Glossary

Find definitions for thousands of medical terms, treatments, and tests -- even health-related abbreviations, prefixes, and suffixes.

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HDL (high-density lipoprotein) is sometimes called "good" cholesterol, because it helps move cholesterol out of the body. HDL does this by binding with cholesterol in the bloodstream and carrying it back to the liver for disposal.

A high HDL level is linked to a lower risk of heart disease.

Last Revised: June 29, 2012

Author: Healthwise Staff

Medical Review: Kathleen Romito, MD - Family Medicine & Robert A. Kloner, MD, PhD - Cardiology & Rakesh K. Pai, MD, FACC - Cardiology, Electrophysiology

A health care agent is a person chosen to make medical decisions for another, should a severe illness or injury occur that makes communication impossible. The document that grants this decision-making power to the person selected is called a medical power of attorney.

A health care agent may also be called a health care proxy or surrogate or an attorney-in-fact.

Although laws vary by state, a health care agent can usually make medical treatment decisions at the end of life or anytime a person is not able to communicate. As soon as a person selects a health care agent, it is important to thoroughly discuss and document medical care preferences, such as when to continue or abandon life-support measures.

Last Revised: December 29, 2011

Author: Healthwise Staff

Medical Review: Anne C. Poinier, MD - Internal Medicine & Shelly R. Garone, MD, FACP - Palliative Medicine

A heart attack happens when blood flow to the heart is blocked. Without blood and the oxygen it carries, part of the heart starts to die. The medical name for a heart attack is myocardial infarction, or MI.

A heart attack is often the result of coronary artery disease, in which fatty deposits called plaque (say "plak") build up inside the heart arteries. When plaque breaks open or ruptures, it can form a clot that blocks blood flow to the heart muscle.

Quick treatment that restores blood flow to the heart can help save lives.

Last Revised: October 9, 2012

Author: Healthwise Staff

Medical Review: E. Gregory Thompson, MD - Internal Medicine & John M. Miller, MD, FACC - Cardiology, Electrophysiology

Heart failure means that your heart muscle doesn't pump as much blood as your body needs. In time, this causes fluid to build up in your body, and you have symptoms like swelling in the legs and feeling out of breath and weak.

Heart failure usually gets worse over time. But treatment can slow the disease and help you feel better and live longer.

Last Revised: October 9, 2012

Author: Healthwise Staff

Medical Review: Rakesh K. Pai, MD, FACC - Cardiology, Electrophysiology & Stephen Fort, MD, MRCP, FRCPC - Interventional Cardiology

A heart murmur is a sound made by blood moving through the chambers and valves of the heart or through the blood vessels near the heart. The sounds can be heard through a stethoscope.

Heart murmurs are common in infants and children and are harmless in most cases. The murmurs usually are not a problem, require no treatment, and go away on their own. Pregnancy, fever, and some types of anemia can also lead to temporary heart murmurs. But some adults have harmless heart murmurs that do not go away.

A heart murmur may sometimes mean there is a more serious problem with the heart walls or heart valves, such as narrowing or leaking of a heart valve (stenosis or regurgitation) or an infection of a heart valve (endocarditis). These problems can cause blood to flow abnormally through the heart valves or chambers, causing a murmur or other sound that the doctor can hear with the stethoscope. These conditions require close monitoring and may require treatment.

Last Revised: July 6, 2012

Author: Healthwise Staff

Medical Review: Rakesh K. Pai, MD, FACC - Cardiology, Electrophysiology & George Philippides, MD - Cardiology & E. Gregory Thompson, MD - Internal Medicine

Heart valve disease occurs when a heart valve is damaged or narrowed and does not control or allow the normal flow of blood through and out of the heart. Causes of heart valve disease include congenital heart disease, an abnormal valve, or a rupture of a valve.

Heart valves operate like one-way gates, helping blood flow in one direction between heart chambers as well as into and out of the heart. A normal heart valve has flaps, called leaflets. When the heart pumps, the leaflets open one way to allow blood to flow through. Between heartbeats, the leaflets should close to form a tight seal so that blood does not leak backwards through the valve.

If the heart valve is damaged, the leaflets may not form a tight seal, and blood may leak backwards through the valve. This leakage is called regurgitation.

