VIDEO ON NERVE ACTION POTENTIAL

This is a video showing how action potentials are generated.

BRADYCARDIA

What Is It?

Bradycardia is an abnormally slow heart rate of less than 60 beats per minute. A normal heartbeat is between 60 and 100 beats per minute.

Here's what happens during a normal heartbeat: The electrical signal that starts a heartbeat comes from the heart's sinus node, the natural pacemaker located in the upper portion of the right atrium. From the sinus node, the heartbeat signal travels to the atrioventricular (A-V) node, located between the atria, and then through the bundle of His (pronounced "hiss") -- a series of modified heart-muscle fibers located between the ventricles -- to the muscles of the ventricles. This triggers a contraction of the ventricles and produces a heartbeat.

Bradycardia, even as low as 50 beats per minute, can be normal in athletes and other people who are physically active. In these people, regular exercise improves the heart's ability to pump blood efficiently, so fewer heart contractions are required to supply the body's needs.

In other cases, bradycardia can be a form of cardiac arrhythmia, a heart-rate abnormality. Cardiac arrhythmia can be caused by a problem in the sinus node, or it can be related to some disturbance in the passage of heartbeat signals through the A-V node and bundle of His. Bradycardia can occur with toxic levels of certain drugs, such as digoxin (Lanoxin) and narcotics. Also, bradycardia sometimes is a side effect of certain medications, including propranolol (Inderal), atenolol (Tenormin), metoprolol (Toprol-XL), sotalol (Betapace), verapamil (Calan, Isoptin, Verelan) and diltiazem (Cardizem, Dilacor-XR). Bradycardia also occurs in some people who have certain medical illnesses not related to the heart, such as:

• An abnormally low level of thyroid hormones (hypothyroidism)
• An abnormally low body temperature (hypothermia)
• A very high blood potassium level
• Lyme disease
• Typhoid fever 

Symptoms

Bradycardia can cause dizziness, weakness, lack of energy or fainting spells.

If bradycardia is caused by a medical illness, there will be additional symptoms that are specific to that illness. For example, people whose bradycardia is due to severe hypothyroidism also can have constipation, muscle cramps, weight gain (often despite poor appetite), very dry skin, hair that is thin and dry, an abnormal sensitivity to cold temperatures and other symptoms related to low levels of thyroid hormones.

Diagnosis.

Your doctor will ask about your family history of heart disease, cardiac arrhythmias and fainting spells. He or she also will review your current symptoms and your personal medical history, including your use of medications that may cause bradycardia.

During the physical examination, your doctor will check your heart rate and rhythm, and your pulse. You may be asked to do some sit-ups or other exercise so that your doctor can see if your heart rate rises normally when your heart is asked to do more work. Your doctor also will check for physical signs of thyroid abnormalities, including an enlarged thyroid gland, very dry skin, and hair that is thin and dry.

To further evaluate your bradycardia, your doctor will order an electrocardiogram (EKG). However, because some forms of bradycardia come and go, a one-time office EKG may be normal. If this is the case, a test called ambulatory electrocardiography may be done. During this test, the patient wears a portable EKG machine called a Holter monitor, usually for 24 hours. If your symptoms are infrequent, you may wear a monitor for longer. You will be taught to press a button to record your EKG reading when your symptoms occur.

Depending on the results of your physical examination, other tests may be necessary to check for medical illnesses that produce bradycardia. For example, if you have symptoms and physical signs of hypothyroidism, your doctor may order blood tests to measure levels of thyroid hormones and TSH, a pituitary gland hormone that stimulates the thyroid. Your doctor also may order blood tests for cholesterol and certain liver enzymes, which often are elevated in people with hypothyroidism.

Expected Duration.

How long bradycardia lasts depends on its cause. For example, normal bradycardia in a well-trained athlete will last as long as the athlete maintains his or her usual level of exercise. When bradycardia occurs as a side effect of medication, it usually will go away as soon as the drug that triggered the bradycardia is used by the body or excreted in the urine. Bradycardia caused by hypothyroidism will go away quickly after treatment with thyroid hormones. Certain forms of bradycardia resulting from cardiac arrhythmias can be cured with a permanent pacemaker.

Prevention.

There are no general guidelines to prevent all forms of bradycardia. When bradycardia occurs as a side effect of medication, the problem can be prevented by either switching the drug or reducing its dose.

Treatment.

In most cases, bradycardia in healthy, well-trained athletes does not need to be treated. In fact, in most people, bradycardia does not require treatment unless patients have symptoms that are clearly due to a slow heartbeat. The following are conditions that produce bradycardia that requires treatment:

Cardiac arrhythmias resulting from sinus node dysfunction. In people with frequent, severe symptoms of sinus-node dysfunction, the main treatment is usually a permanent pacemaker, an implanted device that generates electrical impulses to regulate the heartbeat.

Cardiac arrhythmias resulting from A-V node problems. In people with A-V node problems, the passage of heartbeat signals may be blocked to different degrees. Some patients with less severe degrees of A-V block do not need treatment. Others may simply be monitored with frequent EKGs, especially if they don't have any symptoms and have a heart rate that is adequate for their daily activities. People with the most severe form of A-V block (called third-degree A-V block) can be treated with a special dual-chambered permanent pacemaker, a pacemaker with one electrode in the atrium and one in the ventricle.

