Methemoglobinemia
Background
- Methemoglobin refers to ferric form of Hb (Fe3+) that is unable to bind O2
- Suspect in any patient with cyanosis not responsive to supplemental O2
- Causes[1][2]
- Rarely congenital, NADH reductase deficiency - unable to reduce ferric to ferrous iron (Fe3+ --> Fe2+
- Exposure to oxidizing agent
- Benzocaine and other local anesthetics
- CN kit, nitrites, nitrates, well water (nitrates)
- Antimalarials
- Quinolones
- Dapsone (leprosy, PCP pna, toxoplasmosis, dermatitis herpetiformis, brown recluse bite)
- Aniline dyes
Clinical Features
- Depends on level of metHb
- Pts with anemia or preexisting cardiopulmonary disease will show symptoms sooner
- <20%: asymptomatic, gray-blue cyanosis
- 20-30%: anxiety, HA, weakness, light-headedness, tachycardia
- 50-60%: myocardial ischemia, dysrhythmias, depressed mental status, seizure
- >70%: usually fatal
Differential Diagnosis
Acute dyspnea
Emergent
- Pulmonary
- Airway obstruction
- Anaphylaxis
- Angioedema
- Aspiration
- Asthma
- Cor pulmonale
- Inhalation exposure
- Noncardiogenic pulmonary edema
- Pneumonia
- Pneumocystis Pneumonia (PCP)
- Pulmonary embolism
- Pulmonary hypertension
- Tension pneumothorax
- Idiopathic pulmonary fibrosis acute exacerbation
- Cystic fibrosis exacerbation
- Cardiac
- Other Associated with Normal/↑ Respiratory Effort
- Other Associated with ↓ Respiratory Effort
Non-Emergent
- ALS
- Ascites
- Uncorrected ASD
- Congenital heart disease
- COPD exacerbation
- Fever
- Hyperventilation
- Interstitial lung disease
- Neoplasm
- Obesity
- Panic attack
- Pleural effusion
- Polymyositis
- Porphyria
- Pregnancy
- Rib fracture
- Spontaneous pneumothorax
- Thyroid Disease
- URI
Diagnosis
- Co-oximetry
- Measures relative levels of oxyhemoglobin, carboxyhemoglobin, methemoglobin, reduced hemoglobin
- Usually performed on arterial blood sample, but fingertip CO-oximeter devices are available
- SpO2 will initially be falsely elevated
- Once MetHb >30% → SpO2 will fall to 80-85%
- "Chocolate brown" color of blood
Management
- Methylene Blue
- Treat if symptomatic OR asymptomatic w/ MetHb >25%
- 1-2mg/kg Methylene Blue IV over 5min; repeat dose if no effect
- Improvement seen within 20min
See Also
References
- ↑ Fernandez-Frackelton M, Bocock J: Cyanosis, in Marx JA, Hockberger RS, Walls RM, et al (eds): Rosen’s Emergency Medicine: Concepts and Clinical Practice, ed 7. St. Louis, Mosby, Inc., 2010, (Ch) 29:p 211-216.
- ↑ Mody, A., Silverman, B.: Problems in the Early Neonatal Period, In Fleisher and Ludwig's Textbook of Pediatric Emergency Medicine, Lippincott, WIlliams and Wilkins 2010, Page 995.