Heart valves can also become narrowed, which may block the flow of blood through the heart. This narrowing is called stenosis.

Over time, a damaged valve may lead to enlargement of the heart chambers, heart failure, and atrial fibrillation. It can reduce blood flow to the muscles of the body, including the heart muscle itself, which can result in symptoms or damage.

Treatment for heart valve disease depends on the cause and severity. Close monitoring is sometimes all that is needed for those who have mild or no symptoms, but a doctor may recommend surgery to repair or replace the valve in more serious cases.

Last Revised: January 23, 2013

Author: Healthwise Staff

Medical Review: E. Gregory Thompson, MD - Internal Medicine & Rakesh K. Pai, MD, FACC - Cardiology, Electrophysiology

Heavy use of alcohol is defined as more than 2 drinks a day for men and more than 1 drink a day for women. One drink is 12 fl oz (355 mL) of beer, 5 fl oz (148 mL) of wine, or 1.5 fl oz (44 mL) of hard liquor.

Heavy use of alcohol increases the risk for many health problems.

Last Revised: January 18, 2012

Author: Healthwise Staff

Medical Review: E. Gregory Thompson, MD - Internal Medicine & Peter Monti, PhD - Alcohol and Addiction

Hemochromatosis is a condition that occurs when too much iron builds up in the body. Small amounts of iron are normally stored in the liver and heart, but excess iron will eventually damage these organs.

There are two types of hemochromatosis:

  • Hereditary (genetic) hemochromatosis. The most common form of hemochromatosis is passed down through the genes in families.
  • Acquired (secondary) hemochromatosis. A person may develop acquired hemochromatosis from having many blood transfusions, certain blood disorders (such as thalassemia), or chronic liver disease or from taking excessive or unnecessary iron supplements. In rare cases, a person may develop hemochromatosis if his or her diet contains too much iron.

Hereditary hemochromatosis is one of the most common genetic disorders in white people, especially those of Northern European descent. Excess iron builds up slowly throughout life. Most people with hemochromatosis notice symptoms when they are age 40 to 60. These symptoms include fatigue, joint pain, weakness, excess urination, and weight loss.

If hemochromatosis is recognized early, it can be treated before other problems start. It is treated by removing excess iron from the blood, either by removing blood from the body (phlebotomy) or by taking a medicine (chelating agent) that binds to and removes iron from the body. Hereditary hemochromatosis requires treatment throughout a person's life. Acquired hemochromatosis does not need further treatment after the condition has been corrected.

Last Revised: October 15, 2012

Author: Healthwise Staff

Medical Review: Kathleen Romito, MD - Family Medicine & Brian Leber, MDCM, FRCPC - Hematology

Dialysis is a mechanical process that partly does the work that healthy kidneys would do. Hemodialysis uses a man-made membrane (dialyzer) to filter wastes, remove extra fluid from the blood, restore the proper balance of chemicals in the blood, and eliminate extra fluid (edema) from the body.

Before hemodialysis treatments can begin, a doctor will need to create an access where blood can flow in and out of the body (dialysis access). This is usually done by joining an artery and a vein in the forearm or by using a small tube to connect an artery and a vein.

Hemodialysis is usually done in a hospital or dialysis center on a set schedule. It is usually done 3 days a week and takes 3 to 5 hours a day. In some cases, hemodialysis can be done at home. Home hemodialysis can be done on more days of the week. Some types of home hemodialysis are done during the night.

Last Revised: September 15, 2011

Author: Healthwise Staff

Medical Review: Anne C. Poinier, MD - Internal Medicine & Mitchell H. Rosner, MD - Nephrology

Hemoglobin is the protein in red blood cells that carries oxygen from the lungs to the rest of the body. It also helps carry carbon dioxide back to the lungs, where it can be breathed out of the body.

Abnormally low levels of hemoglobin result in anemia.

Last Revised: August 6, 2012

Author: Healthwise Staff

Medical Review: E. Gregory Thompson, MD - Internal Medicine & Joseph O'Donnell, MD - Hematology, Oncology

Hemophilia is a rare genetic bleeding disorder in which a person inherits problems with certain blood clotting factors, making them unable to work properly. Blood-clotting factors are needed to help stop bleeding after a cut or injury and to prevent spontaneous bleeding.

