 Definition
Myoglobin is a protein found in
muscle. Myoglobin tests are done to evaluate a person who has symptoms
of a heart attack (myocardial infarction) or other muscle damage.
Purpose
Myoglobin holds oxygen inside
heart and skeletal muscle (muscles that attach to and move bones). It
is continually released into the blood in small amounts due to normal
turnover of muscle cells. Kidneys discard the myoglobin into urine.
When muscle is damaged, as in a
heart attack, larger amounts of myoglobin are released and blood
levels rise rapidly. Myoglobin is one of the first tests done to
determine if a person with chest pain is having a heart attack, as it
may be one of the first blood tests to become abnormal.
Damage or injury to skeletal
muscle also causes myoglobin to be released into the blood.
Description
Heart attack must be diagnosed
quickly. Medications to prevent heart damage are effective only within
a limited number of hours. Yet, because of their risk for excessive
bleeding, these medications are given only after a diagnosis of heart
attack is made.
Myoglobin is one of several
cardiac markers used to make the diagnosis. Cardiac markers are
substances in blood whose levels rise in the hours following a heart
attack. Increased levels help diagnose a heart attack; persistent
normal levels rule it out.
Each cardiac marker rises,
peaks, and returns to a normal level according to its own timeline, or
diagnostic window. Myoglobin is useful because it has the earliest
diagnostic window. It is the first marker to rise after chest pain
begins. Myoglobin levels rise within two to three hours, and sometimes
as early as 30 minutes. They peak after six to nine hours. The levels
return to normal within 24-36 hours.
Although a rise in myoglobin
supports a diagnosis of heart attack, it is not conclusive.
Simultaneous skeletal muscle damage could also cause the increase.
Myoglobin rules out, rather than proves, a diagnosis in the following
way. If myoglobin levels have not risen after more than five hours, a
heart attack in unlikely. Normal levels in the first two to three
hours do not rule out an infarction.
The myoglobin test is sometimes
repeated every one to two hours to watch for the rise and peak.
Results are available within 30 minutes.
Myoglobin in large amounts is
toxic to the kidney. When a person has high amounts of myoglobin in
the blood, kidney function must be monitored.
Preparation
This test requires 5 ml of blood.
Collection of the sample takes only a few minutes. A urine myoglobin
test requires 1 ml of urine collected into a urine collection cup.
Aftercare
Discomfort or bruising may occur
at the puncture site or the person may feel dizzy or faint. Pressure
to the puncture site until the bleeding stops reduces bruising. Warm
packs to the puncture site relieve discomfort.
Normal
results
Normal results vary based on the
laboratory and method used.
Abnormal
results
Myoglobin levels and levels of
other cardiac markers are usually considered before finally confirming
a diagnosis of heart attack. A level that has doubled after one to two
hours, even if the level is still in the normal range, indicates a
significant rise that may be due to heart attack.
Increased levels are also found
with skeletal muscle damage or disease, such as an injury, muscular
dystrophy , or polymyositis . Myoglobin levels also rise during renal
failure because kidneys lose their ability to clear myoglobin from
blood.
Key
Terms
- Cardiac
marker
- A substance in the blood
that rises following a heart attack.
- Diagnostic
window
- A cardiac marker's timeline
for rising, peaking, and returning to normal after a heart attack.
- Myoglobin
- A protein that holds oxygen
in heart and skeletal muscle. It rises after damage to either of
these muscle types.
Common
Questions
1. What does heart attack mean?
Heart
attack means that some of the muscle in your heart has died.
A medical term for this is myocardial infarction. Most commonly, a
heart attack starts with a kind of heavy pressure or pain in the
chest, often extending into the neck or left arm. You may have trouble
catching your breath, or you may feel weak and break into a cold
sweat.
A heart attack usually occurs
because one of the blood vessels (called coronary arteries) that bring
blood to your heart muscle is blocked. This usually happens when a
blood clot forms in a blood vessel that is already partially blocked.
The partial blockage, which happens gradually over many years, is
usually caused by too much fat layered in the wall of the blood vessel
(this is often called hardening of the arteriesthe medical term for
this is atherosclerosis).
2. If I
have chest pain, does that mean I am having a heart attack?
Many other problems can cause chest pain, and it is not always
possible to tell just from the type of chest pain whether or not you
are having a heart
attack. Many people have chest pain from straining the muscles in
their chest, and chest pain can occur with some lung problems. Chest
pain can be a warning sign of hardening of the arteries of the heart
(coronary artery disease or CAD).
Chest pain that occurs during
exercise, hard work, or at times of stress, lasts for a few minutes
and goes away with rest is called angina.
If the pain lasts longer than just a few minutes, especially if it
occurs when you are resting, seek immediate medical attention.
3. What
are the other heart attack tests?
Doctors often use more than one test to determine if a person who has
chest pain is having a heart
attack. Troponin can
pick up damage to heart muscles even when there is no other evidence
of a heart attack. Myoglobin and creatine
kinase, two other proteins
found in the heart and in other muscles, do not usually rise in
persons with heart injury unless a heart attack has occurred. Some
doctors use the term mild heart attack if troponin is high but
one or both of the other muscle proteins are not.
4. What
if Im not sure Im having a heart attack?
If you have prolonged chest pain, especially if it does not go away
with rest or if you have been told you have angina,
and the drugs you were prescribed do not ease the pain, seek immediate
medical attention. Many people who have had a heart
attack die without ever having tried to call an ambulance or get
to an emergency room.
5. Is
myoglobin useful to evaluate a stroke, too?
A stroke,
sometimes called a brain attack, occurs when part of the brain
dies. Like a heart
attack, it usually is due to a blockage of one of the blood
vessels that brings blood to the brain. Myoglobin is only found in
muscle cells, not in the brain, so it is not helpful in evaluating a
person who may have had a stroke.
 
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Cardiac
Markers: 
