Methemoglobinemia 

Background:

• Methemoglobin refers to ferric form of hemoglobin (Fe3+) that is unable to bind O2
• Suspect in any patient with cyanosis not responsive to supplemental O2

Causes:

• Rarely congenital, NADH reductase deficiency – unable to reduce ferric to ferrous iron (Fe³⁺ → Fe²⁺)
• Exposure to oxidizing agent
  • Benzocaine and other local/topical anesthetics
  • Nitrites, nitrates, well water (nitrates)
  • Antimalarials
  • Quinolones
  • Dapsone
  • Aniline dyes
  • Phenazopyridine
  • Sodium nitrite toxicity

Evaluation:

• Oximetry is inaccurate because methemoglobin (MetHb) absorbs light at both 660 nm (red) and 940 nm (infrared) wavelengths approximately equally. This causes the pulse oximeter to fail to distinguish it from oxygenated hemoglobin, resulting in a characteristic, fixed, and falsely low SpO2 reading of around 85%,
• Lack of improvement in oxygen saturation with high-flow oxygen plus discrepancy in oxygen saturation >5% between measurements by ABG vs pulse oximetry (“saturation gap”) 
• Diagnosis confirmed by measuring level of methemoglobin in blood
• Normal PaO2
• SpO2 will initially be falsely elevated
  • Once MetHb >30% → SpO2 will fall to 80-85%
  • Mid-80s SpO2 due to light absorption of both oxyHb and deoxyHb
• “Chocolate brown” color of blood

Management:

• High-Flow Oxygen
• Methylene Blue
• Treat if symptomatic OR asymptomatic with MetHb >25%
  • 1-2mg/kg Methylene blue IV over 5min; repeat dose if no effectImprovement seen within 20min
  • Contraindicated in G6Pd deficiency (may cause hemolytic anemia)
• Other treatment modalities
  • Exchange transfusion for symptomatic methemoglobinemia in patient with G6PD deficiency
  • Hyperbaric O2 when methylene blue ineffective or contraindicated
  • IV hydration and bicarbonate for metabolic acidosis

Dispo:

• Admit, likely ICU consider Floor if stable

Blood Transfusion Complications 

1. Acute Hemolytic Transfusion Reaction (AHTR)

Cause: ABO incompatibility (most dangerous; clerical error)

Pathophysiology:

• IgM-mediated → complement activation → intravascular hemolysis 

Presentation (minutes):

• Fever, chills 
• Flank/back pain 
• Hypotension 
• Hemoglobinuria → dark urine 
• DIC possible 

Labs:

• ↑ LDH, ↑ bilirubin 
• ↓ haptoglobin 
• Positive direct Coombs 

Management:

• STOP transfusion immediately 
• Aggressive IV fluids 
• Maintain urine output 
• Treat DIC if present 

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 2. Febrile Non-Hemolytic Transfusion Reaction (FNHTR)

Cause: Cytokines or recipient antibodies to donor leukocytes

Presentation:

• Fever ± chills 
• No hemolysis 

Management:

• Stop transfusion (initially to rule out AHTR) 
• Antipyretics 
• Prevention: leukoreduced blood 

 3. Allergic Reactions

Mild:

• Urticaria, pruritus 

Management:

• Antihistamines → may restart transfusion 

Severe (Anaphylaxis):

Cause: IgA deficiency with anti-IgA antibodies

Presentation:

• Hypotension 
• Bronchospasm 
• Angioedema 

Management:

• Epinephrine 
• Avoid future IgA-containing products 

 4. Transfusion-Related Acute Lung Injury (TRALI)

Cause: Donor anti-leukocyte antibodies → neutrophil activation

Timing:

• Within 6 hours 

Presentation:

• Acute hypoxia 
• Noncardiogenic pulmonary edema 
• Bilateral infiltrates 

Key distinction:

• Normal BNP 
• No volume overload 

Management:

• Supportive (oxygen, ventilatory support) 

5. Transfusion-Associated Circulatory Overload (TACO)

Cause: Volume overload

Presentation:

• Dyspnea 
• Hypertension 
• Pulmonary edema 

Key distinction from TRALI:

• Elevated BNP 
• Signs of fluid overload 

Management:

• Diuretics 
• Slow transfusion rate 

 6. Delayed Hemolytic Transfusion Reaction

Cause: IgG antibodies (e.g., Kidd, Duffy)

Timing:

• Days to weeks 

Presentation:

• Mild fever 
• Anemia 
• Jaundice 

Management:

• Usually supportive 

7. Transfusion-Associated Graft-versus-Host Disease (TA-GVHD)

Cause: Donor T cells attack recipient

Risk:

• Immunocompromised 

Presentation (days–weeks):

• Fever 
• Rash 
• Diarrhea 
• Pancytopenia 

Prevention:

• Irradiated blood products 

Prognosis:

• Often fatal 

 8. Infectious Complications

Examples:

• Bacterial (platelets highest risk) 
• Viral: HIV, Hepatitis B/C (rare now) 

Presentation:

• Fever, sepsis 

9. Metabolic Complications

Hypocalcemia

• Citrate binds calcium 
• → Tetany, arrhythmias 

Hyperkalemia

• Stored blood leaks K⁺ 

Hypothermia

• Large-volume transfusion