Methemoglobinemia Notes

NOTES NOTES METHEMOGLOBINEMIA METHEMOGLOBINEMIA osms.it/methemoglobinemia PATHOLOGY & CAUSES ▪ ↑ methemoglobin levels in blood > 10% ▫ Normal methemoglobin level < 1% ▪ Methemoglobin ▫ Hemoglobin form: contains ferric (Fe3+) iron → ↓ oxygen affinity ▫ Overwhelming oxidative stress (in red blood cells) → iron oxidation within heme (Fe2+ → Fe3+) → hemoglobin converted to methemoglobin ▫ Oxygen binding → ↑ affinity in remaining heme sites that may have been in ferrous (Fe2+) state in heme tetramer → left shift in oxygen curve → overall oxygen to tissue release ability ↓ → tissue hypoxia ▪ Healthy cells ▫ Spontaneous methemoglobin conversion rapidly ↓ by protective enzymes ▫ Nicotinamide adenine dinucleotidereduced (NADH) methemoglobin reductase (diaphorase I) ▫ Nicotinamide adenine dinucleotide phosphate (NADPH) methemoglobin reductase ▫ Ascorbic acid ▫ Glutathione enzyme system TYPES ▪ Industrial chemicals ▫ Aniline dyes, chlorates, bromates, nitrates Congenital methemoglobinemia ▪ Recessive gene → enzyme diaphorase I deficiency → inefficient methemoglobin ↓ → accumulation ▪ Abnormal hemoglobin variants (HbM/HbH) → hemoglobin not enzymatic-reduction amenable ▪ Diaphorase I cofactor deficiency ▫ Impaired upstream enzymes → insufficient diaphorase I cofactors production → impaired enzymatic reduction ▫ Pyruvate kinase deficiency ▫ Glucose-6-phosphate dehydrogenase deficiency SIGNS & SYMPTOMS ▪ Arterial blood may develop bluish/ chocolate-brown tinge ▪ Healthy individuals ▫ Symptoms may only manifest at methemoglobin levels > 15% ▫ Cyanosis → dyspnea, mental status change, headache, fatigue, exercise intolerance, dizziness ▪ Severe methemoglobinemia (> 50%) → seizure, coma, death Acquired methemoglobinemia ▪ ↑ methemoglobin formation ▪ Drugs ▫ Antibiotics, local anesthetics, antiemetics OSMOSIS.ORG 413

DIAGNOSIS DIAGNOSTIC IMAGING TREATMENT MEDICATIONS Co-oximetry ▪ Analyses blood-spectrum absorption → methemoglobin peak absorbance at 631nm ▪ Specific ▫ Methylene blue: intravenous reducing agent → reduces heme group from methemoglobin → hemoglobin LAB RESULTS OTHER INTERVENTIONS ▪ Arterial blood gas ▫ Clinical cyanosis in normal arterial PO2 presence ▪ Enzyme assays ▫ Enzyme assays for specific enzymes involved in maintaining hemoglobin in reduced state may be utilized 414 OSMOSIS.ORG ▪ Supplemental oxygen ▪ Vitamin C (ascorbic acid) may improve cyanosis in chronic cases