Hypothyroidism. Hypothyroidism is treated with replacement doses of thyroid hormones. Synthetic forms of these hormones are available -- levothyroxine (Synthroid, Levothroid and others), liothyronine (Cytomel, Triostat) or liotrix (Thyrolar) -- as well as thyroid extract.

Lyme disease. Treatment with antibiotics clears the infection and the bradycardia almost always resolves.

When to Call a Professional.

Call your doctor if your pulse is less than 60 beats per minute. This is especially important if you have had dizziness, weakness, lack of energy or fainting spells.

Prognosis.

In well-trained athletes with normal bradycardia, the slow heartbeat is often a sign of overall good health. In other people with bradycardia, the outlook depends on the underlying disorder. For example, the prognosis is usually good in people with hypothyroidism, since treatment with thyroid hormones can relieve bradycardia and other symptoms related to low hormone levels. In some elderly patients, hormone dosages may need to be increased very slowly over several weeks to prevent straining the heart. At one time, the prognosis for people with third-degree A-V block was poor, with 50% of patients dying within one year of diagnosis. Now, however, the availability of permanent pacemakers has improved the prognosis for many people with cardiac arrhythmias.

PHYSIOLOGICAL CONDITION OF MYASTHENIA GRAVIS

Myasthenia gravis is a rare, long-term condition that causes certain muscles to become weak.

It mainly affects muscles that are controlled voluntarily - often those controlling eye and eyelid movement, facial expression, chewing, swallowing and talking.

Sometimes, the muscles that control breathing, neck and limb movements are also affected.

The muscle weakness associated with myasthenia gravis is usually worse during physical activity and improves with rest.


What causes myasthenia gravis?





Myasthenia gravis is an autoimmune condition that affects the nerves and muscles. Autoimmune conditions are those caused by the body's immune system mistakenly attacking healthy tissue.

In myasthenia gravis, the immune system produces antibodies (proteins) that block or damage muscle receptor cells.

This prevents messages being passed from the nerve endings to the muscles, which results in the muscles not contracting (tightening) and becoming weak.

It is not fully understood why some people's immune systems produce antibodies that attack the muscle receptor cells.

Diagnosing myasthenia gravis






The process of diagnosing myasthenia gravis can take a long time because muscle weakness is a symptom of many different conditions.

Treating myasthenia gravis

There is no cure for myasthenia gravis but treatments are available to help control the symptoms and improve muscle weakness.

Medication can be used to improve communication between the nerves and muscles and increase muscle strength.

Immunosuppressants are a type of medication used to improve muscle strength by controlling the production of abnormal antibodies.

In some cases of myasthenia gravis, surgery may be recommended to remove the thymus gland (a thymectomy). The thymus gland is found underneath the breastbone and is part of the immune system. It is often abnormal in people with myasthenia gravis.


OutlookIn most cases, treatment for myasthenia gravis significantly improves muscle weakness and a person with the condition is able to lead a relatively normal life.

Some people may experience a temporary or permanent period of remission (where there are no longer symptoms) and treatment can be stopped.

Permanent remissions occur in about a third of all people who have a thymectomy (surgery to remove the thymus gland).

TACHYCARDIA

Supraventricular tachycardia (SVT) is a heart condition featuring episodes of an abnormally fast heart rate.

The heart will suddenly start racing, then stop or slow down abruptly.

Episodes can last for seconds, minutes, hours or (in rare cases) days. They may occur regularly, several times a day, or very infrequently, once or twice a year.

The heart rate may be as high as 250 beats per minute, but is usually between 140 and 180 (a normal heartbeat should be 60-100 beats per minute at rest).
What happens

When the heart beats normally, its muscular walls contract (tighten and squeeze) to force blood out and around the body. They then relax, so the heart can fill with blood again (see How the heart works). This process is repeated for every heartbeat.

In SVT, the heart muscle is contracting so fast that it cannot relax between contractions. This reduces the amount of blood being pumped around the body, which can make you feel dizzy and short of breath (see Supraventricular tachycardia - symptoms).

You usually feel heart palpitations (noticeable heartbeats) and a fast pulse.
Why it happens

SVT is caused by abnormal electrical impulses that start suddenly in the upper chambers of your heart (the atria). These impulses override your heart's natural rhythm (see Supraventricular tachycardia - causes).

It is often a short circuit in the electrical system of your heart that causes these spontaneous impulses. The short circuit causes an electrical signal to travel continuously around in a circle. Each time the signal completes the circuit, the impulse spreads out to the rest of your heart, forcing it to beat rapidly.

SVT attacks often happen for no obvious reason. However, they may be triggered by a change in posture, exertion, emotional upset, coffee or alcohol.
Who is affected

SVT can occur in anyone at any age, but it often occurs for the first time in children or young adults.
Outlook

In the vast majority of cases, attacks of SVT are harmless, do not last long and settle on their own without treatment.

If necessary, SVT can be treated with drugs that correct the abnormal heart rate. It can also be permanently treated with a very effective surgical procedure called catheter ablation, which interrupts the abnormal electrical circuits (see Supraventricular tachycardia - treatment).