The hemophilia gene can contain many different errors, leading to different degrees of abnormality in the amount of clotting factor produced. People who have hemophilia are at risk of abnormal bleeding throughout the body, especially in the joints and muscles, which may lead to disabling joint problems.

Hemophilia occurs almost exclusively in men. The disease can be passed from a mother who is a carrier of the genetic defect (but who does not have the disease) to her son. Rarely, a girl can have hemophilia. This occurs only if she inherits a defective gene from both her mother and her father.

Symptoms of hemophilia are usually first noticed during infancy or childhood. However, some people who have milder forms of hemophilia may not develop symptoms until later in life.

The following are signs of hemophilia that may be noticed shortly after birth:

  • Bleeding into the muscle, resulting in a deep bruise after receiving a routine vitamin K shot
  • Prolonged bleeding after a boy is circumcised
  • In rare cases, prolonged bleeding after the umbilical cord is cut at birth

Other symptoms of hemophilia include easy bruising, frequent nosebleeds, blood in the urine, and bleeding after dental work.

Some people who have hemophilia may need to inject (infuse) themselves with clotting factors to prevent uncontrolled bleeding. They may need to do this either regularly or only before activities or situations (such as surgery) when injury or bleeding may occur.

Last Revised: August 3, 2011

Author: Healthwise Staff

Medical Review: E. Gregory Thompson, MD - Internal Medicine & Brian Leber, MDCM, FRCPC - Hematology

Hepatitis is a disease that causes inflammation of the liver and interferes with its normal function. Hepatitis can be caused by infection (usually by a virus), excessive alcohol use, medicine, or a problem with the immune system.

The three most common types of viral hepatitis are hepatitis A, hepatitis B, and hepatitis C. Viral hepatitis is contagious. All three types of viral hepatitis (A, B, and C) can be spread through contact with body fluids. Hepatitis A can also spread when people consume food or water contaminated by stool (feces) containing the virus.

Symptoms of hepatitis can last for weeks to months. They include:

  • Yellowing of the skin and eyes (jaundice).
  • Weight loss and lack of appetite.
  • Discomfort in the upper right abdomen.
  • Nausea and vomiting.
  • Brownish urine.
  • Fatigue.

Some types of hepatitis can cause serious, long-term complications, such as severe and permanent liver damage.

Last Revised: October 29, 2012

Author: Healthwise Staff

Medical Review: Kathleen Romito, MD - Family Medicine & W. Thomas London, MD - Hepatology

Hepatitis B is a virus that infects the liver. Most adults who get it have it for a short time and then get better. But sometimes the virus causes a long-term infection, called chronic hepatitis B. Over time, this can lead to liver damage or liver cancer.

The virus spreads through the blood of an infected person or through sexual contact with an infected person.

Last Revised: October 29, 2012

Author: Healthwise Staff

Medical Review: Kathleen Romito, MD - Family Medicine & W. Thomas London, MD - Hepatology

A hernia is tissue from inside the abdomen that bulges out through a weak spot in the muscles of the abdominal (belly) wall. The weak spot may have been present since birth or may develop after surgery or from violent or ongoing coughing, lifting heavy objects, or aging.

There are several types of hernias:

  • A femoral hernia appears as a bulge on the top of the thigh.
  • A hiatal hernia is a part of the stomach bulging up through the diaphragm (the wall of muscle that separates the abdominal cavity from the chest cavity).
  • An incisional hernia occurs after surgery to the wall of the abdomen.
  • An inguinal hernia occurs when a small portion of the bowel bulges into the groin.
  • An umbilical hernia appears in the belly button. A periumbilical hernia is similar to an umbilical hernia, but it occurs next to the belly button.

A person with a hernia often feels pain, pressure, or burning or feels like something has given way.

Last Revised: November 15, 2012

Author: Healthwise Staff

Medical Review: E. Gregory Thompson, MD - Internal Medicine & Kenneth Bark, MD - Surgery, Colon and Rectal

The bones that form your spine are cushioned by small discs that act as shock absorbers and keep the spine flexible. When a disc is damaged, it may bulge or break open. This is called a herniated disc, slipped disc, or ruptured disc. It may push on the nerves and cause pain, numbness, or weakness in the area where that nerve travels. Most herniated discs happen in the lower back.

