メチレンブルー
メチレンブルーは**チアジン系色素**であり、主に反芻動物において硝酸塩や塩素酸塩などの酸化剤によって引き起こされる**メトヘモグロビン血症**の**解毒剤**として使用されます。また、シアン化物中毒の補助的または代替療法として用いられることもあります。 小動物医療においては、犬の膵臓の**膵島細胞腫（インスリノーマ）**を優先的に染色するための術中静脈内染色剤として独自に使用され、外科的切除を補助します。 **臨床のポイント：** * メチレンブルーには逆説的な用量依存性効果があります。低用量ではメトヘモグロビン血症を治療する還元剤として働きますが、高用量では酸化剤として働き、逆にメトヘモグロビン血症を*引き起こす*可能性があります。 * 猫はこの薬物による酸化損傷に対して非常に敏感であるため、猫での使用は非常に議論の余地があり、一般的に禁忌とされています。
作用機序: 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**.
動物種別の用量
- 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.
投与経路
禁忌
- 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
有害事象
- 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
薬物相互作用
- 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
モニタリング
- 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
過量投与
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.
VetSheet の薬剤リファレンスは、獣医療従事者向けの臨床意思決定支援を目的としており、専門的判断やメーカーの最新添付文書に代わるものではありません。