亞甲藍
亞甲藍是一種**噻嗪類染料**，主要作為**解毒劑**，用於治療反芻動物因硝酸鹽或氯酸鹽等氧化劑引起的**變性血紅素血症 (Methemoglobinemia)**。它偶爾也作為氰化物中毒的輔助或替代療法。 在小動物醫學中，它被獨特地用作術中靜脈染色劑，以優先標記狗胰臟的**胰島細胞瘤 (Insulinomas)**，從而輔助外科切除。 **臨床要點：** * 亞甲藍具有矛盾的劑量依賴性效應：在低劑量下，它作為還原劑治療變性血紅素血症；但在高劑量下，它作為氧化劑，反而會*導致*變性血紅素血症。 * 貓對此藥物的氧化損傷極度敏感，因此在貓科動物中使用具高度爭議性，通常被視為禁忌。
作用機制: 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**.
各物種劑量
- 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.
給藥途徑
禁忌症
- 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
不良反應
- 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
藥物相互作用
- 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
監測
- 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
過量
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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