Download Blood Disorders

Blood Disorders
Methemoglobinemia
• a disorder characterized
by the presence of a
higher than normal
level of methemoglobin
(metHb) in the blood
• 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
• The oxygen–
hemoglobin
dissociation curve is
therefore shifted to the
left. When
methemoglobin
concentration is
elevated in red blood
cells, tissue hypoxia can
occur.
• 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
(Fe2+) of the heme group of the hemoglobin molecule
is oxidized to the ferric state (Fe3+).
• 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.
– Infants under 6 months of age are particularly
susceptible to methemoglobinemia caused by nitrates
ingested in drinking water (called blue-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) oxygencarrying state.