Last Revised: October 9, 2012

Author: Healthwise Staff

Medical Review: William H. Blahd, Jr., MD, FACEP - Emergency Medicine & Robert B. Keller, MD - Orthopedics

A hiatal hernia occurs when a small portion of the stomach pushes upward through the diaphragm, a sheetlike muscle that separates the lungs from the abdomen. Usually this doesn't cause any symptoms, but it increases the risk of stomach acid backing up into the esophagus (reflux), which can lead to heartburn.

Normally the entire stomach sits below the diaphragm. The esophagus passes through an opening in the diaphragm called the hiatus before it enters the stomach. Weakened tissues within and around the hiatus allow a hiatal hernia to develop.

A hiatal hernia that is not causing symptoms does not usually need any treatment. Treatment for a hiatal hernia that causes heartburn is the same as for gastroesophageal reflux disease (GERD). This may include home treatment with nonprescription antacids, acid reducers, or acid blockers; prescription medicines; or, in severe cases, surgery.

Last Revised: March 6, 2012

Author: Healthwise Staff

Medical Review: Adam Husney, MD - Family Medicine & Peter J. Kahrilas, MD - Gastroenterology

Blood pressure is a measure of how hard the blood pushes against the walls of your arteries as it moves through your body. Blood pressure readings consist of an upper number and a lower number (such as 120 over 90 or 120/90). High blood pressure is 140/90 or higher.

When blood pressure is high, it starts to damage the blood vessels, heart, and kidneys. This can lead to heart attack, stroke, and other problems.

High blood pressure is also called hypertension. It can be managed with lifestyle changes and medicines.

Last Revised: September 26, 2012

Author: Healthwise Staff

Medical Review: Adam Husney, MD - Family Medicine

Cholesterol is a type of fat (lipid) in your blood. Your body needs some cholesterol. But if you have too much, it can build up in your arteries and increase your risk of heart disease and stroke.

You can get high cholesterol by eating foods that have too much cholesterol and saturated fat or by having an inherited condition that causes high cholesterol.

Last Revised: October 9, 2012

Author: Healthwise Staff

Medical Review: Kathleen Romito, MD - Family Medicine & Robert A. Kloner, MD, PhD - Cardiology & Rakesh K. Pai, MD, FACC - Cardiology, Electrophysiology

HIV (human immunodeficiency virus) is a virus that attacks the immune system, the body's natural defense system. Without a strong immune system, the body has trouble fighting off disease.

HIV is treated with medicines that slow or stop the damage to the immune system. If it's not treated, in time HIV will cause AIDS (acquired immunodeficiency syndrome).

Last Revised: October 9, 2012

Author: Healthwise Staff

Medical Review: E. Gregory Thompson, MD - Internal Medicine & Peter Shalit, MD, PhD - Internal Medicine

Hives (urticaria) are an allergic reaction of the skin that may last a few minutes or can persist for a few days. They appear as raised, red, itchy bumps (wheals) of different shapes and sizes, with defined red margins and pale centers.

Hives may appear and then disappear at random and seem to move from place to place on the skin. Hives may range in size from less than 0.25 in. (0.6 cm) to 3 in. (7.6 cm) across or larger. Patches of hives may combine to form raised, reddened skin over large areas of the body.

Hives may appear as a reaction to a medication, food, or infection. A single area of swelling often occurs after an insect bite at the site of the bite. Other possible causes include contact with plants, things you breathe in (inhalants), stress, makeup, and exposure to heat, cold, or sunlight. Often a cause cannot be found.

Hives are often minor, but they can also be the first sign of a life-threatening allergic reaction (anaphylaxis) that requires emergency care.

Last Revised: April 29, 2011

Author: Healthwise Staff

Medical Review: William H. Blahd, Jr., MD, FACEP - Emergency Medicine & H. Michael O'Connor, MD - Emergency Medicine

Homocysteine is an amino acid normally found in small amounts in the blood. Abnormal levels may mean that you are not getting enough vitamins like folic acid, vitamin B6, and vitamin B12.

Homocysteine tests might be used to check for other conditions or diseases.

