a doctor's examination

Fainting sometimes preceded by disturbances in sensation, vision, or other functions (called the aura)

Sensitivity to light

Only a doctor's examination

Standing for a long time

A cause that is apparent based on the history

No other symptoms

Only a doctor's examination

Pregnancy

In healthy women of childbearing age

No other symptoms

Usually in women with an early or unrecognized pregnancy

Urine pregnancy test

Hyperventilation

Often tingling around the mouth or in the fingers before fainting

Usually during or in response to an emotional situation

Rapid breathing, which may not be noticed by the person or by others

Typically in younger people

Only a doctor's examination

Drugs used to lower the blood pressure (but rarely beta-blockers), such as loop diuretics and nitrates

Light-headedness, followed by fainting within several minutes of sitting up or standing

A drop in blood pressure when standing, detected during the examination

A doctor's examination

Sometimes tilt table testing

Drugs that can cause irregular and rapid heart rate by affecting the heart's electric activity such as antipsychotics (mainly phenothiazines), some antiarrhythmic drugs, some antidepressants, and some antibiotics

Sometimes palpitations and light-headedness

Sudden loss of consciousness

A doctor's examination

ECG

Malfunction of the autonomic nervous system (which regulates internal body processes that require no conscious effort, such as blood pressure)

Light-headedness, followed by fainting within several minutes of sitting up or standing

A drop in blood pressure when standing, detected during the examination

A doctor's examination

Sometimes tilt table testing

Deconditioning caused by bed rest for many days

Light-headedness, followed by fainting within several minutes of sitting up or standing

A drop in blood pressure when standing, detected during the examination

A doctor's examination

Sometimes tilt table testing

Anemia

Light-headedness, followed by fainting within several minutes of sitting up or standing

Chronic fatigue

Sometimes dark stools or heavy menstrual periods

A complete blood count

Stool tests to check for blood

* Features include symptoms and the results of the doctor's examination. Features mentioned are typical but not always present.

† In all people who have fainted, ECG is done, and oxygen levels in the blood may be measured with a sensor placed on a finger (pulse oximetry).

CT = computed tomography; ECG = electrocardiography; MRI = magnetic resonance imaging.