Blood Disorders

Methemoglobinemia a disorder characterized by the presence of a higher than normal level of methemoglobin (metHb) in the bloodmethemoglobinblood

Methemoglobin is an oxidized form of hemoglobin that has a decreased affinity for oxygen, resulting in an increased affinity of oxygen to other heme sites and overall reduced ability to release oxygen to tissues hemoglobin

The oxygen– hemoglobin dissociation curve is therefore shifted to the left. When methemoglobin concentration is elevated in red blood cells, tissue hypoxia can occur.oxygen– hemoglobin dissociation curvered blood cellstissue hypoxia

Normal methemoglobin levels are <1% Elevated levels of methemoglobin in the blood are caused when the mechanisms that defend against oxidative stress within the red blood cell are overwhelmed and the oxygen carrying ferrous ion (Fe 2+ ) of the heme group of the hemoglobin molecule is oxidized to the ferric state (Fe 3+ ). oxidative stressferrous ion (Fe 2+ )hemeferric state (Fe 3+ ) This converts hemoglobin to methemoglobin, resulting in a reduced ability to release oxygen to tissues and thereby hypoxia. This can give the blood a bluish or chocolate-brown color.

2 types Congenital – Genetic defect present at birth (recessive gene) Aquired – Exposure to exogenous oxidizing drugs and their metabolites (such as benzocaine, dapsone and nitrates) may accelerate the rate of formation of methemoglobin up to one-thousandfold, overwhelming the protective enzyme systems and acutely increasing methemoglobin levels.benzocainedapsone nitrates – Infants under 6 months of age are particularly susceptible to methemoglobinemia caused by nitrates ingested in drinking water (called blue-baby syndromeblue-baby syndrome

Treatment Methemoglobinemia can be treated with supplemental oxygen and methylene blue1% solution 1 to 2 mg/kg administered intravenously slowly over five minutes followed by IV flush with normal saline. Methylene blue restores the iron in hemoglobin to its normal (reduced) oxygen- carrying state.methylene bluereduced