Last Revised: January 15, 2013

Author: Healthwise Staff

Medical Review: E. Gregory Thompson, MD - Internal Medicine & George Philippides, MD - Cardiology

A hormone, such as insulin or estrogen, is a substance released by an organ or tissue that controls the activity of organs or cells in another part of the body. The organs or glands that release hormones are part of the endocrine system.

Hormones may also be taken as medicines, such as birth control pills or estrogen therapy.

Last Revised: May 17, 2012

Author: Healthwise Staff

Medical Review: E. Gregory Thompson, MD - Internal Medicine & Alan C. Dalkin, MD - Endocrinology

Hormone therapy (HT) is a general term for the use of man-made (synthetic) estrogen, with or without progestin, to treat symptoms caused by the changing hormone levels that occur before and after menopause. Hormone therapy carries some health risks, and its use should be discussed carefully with a health professional.

Hormone therapy includes:

  • Birth control pills containing estrogen and progestin. These can be used before menopause to treat perimenopausal symptoms and prevent pregnancy. Progestin-only pills can be used to treat heavy menstrual bleeding linked with perimenopause.
  • Estrogen and progestin are the form of hormone therapy (HT) most frequently used for treating menopausal symptoms in woman who have a uterus. The dose of estrogen and progestin is less than in a birth control pill.
  • Estrogen therapy (ET), which refers to the use of estrogen without progestin. Most women who use ERT have had their uterus removed (hysterectomy).

When taken after menopause, hormone therapy is also called hormone replacement therapy. But more experts are using the term "hormone therapy" (HT) to avoid the misleading message that women should have premenopausal levels ("replacement") of hormones after menopause.

Last Revised: April 26, 2012

Author: Healthwise Staff

Medical Review: Anne C. Poinier, MD - Internal Medicine & Carla J. Herman, MD, MPH - Geriatric Medicine

Hormone therapy is used to change the way hormones stimulate cancer growth. These medicines either block the effects hormones have on the cancer cells or block the production of the hormones.

If tests show that the breast cancer cells have estrogen and progesterone receptors (ER/PR-positive), hormone therapy may be used. Tamoxifen and aromatase inhibitors are the most commonly used hormonal therapies. Other hormonal therapies include progestins, such as megestrol (Megace), and antiestrogen, such as fulvestrant.

Last Revised: June 28, 2011

Author: Healthwise Staff

Medical Review: Sarah Marshall, MD - Family Medicine & Douglas A. Stewart, MD - Medical Oncology

Hospice care provides medical services, emotional support, and spiritual resources for people who are in the late stages of an incurable illness, such as cancer or Alzheimer's disease. Hospice care also helps family members manage the practical details and emotional challenges of caring for a dying loved one.

Hospice services are provided by a team of caregivers that may include health professionals, volunteers, and spiritual advisors. Services typically include:

  • Basic medical care with a focus on pain and symptom control.
  • Access to a member of the hospice team 24 hours a day, 7 days a week.
  • Medical supplies and equipment, as needed.
  • Counseling and social support. These services are available, as needed, for both the person in hospice care and for anyone in his or her family.
  • Guidance with the difficult, but normal, issues of life completion and closure.
  • A break (respite care) for caregivers, family, and others who regularly care for the person.
  • Volunteer support, such as preparing meals or running errands.

Last Revised: December 29, 2011

Author: Healthwise Staff

Medical Review: Anne C. Poinier, MD - Internal Medicine & Shelly R. Garone, MD, FACP - Palliative Medicine

A hot flash is a sudden sensation of intense body heat, often with profuse sweating and reddening of the head, neck, and chest. These symptoms can be accompanied by mild to severe heart palpitations, anxiety, irritability and, in rare cases, panic.

Hot flashes are the most common symptom of a woman's changing estrogen levels around the time of menopause. They strike unexpectedly, often at night, and usually last several seconds to minutes. Hot flashes:

  • Affect some women during perimenopause, when estrogen levels are changing.
  • Most commonly affect women during the first 1 to 2 years after menopause, when estrogen levels have dropped below a certain point. Women who become menopausal from chemotherapy, from surgical removal of the ovaries (oophorectomy) during hysterectomy, or from antiestrogen treatment for breast cancer are especially likely to have severe hot flashes.
  • Continue to affect some women for 5 years or more after menopause.
  • Can happen normally during stress or embarrassment for women of all ages.

