Methaemoglobin
Encyclopedia
Methemoglobin is a form of the oxygen
-carrying metalloprotein
hemoglobin
, in which the iron
in the heme
group is in the Fe3+ (ferric
) state, not the Fe2+ (ferrous
) of normal hemoglobin. Methemoglobin cannot release bound oxygen, unlike oxyhemoglobin. It is a bluish chocolate-brown
in color. The NADH-dependent enzyme methemoglobin reductase (diaphorase I) is responsible for converting methemoglobin back to hemoglobin.
Normally one to two percent of people's hemoglobin is methemoglobin; a higher percentage than this can be genetic or caused by exposure to various chemicals and depending on the level can cause health problems known as methemoglobinemia
. A higher level of methemoglobin will tend to cause a pulse oximeter
to read closer to 85% regardless of the true level of oxygen saturation.
is administered to treat cyanide
poisoning. It works by converting hemoglobin
to methemoglobin, which allows for the binding of cyanide and the formation of non-toxic cyanomethemoglobin.
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
-carrying metalloprotein
Metalloprotein
Metalloprotein is a generic term for a protein that contains a metal ion cofactor. Metalloproteins have many different functions in cells, such as enzymes, transport and storage proteins, and signal transduction proteins. Indeed, about one quarter to one third of all proteins require metals to...
hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
, in which the iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
in the heme
Heme
A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic group; these are...
group is in the Fe3+ (ferric
Ferric
Ferric refers to iron-containing materials or compounds. In chemistry the term is reserved for iron with an oxidation number of +3, also denoted iron or Fe3+. On the other hand, ferrous refers to iron with oxidation number of +2, denoted iron or Fe2+...
) state, not the Fe2+ (ferrous
Ferrous
Ferrous , in chemistry, indicates a divalent iron compound , as opposed to ferric, which indicates a trivalent iron compound ....
) of normal hemoglobin. Methemoglobin cannot release bound oxygen, unlike oxyhemoglobin. It is a bluish chocolate-brown
Impossible colors
Impossible colors or forbidden colors are hues that cannot be perceived in ordinary viewing conditions from light that is a combination of various intensities of the various frequencies of visible light. Examples of impossible colors are bluish-yellow and reddish-green...
in color. The NADH-dependent enzyme methemoglobin reductase (diaphorase I) is responsible for converting methemoglobin back to hemoglobin.
Normally one to two percent of people's hemoglobin is methemoglobin; a higher percentage than this can be genetic or caused by exposure to various chemicals and depending on the level can cause health problems known as methemoglobinemia
Methemoglobinemia
Methemoglobinemia is a disorder characterized by the presence of a higher than normal level of methemoglobin in the blood. Methemoglobin is an oxidized form of hemoglobin that has an increased affinity for oxygen, resulting in a reduced ability to release oxygen to tissues. The oxygen–hemoglobin...
. A higher level of methemoglobin will tend to cause a pulse oximeter
Pulse oximeter
A pulse oximeter is a medical device that indirectly monitors the oxygen saturation of a patient's blood and changes in blood volume in the skin, producing a photoplethysmograph. It is often attached to a medical monitor so staff can see a patient's oxygenation at all times...
to read closer to 85% regardless of the true level of oxygen saturation.
Common causes
- Reduced cellular defense mechanisms
- Children younger than 4 months exposed to various environmental agents
- Cytochrome b5 reductaseCytochrome b5 reductaseCytochrome-b5 reductase is a NADH-dependent enzyme that converts methemoglobin to hemoglobin...
deficiency - G6PD deficiencyGlucose-6-phosphate dehydrogenase deficiencyGlucose-6-phosphate dehydrogenase deficiency is an X-linked recessive hereditary disease characterised by abnormally low levels of glucose-6-phosphate dehydrogenase , a metabolic enzyme involved in the pentose phosphate pathway, especially important in red blood cell metabolism. G6PD deficiency is...
- Hemoglobin M disease
- Pyruvate kinase deficiencyPyruvate kinase deficiencyPyruvate kinase deficiency, also called erythrocyte pyruvate kinase deficiency, is an inherited metabolic disorder of the enzyme pyruvate kinase which affects the survival of red blood cells and causes them to deform into echinocytes on peripheral blood smears.Both autosomal dominant and recessive...
- Various pharmaceutical compounds
- Local anesthetic agents, especially prilocaine as used in the Bier blockIntravenous regional anesthesiaIntravenous regional anesthesia or Bier block anesthesia is a common anesthetic technique for surgical procedures on the body's extremities where a local anesthetic is injected intravenously. The technique usually involves exsanguination, which forces blood out of the extremity, followed by the...
- Amyl nitriteAmyl nitriteAmyl nitrite is the chemical compound with the formula C5H11ONO. A variety of isomers are known, but they all feature an amyl group attached to the nitrito functional group. The alkyl group is unreactive and the chemical and biological properties are mainly due to the nitrite group...
, chloroquineChloroquineChloroquine is a 4-aminoquinoline drug used in the treatment or prevention of malaria.-History:Chloroquine , N'--N,N-diethyl-pentane-1,4-diamine, was discovered in 1934 by Hans Andersag and co-workers at the Bayer laboratories who named it "Resochin". It was ignored for a decade because it was...
