1. Good Morning and Welcome Applicants
February 3, 2010

9. Methemoglobinemia Normal Fe+2 to Fe3+ Rare cause of cyanosis Oxidation
Fe3+ is unable to bind oxygen
Remaining Fe has higher O2 affinity
Impaired O2 delivery to tissues
Rare cause of cyanosis

10. Methemoglobinemia Hereditary NADH cytochrome b5 reductase deficiency
Hemoglobin M disease AD
Structural alteration of alpha or beta globin chains
NADH – physiologic pathway to reduce methemoglobin back to hemoglobin, patients that are heterozygous can develop symptoms as well
M disease – forms metHb that resists reduction, will not be cured by methylene blue

11. Methemoglobinemia Acquired
Exogenous agents that increase methemoglobin formation
Topical anesthetic agents
Benzocaine
Acute diarrhea and metabolic acidosis
Babies 2-6 months
Abnormal absorption of nitrites

13. Methemoglobinemia Presentation Cyanosis (8-12%) 30-50% Dyspnea
Dizziness
Tachycardia
Fatigue HA
Cyanosis – different absorbance spectrum of methemoglobin compared with oxyhemoglobin

14. Methemoglobinemia Presentation Congenital 50-70% >70% Asymptomatic
Severe lethargy
Stupor
>70%
Death
Congenital
Asymptomatic

15. Methemoglobinemia Suspicion Clinical cyanosis Normal arterial pO2
“chocolate brown blood”
Pulse oximetry unreliable
Calculated saturation on the gas may be significantly different from the pulse oximetry

16. Methemoglobinemia Diagnosis Co-oximetry Genetic testing
Respiratory therapy
False positives with methylene blue with some methods
Genetic testing

17. Methemoglobinemia Treatment - Acquired
Recognition and discontinuation of offending agents
Symptomatic or >20%
Methylene blue 1-2mg/kg
May repeat 1 hr later (max dose 7mg/kg)
Reducing agents
Ascorbid acid
Glutathione
Riboflavin
Treatment for acquired

18. The major pathway for methemoglobin reduction is via cytochrome b5 reductase (thick arrows). An alternative pathway, which requires an exogenous electron acceptor such as methylene blue, is via NADPH methemoglobin reductase. Only a small amount of methemoglobin is reduced via nonenzymatic pathways (dashed arrow).

19. Methemoglobinemia Treatment G6PD deficiency Other
May not respond to treatment
Other
Hyperbaric oxygen
Exchange Transfusion
G6PD – G6PD def patients cannot produce NADPH which is needed to reduce methemoglobin, needs methylene blue or riboflavin as cofactors to react. Also MB may induce hemolysis in these patients.

20. Methemoglobinemia Lessons
Always consider in differential diagnosis of cyanosis
Caution with use of topical anesthetics
History
Cyanosis
Blue nail beds
Blue lips
Family History