Methemoglobin (British:methaemoglobin, shortenedMetHb) (pronounced "met-hemoglobin") is ahemoglobin in the form ofmetalloprotein, in which theiron in theheme group is in the Fe3+ (ferric) state, not the Fe2+ (ferrous) of normal hemoglobin. Sometimes, it is also referred to as ferrihemoglobin.[2] Methemoglobin cannot bindoxygen, which means it cannot carry oxygen to tissues. It is bluish chocolate-brown in color. In humanblood a trace amount of methemoglobin is normally produced spontaneously, but when present in excess the blood becomes abnormally dark bluish brown. TheNADH-dependent enzymemethemoglobin reductase (a type of diaphorase) is responsible for converting methemoglobin back tohemoglobin.
Normally one to two percent of a person'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 asmethemoglobinemia. A higher level of methemoglobin will tend to cause apulse oximeter to read closer to 85% regardless of the true level ofoxygen saturation.
The word methemoglobin derives from theAncient Greek prefixμετα- (meta-: behind, later, subsequent) and the wordhemoglobin.
The namehemoglobin is itself derived from the wordsheme andglobin, eachsubunit of hemoglobin being aglobular protein with an embeddedheme group.
Amyl nitrite is administered to treatcyanide poisoning. It works by convertinghemoglobin to methemoglobin, which allows for the binding ofcyanide (CN–)anions byferric (Fe3+)cations and the formation ofcyanomethemoglobin. The immediate goal of forming this cyanide adduct is to prevent the binding of free cyanide to thecytochrome a3 group incytochrome c oxidase.[9]
Methemoglobin is expressed as a concentration or a percentage. Percentage of methemoglobin is calculated by dividing the concentration of methemoglobin by the concentration of total hemoglobin. Percentage of methemoglobin is likely a better indicator of illness severity than overall concentration, as underlying medical conditions play an important role. For example, a methemoglobin concentration of 1.5 g/dL may represent a percentage of 10% in an otherwise healthy patient with a baseline hemoglobin of 15 mg/dL, whereas the presence of the same concentration of 1.5 g/dL of methemoglobin in an anemic patient with a baseline hemoglobin of 8 g/dL would represent a percentage of 18.75%. The former patient will be left with a functional hemoglobin concentration of 13.5 g/dL and potentially remain asymptomatic while the latter patient with a functional hemoglobin concentration 6.5 g/dL may be severely symptomatic with a methemoglobin of less than 20%.[10]
This may be further compounded by the "functional hemoglobin's" decreased ability to release oxygen in the presence of methemoglobin.Anemia,congestive heart failure,chronic obstructive pulmonary disease, and essentially any pathology that impairs the ability to deliver oxygen may worsen the symptoms of methemoglobinemia.[10]
Increased levels of methemoglobin are found in blood stains. Upon exiting the body,bloodstains transit from bright red to dark brown, which is attributed to oxidation of oxy-hemoglobin (HbO2) to methemoglobin (met-Hb) andhemichrome (HC).[11]
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