, dapsoneDapsoneDapsone is a medication most commonly used in combination with rifampicin and clofazimine as multidrug therapy for the treatment of Mycobacterium leprae infections . It is also second-line treatment for prophylaxis against Pneumocystis pneumonia caused by Pneumocystis jiroveci Dapsone...
, nitrates, nitrites, nitroglycerin, nitroprusside, phenacetinPhenacetinPhenacetin is an analgesic, once widely used; its use has declined because of its adverse effects.-History:Phenacetin was introduced in 1887, and was used principally as an analgesic, and was one of the first synthetic fever reducers to go on the market...
, phenazopyridinePhenazopyridinePhenazopyridine is a chemical which, when excreted into the urine, has a local analgesic effect. It is often used to alleviate the pain, irritation, discomfort, or urgency caused by urinary tract infections, surgery, or injury to the urinary tract...
, primaquinePrimaquinePrimaquine is a medication used in the treatment of malaria and Pneumocystis pneumonia. It is a member of the 8-aminoquinoline group of drugs that includes tafenoquine and pamaquine.-Radical cure:...
, quinones and sulfonamides
- Local anesthetic agents, especially prilocaine as used in the Bier block
- Environmental agents
- Aromatic amineAmineAmines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
s (e.g. p-nitroaniline4-Nitroaniline4-Nitroaniline, p-nitroaniline or 1-amino-4-nitrobenzene is an organic compound with the formula C6H6N2O2. It is an organic chemical compound, consisting of a phenyl group attached to an amino group which is para to a nitro group. The chemical structure of p-nitroaniline is shown at the right...
, patient case) - ArsineArsineArsine is the chemical compound with the formula AsH3. This flammable, pyrophoric, and highly toxic gas is one of the simplest compounds of arsenic...
- ChlorobenzeneChlorobenzeneChlorobenzene is an aromatic organic compound with the chemical formula C6H5Cl. This colorless, flammable liquid is a common solvent and a widely used intermediate in the manufacture of other chemicals.-Uses:...
- Chromates
- Nitrates/nitrites
- Aromatic amine
- In cats
- Ingestion of ParacetamolParacetamolParacetamol INN , or acetaminophen USAN , is a widely used over-the-counter analgesic and antipyretic . It is commonly used for the relief of headaches and other minor aches and pains and is a major ingredient in numerous cold and flu remedies...
(i.e. acetaminophen, tylenol)
- Ingestion of Paracetamol
Therapeutic uses
Amyl nitriteAmyl nitrite
Amyl nitrite is the chemical compound with the formula C5H11ONO. A variety of isomers are known, but they all feature an amyl group attached to the nitrito functional group. The alkyl group is unreactive and the chemical and biological properties are mainly due to the nitrite group...
is administered to treat cyanide
Cyanide
A cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom. Cyanides most commonly refer to salts of the anion CN−. Most cyanides are highly toxic....
poisoning. It works by converting hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
to methemoglobin, which allows for the binding of cyanide and the formation of non-toxic cyanomethemoglobin.
Methemoglobin saturation
Methemoglobin saturation is expressed as the percentage of hemoglobin in the methemoglobin state; That is MetHb as a proportion of Hb.- 1-2% Normal
- Less than 10% metHb - No symptoms
- 10-20% metHb - Skin discoloration only (most notably on mucus membranes)
- 20-30% metHb - AnxietyAnxietyAnxiety is a psychological and physiological state characterized by somatic, emotional, cognitive, and behavioral components. The root meaning of the word anxiety is 'to vex or trouble'; in either presence or absence of psychological stress, anxiety can create feelings of fear, worry, uneasiness,...
, headacheHeadacheA headache or cephalalgia is pain anywhere in the region of the head or neck. It can be a symptom of a number of different conditions of the head and neck. The brain tissue itself is not sensitive to pain because it lacks pain receptors. Rather, the pain is caused by disturbance of the...
, dyspneaDyspneaDyspnea , shortness of breath , or air hunger, is the subjective symptom of breathlessness.It is a normal symptom of heavy exertion but becomes pathological if it occurs in unexpected situations...
on exertion - 30-50% metHb - Fatigue, confusionConFusionConFusion is an annual science fiction convention organized by the Stilyagi Air Corps and its parent organization, the Ann Arbor Science Fiction Association. Commonly, it is held the third weekend of January. It is the oldest science fiction convention in Michigan, a regional, general SF con...
, dizzinessDizzinessDizziness refers to an impairment in spatial perception and stability. The term is somewhat imprecise. It can be used to mean vertigo, presyncope, disequilibrium, or a non-specific feeling such as giddiness or foolishness....
, tachypneaTachypneaTachypnea means rapid breathing. Any rate between 12-20 breaths per minute is normal. Tachypnea is a respiration rate greater than 20 breaths per minute. - Distinction from other breathing terms :...
, palpitations - 50-70% metHb - ComaComaIn medicine, a coma is a state of unconsciousness, lasting more than 6 hours in which a person cannot be awakened, fails to respond normally to painful stimuli, light or sound, lacks a normal sleep-wake cycle and does not initiate voluntary actions. A person in a state of coma is described as...
, seizures, arrhythmias, acidosisAcidosisAcidosis is an increased acidity in the blood and other body tissue . If not further qualified, it usually refers to acidity of the blood plasma.... - Greater than 70% metHb - DeathDeathDeath is the permanent termination of the biological functions that sustain a living organism. Phenomena which commonly bring about death include old age, predation, malnutrition, disease, and accidents or trauma resulting in terminal injury....