Methylene Blue
Methylene blue is a **thiazine dye** primarily used as an **antidote** for treating **methemoglobinemia** in ruminants, typically secondary to oxidative agents like nitrates or chlorates. It is also occasionally employed as an adjunctive or alternative therapy for cyanide toxicity. In small animal medicine, it is uniquely utilized as an intra-operative intravenous stain to preferentially identify **islet-cell tumors** (insulinomas) of the pancreas in dogs, aiding in their surgical removal. **Clinical Pearls:** * Methylene blue has a paradoxical dose-dependent effect: at low doses, it acts as a reducing agent to treat methemoglobinemia, but at high doses, it acts as an oxidizing agent and can *cause* methemoglobinemia. * Cats are exquisitely sensitive to oxidative damage from this drug, making its use highly controversial and generally contraindicated in feline patients.
Mecanismo: Methylene blue acts as an artificial electron acceptor and donor in red blood cells. * **Reduction Pathway:** In the tissues, methylene blue interacts with the enzyme **NADPH-methemoglobin reductase**. It is rapidly reduced to **leucomethylene blue**. * **Hemoglobin Conversion:** Leucomethylene blue serves as a reducing agent (electron donor) → converts **methemoglobin (Fe3+)** back to functional oxygen-carrying **hemoglobin (Fe2+)**. * **Oxidation Pathway (High Doses):** Methylene blue is inherently an oxidizing agent. If administered at high doses (species dependent), it overwhelms the reduction pathway and directly oxidizes hemoglobin → **causes methemoglobinemia**.
Dosificación por especie
- For methemoglobin-producing toxins (nitrites, nitrates, chlorates) · 4-15 mg/kg (Using a 1% solution) · IV · q6h · As needed
- For cyanide toxicity · 4-6 grams per 454 kg (1000 lb.) of body weight · IV · Once · Once
- For methemoglobin-producing toxins (nitrites, nitrates, chlorates) · 4-15 mg/kg (Using a 1% solution) · IV · q6h · As needed
- For cyanide toxicity · 4-6 grams per 454 kg (1000 lb.) of body weight · IV · Once · Once
- Methemoglobinemia secondary to phenol exposure · 1.5 mg/kg · IV · Single, slow infusion · Once · May use with 20 mg/kg ascorbic acid PO.
- Methemoglobinemia · 1-1.5 mg/kg · IV · one time only · Once
- Severe methemoglobinemia · 1 mg/kg as a 1% solution · IV · given slowly over several minutes · May be repeated if necessary · Use cautiously (can cause Heinz body anemia). Measure hematocrit for 3 days.
- For methemoglobin-producing toxins (nitrites, nitrates, chlorates) · 4-15 mg/kg (Using a 1% solution) · IV · q6h · As needed
- For nitrate poisoning · 5-15 mg/kg as a 1% solution in physiologic saline · IV · Once · Repeat treatment at a lower dose may be required in severe cases · In animals that do not succumb, recovery occurs by 24 hours.
Vías de administración
Contraindicaciones
- Cats (considered contraindicated by most clinicians due to severe sensitivity)
- Renal insufficiency
- Hypersensitivity to methylene blue
- Intraspinal (intrathecal) injection
- Lactating dairy animals
- Severe renal impairment
- Patients with G6PD deficiency (can cause severe hemolysis)
- Use with extreme caution in cats due to high susceptibility to Heinz body formation
Efectos adversos
- Heinz body anemia
- Red cell morphological changes
- Methemoglobinemia (at high doses)
- Decreased red blood cell life span
- Necrotic abscesses (if injected SC or extravasated IV)
- Green-blue discoloration of urine
- Heinz body anemia (especially in cats)
- Paradoxical methemoglobinemia (at high doses)
- Tissue necrosis (if extravasated)
- Blue/green discoloration of urine, feces, and mucous membranes
- Nausea and vomiting
Interacciones farmacológicas
- SSRIs (e.g., Fluoxetine) · Methylene blue is a potent reversible monoamine oxidase inhibitor (MAOI); concurrent use can precipitate fatal serotonin syndrome. · major
- Tricyclic Antidepressants (TCAs) · Increased risk of serotonin toxicity. · major
Monitorización
- Methemoglobinemia levels
- Red cell morphology
- Red cell indices
- Hematocrit
- Hemoglobin
- Methemoglobin levels
- Complete Blood Count (CBC) for Heinz bodies and hemolysis
- Arterial blood gas (ABG)
- Renal function
- Signs of serotonin syndrome if on concurrent serotonergic drugs
Sobredosis
The LD50 for IV administered 3% methylene blue is approximately 43 mg/kg in sheep. High doses can paradoxically cause methemoglobinemia due to the drug's inherent oxidizing properties.
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