Hot flashes are less commonly caused by thyroid problems, cancers, and psychological stress. Men commonly have hot flashes when taking hormone therapy for prostate cancer.

Several medicines are available to treat hot flashes.

Last Revised: April 26, 2012

Author: Healthwise Staff

Medical Review: Anne C. Poinier, MD - Internal Medicine & Carla J. Herman, MD, MPH - Geriatric Medicine

Huntington's disease is a rare inherited (genetic) disorder that causes parts of the brain to break down and lose some normal functions (degeneration). It is also called Huntington's chorea.

Symptoms of the disease usually develop after age 40 and include rapid, jerky movements (twitches in the face and jerks of the arms) that cannot be controlled (chorea) and the gradual loss of mental abilities (dementia), leading to personality changes, behavior problems, and memory loss.

There is no known cure for the disease. Treatment with medications may help control the involuntary movements.

Last Revised: November 3, 2011

Author: Healthwise Staff

Medical Review: Sarah Marshall, MD - Family Medicine & Siobhan M. Dolan, MD, MPH - Reproductive Genetics

Hyperbaric oxygen therapy is a treatment to increase a person's blood oxygen level, which can prevent tissue death, promote healing, and help fight infection. This treatment involves a person being in an enclosed chamber while 100% oxygen is pumped in at high pressure.

Hyperbaric oxygen therapy may be used to treat conditions such as severe carbon monoxide poisoning, some types of infections, decompression sickness, burns, extreme blood loss, and injuries that cut off oxygen supply to the muscles and other soft tissues.

Large medical centers can often provide hyperbaric oxygen therapy, but it may not be available in smaller hospitals.

Last Revised: March 1, 2012

Author: Healthwise Staff

Medical Review: Anne C. Poinier, MD - Internal Medicine & R. Steven Tharratt, MD, MPVM, FACP, FCCP - Pulmonology, Critical Care Medicine, Medical Toxicology

A hyperosmolar state develops when a person with type 2 diabetes has very high blood sugar—usually 600 milligrams per deciliter (mg/dL) or more. Sometimes this condition is the first time a person learns that he or she has type 2 diabetes.

A hyperosmolar state can develop when a person is very sick and/or dehydrated, such as from the flu, a severe infection, a heart attack, or water pills (diuretics). If the person does not drink enough liquids, he or she may become confused. Older people are at increased risk for developing a hyperosmolar state.

Symptoms of a hyperosmolar state include:

  • Increased urination for several days.
  • Dehydration, which develops because the person doesn't drink enough liquids.
  • A change in alertness from generalized fatigue to stupor, coma, or seizures. These changes may be mistaken for a stroke or mental illness.

Hyperosmolar state is treated in a hospital with insulin to reduce the blood sugar level and extra fluids through a vein (IV) to replace the lost fluids.

The best way to prevent a hyperosmolar state is to treat high blood sugar levels early and drink enough liquids.

Last Revised: June 29, 2011

Author: Healthwise Staff

Medical Review: E. Gregory Thompson, MD - Internal Medicine & Jennifer Hone, MD - Endocrinology, Diabetes and Metabolism

Hyperthyroidism means that your body has too much thyroid hormone, which controls how your body uses energy. Too much thyroid hormone can make you lose weight quickly, have a fast heartbeat, sweat a lot, or feel nervous and moody. If it isn't treated, it can cause serious problems.

Last Revised: October 9, 2012

Author: Healthwise Staff

Medical Review: E. Gregory Thompson, MD - Internal Medicine & David C.W. Lau, MD, PhD, FRCPC - Endocrinology

Hypertrophic cardiomyopathy is a genetic disease in which the heart muscle grows abnormally, making the heart muscle thicken. The thickened heart muscle can interfere with the heart's electrical system, which increases the risk for life-threatening abnormal heartbeats (arrhythmias) and, rarely, sudden death.

Symptoms, such as shortness of breath and chest pain, may occur at any time of life. But some people never have symptoms, even though the condition may have been present for some time. In some cases, the thickened heart muscle is unable to relax between heartbeats, and the heart muscle itself does not get enough blood or oxygen, which causes chest pain. In rare cases, the thickened heart muscle becomes unable to pump enough blood to meet the body's needs, resulting in heart failure.

