Methemoglobinemia

Description:

Normal conditions in blood contain insignificant quantities of the derivatives of hemoglobin not capable to transfer oxygen, so-called disgemoglobina: carboxyhaemoglobin, methemoglobin and sulfhemoglobin. At increase in maintenance of disgemoglobin kislorodotransportny function of blood significantly suffers. Such disgemoglobina as carboxyhaemoglobin (SONB) and a methemoglobin (MetHb) are clinically most significant. The methemoglobin is constantly formed as a result of normal metabolism of cells of an organism. There is an endogenous mechanism of regulation of level of a methemoglobin in blood allowing to support a share of this fraction not higher than 1,0 — 1,5% of the general Hb. Unlike the carboxyhaemoglobin formed as a result of inclusion of carbon monoxide in structure of a molecule of hemoglobin, a methemoglobin differs from hemoglobin only in existence in gem of the oxidized trivalent Fe iron +++ instead of iron of bivalent Fe ++. At the nature there is a set of the connections capable to oxidize Fe ++ in Fe +++ in a hemoglobin molecule. In addition to external, also endogenous influences, and also inborn disturbances of mechanisms of regulation of level of a methemoglobin are known.

Methemoglobinemia symptoms:

Clinical manifestations of a methemoglobinemia rather clearly correlate with fraction of a methemoglobin in the blood measured by means of multiwave to-oksimetra. Modern to-oksimetry, being a part of analyzers of blood gases and acid-base balance, allow to carry out the analysis by method of an absorbing spektrofotometriya on the 128th length of waves with a step to 1,5 nanometers.
<3 Отсутствие клинических проявлений
3—15 Grayish shade of integuments
15—30 Cyanosis, chocolate-brown coloring of blood
30—50 Диспноэ, headache, weakness, dizziness, faints, indicator of a pulsoksimetr of SpO2 about 85%
50—70 A tachypnea, a metabolic acidosis, arrhythmias, spasms, oppression of TsNS up to a coma
> 70 Expressed clinics of a hypoxia, death

At assessment of clinical symptomatology of a methemoglobinemia it is necessary to consider considerable vulnerability of children of early age. Children up to 6 months rather often have diarrhea of various etiology. Also it is necessary to consider possible influence of such oxidizing agents as surface anesthetics or tooth gels, streptocides, naphthalene - the containing balls, etc. A number of authors noted absence of cyanosis at children with the expressed methemoglobinemia against the background of diarrhea. Other authors note specific intolerance of proteins of nutritious mixes as the main reason for clinically significant methemoglobinemia.

Methemoglobinemia reasons:

Inborn

    * HbM
    * Deficit of a methemoglobin reductase (cytochrome-b5-reductase)

Acquired (medicamentous influences)

    * Amyle nitrite
    * Novocaine
    * Lidocaine / прилокаин
    * Dapsone
    * Nitroglycerine
    * Sodium nitroprussidum
    * Acetphenetidiene
    * Fenazopiridin
    * Metoclopramidum
    * Sulfonamides
    * Quinones (chlorquinone, примаквин)
    * Nitrogen oxide
    * Other.

Acquired (chemical agents)

    * Aniline dyes
    * Butyl nitrite
    * Benzene chloride
    * Isobutyl nitrite
    * Naftalen
    * Nitrophenol
    * Silver nitrate
    * Trinitrotoluene
    * Foodstuff and drinking water with the high content of nitrates

Endogenous reasons (characteristic of newborns and children of the first year of life)

    * Reduced activity of a methemoglobin reductase (cytochrome-b5-reductase) in comparison with adults (the norm for adult 10 — 20 U/g, at children aged up to 4 months makes no more than 60%);
    * Diarrhea (intolerance of a number of proteins, virus and bacterial coloenteritis, etc.);
    * The states causing a metabolic acidosis;
    * Colonization of intestines the nitroforming bacteria.

The oxidizing substances are capable to cause a methemoglobinemia or direct oxidation of iron of hemoglobin, or owing to education of free radicals. In addition to influence of metgemoglobinobrazuyushchy medicines children of the first year of life are predisposed to development of a methemoglobinemia at reception of products and drinking water with the high content of nitrates. The indestinal flora transforming nitrates to nitrites also promotes increase in formation of a methemoglobin at children's age. Besides, only by 4 months of life of the child cytochrome-b5-reductase reaches the level of activity of an adult individual. It is also necessary to note that the fetalis hemoglobin characteristic of newborns, is exposed to oxidation in comparison with hemoglobin of adults easier.

Newborns are more often subject to diarrhea which can lead to development of a metabolic acidosis. It is known that in the conditions of a metabolic acidosis the fermental system of recovery of hemoglobin is capable to lose to 50% of the activity. The methemoglobinemia connected with diarrhea is caused by a combination of factors even in the absence of system acidosis. In this case transformation of nitrates into nitrites under the influence of gram-negative bacteria, and also idiopathic hyper sensitive reaction to the certain proteins which are contained in nutritious mixes plays a role.

Treatment of the Methemoglobinemia:

Treatment of a metgmoglobinemiya is based on recovery of the oxidized trivalent iron to bivalent. Method of the choice is intravenous administration of methylene blue 1 — 2 mg/kg in a dose within 3-5 minutes. Methylene blue is recommended for introduction to patients with FmetHb of 20% in the presence of clinical symptomatology, in the absence of symptoms — at the FmetHb level of 30%. Improvement usually occurs within 1 hour. In the absence of improvement repeated introduction of methylene blue 1 mg/kg in a dose is admissible. It is necessary to remember what methylene blue in itself is the oxidizing agent and can cause development of hemolitic anemia, especially at exceeding of its dose over 4 mg/kg or in patients with deficit glyukozo-6-fosfatdegidrogenazy. At the accompanying existence of a methemoglobinemia and deficit glyukozo-6-fosfatdegidrogenazy therapy by methylene blue can be inefficient as these patients have an insufficiency of the NADF-cofactor. Alternative method of treatment of these patients is the exchange hemotransfusion. Also introduction of N-Acetylcysteinum as predecessor of glutathione or glucose as cofactor of synthesis of NADF is applied. Useful also is an intravenous administration of such antioxidant as ascorbic acid in a dose of 1 — 2 g.

In conclusion it should be noted that active implementation in practice of work of intensive care units modern to-oksimetrov allows to find out the reason of development of hypoxemic states and to define disgemoglobinemiya degree, in particular methemoglobinemias, in many urgentny situations which were earlier remaining in the field of "terra incognita".