ブトルファノール
**ブトルファノール**は、複数の動物種にわたって獣医療で広く使用されている合成オピオイド部分アゴニストです。 * **鎮痛作用**: 軽度から中等度の内臓痛に対する鎮痛効果を提供します。「天井効果（シーリング効果）」があるため、高用量でも鎮痛効果は増強されず、重度の痛み（大がかりな整形外科手術など）には適していません。 * **鎮咳作用**: 犬の慢性的な非湿性咳嗽（伝染性気管気管支炎、気管虚脱など）の抑制に非常に効果的です。 * **拮抗薬として**: フェンタニル、メタドン、モルヒネなどの純粋なμ（ミュー）アゴニストによる重度な呼吸抑制や中枢神経抑制を特異的に拮抗（リバース）しつつ、κ（カッパ）受容体を介したある程度の鎮痛効果を維持することができます。 * **鎮静作用**: 相乗的な鎮静や化学的保定を目的として、α2アゴニスト（デクスメデトミジンやキシラジンなど）およびベンゾジアゼピン系薬物と頻繁に併用されます。 > **臨床のポイント**: ブトルファノールは小動物において優れた鎮咳薬および軽度の鎮静薬ですが、作用時間が非常に短い（犬での鎮痛作用は通常1時間未満）ため、主要な鎮痛薬としての有用性は限られています。対照的に、馬（疝痛など）においては非常に効果的で一般的に使用される内臓鎮痛薬です。
作用機序: Butorphanol exerts its effects by interacting with specific opioid receptors in the central nervous system: * **Kappa (κ) and Sigma (σ) Agonist**: Activation of κ-receptors in the limbic system and spinal cord provides visceral analgesia and sedation. * **Mu (μ) Antagonist**: Competitively binds to and blocks μ-receptors. This antagonism is responsible for its ability to reverse pure μ-agonist drugs and is the reason for its analgesic "ceiling effect." * **Antitussive Mechanism**: Directly suppresses the **medullary cough center**, elevating the threshold to stimuli (like CO2) without depressing respiratory center sensitivity as profoundly as pure agonists. * **Cellular Pathway**: Binds to κ-receptors → inhibits adenylate cyclase → decreases intracellular cAMP → closes voltage-gated calcium channels and opens potassium channels → hyperpolarization and reduced neuronal excitability.
動物種別の用量
- As an analgesic · 0.1-0.5 mg/kg IV, IM, SQ · IV/IM/SC · PRN · Provides only mild to moderate analgesia; duration of sedative action 2-4 hours, but analgesic action may be 1 hour or less.
- As a postoperative CRI · Loading dose of 0.1-0.2 mg/kg IV, then a CRI of 0.1-0.2 mg/kg/hr · IV · CRI · Usually in combination with ketamine (loading dose 0.1 mg/kg IV, CRI 0.4 mg/kg/hr).
- As an epidural analgesic · 0.25 mg/kg diluted with preservative-free saline (0.2 mL) or local anesthetic epidurally · Epidural · Once · 2-4 hours · Onset of action less than 30 minutes.
- As reversal agent for mu-agonist opiates · 0.05-0.1 mg/kg IV · IV · Once · Does not completely reverse analgesic effects.
- In combination as an immobilizing agent (short procedures) · butorphanol 0.2 mg/kg; medetomidine 0.001-0.015 mg/kg; midazolam 0.05-0.2 mg/kg · IM · Once · For difficult cats and short procedures.
- In combination as an immobilizing agent (more sedation) · butorphanol 0.2 mg/kg; medetomidine 0.015-0.02 mg/kg; midazolam 0.05-0.2 mg/kg; add ketamine 1-5 mg/kg when insufficient sedation · IM · Once
- As a sedative/analgesic · Butorphanol alone 0.05-0.1 mg/kg IM, SC. Butorphanol/Xylazine: Butorphanol 0.2 mg/kg + Xylazine 2 mg/kg IM · IM/SC · Once
- For injectable anesthesia · Butorphanol 0.1 mg/kg, Ketamine 5 mg/kg, medetomidine 80 micrograms/kg. Combine in one syringe and give IM. · IM · Once · May need to supplement with isoflurane (0.5-1.5%) for abdominal surgery.
投与経路
禁忌
- Known hypersensitivity to butorphanol
- Lower respiratory tract conditions with copious mucous production (suppressing cough prevents clearance)
- Caution in patients with head trauma, increased CSF pressure, or severe CNS dysfunction (e.g., coma)
- Caution in severe liver disease, renal insufficiency, hypothyroidism, or Addison's disease
- Caution in dogs with heartworm disease (safety not established)
有害事象
- Sedation and ataxia
- Anorexia or diarrhea (rare in small animals)
- Respiratory depression (mild compared to pure agonists)
- CNS excitement, head tossing, and increased ambulation (especially in horses at high doses or rapid IV administration)
- Decreased gastrointestinal motility and potential ileus (horses)
- Nystagmus, salivation, seizures, and hyperthermia (at massive overdoses in horses)
薬物相互作用
- Other CNS Depressants (anesthetics, antihistamines, phenothiazines, barbiturates, tranquilizers) · May cause increased CNS or respiratory depression; dosage may need to be decreased.
- Erythromycin · Could potentially decrease the metabolism of butorphanol, prolonging its effects.
- Fentanyl (and other pure opiate agonists) · Butorphanol may antagonize analgesic effects, but will also reverse sedative and respiratory depressant effects.
- Pancuronium · May cause increased conjunctival changes when used concurrently.
- Theophylline · Could potentially decrease the metabolism of butorphanol.
- Anaesthetic agents · Reduces the doses of other drugs required for induction and maintenance of anaesthesia · moderate
- Full mu-opioid agonists (e.g., methadone, fentanyl) · Addition of butorphanol will reduce analgesia produced from the full mu agonist; combination is not recommended for analgesia · major
- Acepromazine · Synergistic sedation · moderate
- Alpha-2 agonists · Synergistic sedation and analgesia · moderate
- Full mu-agonist opioids (e.g., methadone, fentanyl) · Butorphanol's mu-antagonist properties may block or partially reverse the analgesic effects of full mu-agonists. Higher doses of full mu-agonists may be required. · major
モニタリング
- Analgesic and/or antitussive efficacy
- Respiratory rate and depth
- Appetite and bowel function
- CNS effects (sedation vs. excitement)
過量投与
Acute life-threatening overdoses are unlikely (LD50 in dogs is 50 mg/kg). However, because veterinary injection comes in two highly different strengths (0.5 mg/mL and 10 mg/mL), inadvertent overdoses can occur in small animals. **Clinical Signs**: CNS effects (profound sedation or excitement), cardiovascular changes, and respiratory depression. **Treatment**: * Administer **intravenous naloxone** immediately to reverse opioid effects. * Provide supportive measures: IV fluids, oxygen therapy, vasopressors, and mechanical ventilation if required. * If seizures occur and persist, use **diazepam** for control.
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