Last Revised: July 23, 2012

Author: Healthwise Staff

Medical Review: Rakesh K. Pai, MD, FACC - Cardiology, Electrophysiology & Robert A. Kloner, MD, PhD - Cardiology & George Philippides, MD - Cardiology

Hyperventilation is breathing that is deeper and more rapid than normal, which causes the amount of carbon dioxide (CO2) in the blood to drop too low. This may result in lightheadedness, a rapid heartbeat, shortness of breath, numbness or tingling in the hands or feet, anxiety, fainting, and sore chest muscles.

Some causes of hyperventilation include extreme anxiety, asthma, emphysema, head injury, fever, exposure to altitude above 6000 ft (2000 m), and some medicines.

In many cases, hyperventilation can be controlled with home treatment, such as focusing on proper breathing techniques.

Last Revised: August 29, 2012

Author: Healthwise Staff

Medical Review: William H. Blahd, Jr., MD, FACEP - Emergency Medicine & David Messenger, MD

Hypoglycemia is a low level of blood sugar. A person may tremble, feel nervous or jittery, break out in a cold sweat, have a headache, or feel sick to his or her stomach.

Blood sugar levels are measured in mg/dL. A fasting blood sugar level of 70 to 99 mg/dL is normal, 50 to 70 mg/dL is mildly low, and less than 50 mg/dL is very low.

If blood sugar, also called glucose, continues to fall, a person may experience mood changes, such as irritability, anxiety, restlessness, anger, or confusion. And he or she may have symptoms such as weakness, blurred vision, dizziness, fatigue, and poor coordination.

Hypoglycemia may also result from taking certain medicines or drinking alcohol. It can also be caused by certain health problems, such as not processing carbohydrates properly or having an enzyme deficiency. Sometimes the cause is unknown.

Treatment for a sudden (acute) episode of hypoglycemia involves eating or drinking some form of sugar to restore blood sugar to a normal level. Episodes of hypoglycemia caused by a long-term (chronic) health condition are treated the same way. But to prevent future episodes of hypoglycemia, treatment or cure of the long-term condition is needed.

Last Revised: March 16, 2011

Author: Healthwise Staff

Medical Review: E. Gregory Thompson, MD - Internal Medicine & Alan C. Dalkin, MD - Endocrinology

Hypoglycemia (or hypoglycemic) unawareness is the inability to recognize early symptoms of low blood sugar until they become severe. Once symptoms reach this stage, urgent treatment is needed to prevent further progression and life-threatening health problems, such as a seizure or stroke.

Severe symptoms of low blood sugar include confusion, slurred speech, unsteadiness when standing or walking, muscle twitching, and personality changes. People with diabetes who tightly control their blood sugar levels are more likely to have episodes of low blood sugar. Frequent and severe low blood sugar episodes are likely to evolve into hypoglycemia unawareness. Once a person has had one hypoglycemia unawareness episode, more are likely to occur.

Last Revised: September 20, 2012

Author: Healthwise Staff

Medical Review: E. Gregory Thompson, MD - Internal Medicine & Jennifer Hone, MD - Endocrinology, Diabetes and Metabolism

Hypoplastic left heart syndrome is a heart defect that is present at birth (congenital) in which the left side of the heart and aorta are very small or underdeveloped. The small left ventricle makes the right ventricle responsible for pumping blood to the lungs and body.

Over time, too little blood may be pumped to the body, leading to heart failure. Without a series of major heart operations, this defect almost always causes death.

Last Revised: October 11, 2011

Author: Healthwise Staff

Medical Review: John Pope, MD - Pediatrics & Larry A. Latson, MD - Pediatric Cardiology

Hypothyroidism happens when your thyroid gland doesn't make enough thyroid hormone. Thyroid hormone controls the way your body uses energy. A low thyroid level can make you feel tired and weak.

Medicine can correct your thyroid level. Most people need to keep taking the thyroid medicine throughout their lives.

Last Revised: September 27, 2012

Author: Healthwise Staff

Medical Review: Kathleen Romito, MD - Family Medicine & Matthew I. Kim, MD - Endocrinology