Carbon monoxide poisoning

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Alternative names
Methemoglobin; Carboxyhemoglobin; Sulfhemoglobin; Hemoglobin derivatives

Definition
This is a blood test to detect and measure the percentage of abnormal hemoglobin derivatives that result from drugs such as nitroglycerin or poisoning by carbon monoxide.

How the test is performed

Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

The samples are then sent to the lab where the percentage of total hemoglobin vs. derivatives is evaluated.

How to prepare for the test
If your child is to have this test performed it may be helpful to explain how the test will feel, and even practice or demonstrate on a doll. The more familiar your child is with what will happen to them, and the purpose for the procedure, the less anxiety they will feel.

How the test will feel
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed

Hemoglobin is the substance in red blood cells that transports oxygen and carbon dioxide between the lungs and body tissues.

If certain chemicals or drugs are introduced into the blood stream, they can alter the hemoglobin so it no longer functions as a transport medium. Measuring abnormal hemoglobins can be helpful in diagnosing and monitoring exposure to these agents.

Carboxyhemoglobin is hemoglobin bound to carbon monoxide instead of oxygen or carbon dioxide. Carbon monoxide has a much higher affinity for hemoglobin than does oxygen (210 times more), and high percentages of carboxyhemoglobin in the blood impairs the normal transport of oxygen by the blood.

Sulfhemoglobin results from the union of hemoglobin with medications such as phenacetin or sulfonamides. This form of hemoglobin is unable to transport oxygen, and is untreatable. The only solution is to wait until the affected red blood cells are destroyed as part of their normal life cycle.

Methemoglobin occurs when the iron that is part of hemoglobin is altered such that it does not carry oxygen well. Certain compounds introduced into the blood stream can cause this problem:

• sulfonamides
  
• chlorates
  
• nitrates
  
• nitrites
  
• aniline
  
• phenacetin

There is also a congenital form of this disorder caused by a deficiency of NADH cytochrome b5 reductase, and another form caused by hemoglobin M disease.

Normal Values
The following values represent the percentage of hemoglobin derivatives based on total hemoglobin:

• carboxyhemoglobin: 3% (but may be as high as 15% in smokers)
  
• methemoglobin: less than 3%
  
• sulfhemoglobin: undetectable

What abnormal results mean
As abnormally high levels of hemoglobin derivatives rise, there are increasingly detrimental effects. Since these forms of hemoglobin do not permit the transfer of oxygen, the effects on the body are related to lack of oxygen to the tissues, which can result in tissue death.

The following values, except sulfhemoglobin, represent the percentage of hemoglobin derivatives based on total hemoglobin:

Carboxyhemoglobin:

• 20% symptoms of carbon monoxide poisoning begin to appear
  
• 30% severe carbon monoxide poisoning present
  
• 50-80% results in fatal carbon monoxide poisoning

Methemoglobin:

• 10-25% results in cyanosis
  
• 35-40% results in shortness of breath and headache
  
• over 60% results in lethargy and stupor
  
• over 70% results in death

Sulfhemoglobin

• values of 10 gm/dl result in cyanosis, but do not produce toxic effects

Note:

• lower levels are hemoglobin derivatives produce the same effects in children.
  
• gm/dl = grams per deciliter

What the risks are
Risks associated with having blood drawn are slight:

• excessive bleeding
  
• fainting or feeling light-headed
  
• Hematoma (blood accumulating under the skin)
  
• infection (a slight risk any time the skin is broken)
  
• multiple punctures to locate veins

Special considerations
Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

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