キシラジン
キシラジンは強力な**α2アドレナリン受容体作動薬**であり、獣医学において鎮静、鎮痛、筋弛緩薬として広く使用されています。また、猫における確実な**催吐薬**（嘔吐を誘発する薬）としても頻繁に利用されます。 > **臨床のポイント:** キシラジンに対する感受性には著しい種差があります。**反芻動物は極めて感受性が高く**、馬と同じ効果を得るために必要な用量は約10分の1です。逆に、豚は耐性が高く、反芻動物の20〜30倍の用量を必要とします。 優れた内臓鎮痛効果（特に馬において）を提供しますが、徐脈、不整脈、心拍出量の減少など、心血管系に対する顕著な抑制作用があるため、小動物での使用は通常、若くて健康な患者に限られます。食用動物への使用はFDAによって承認されていませんが、厳格な休薬期間を設けた上で適応外使用されることがあります。
作用機序: Xylazine stimulates presynaptic and postsynaptic **alpha-2 adrenergic receptors** in the central and peripheral nervous systems. * **Sedation & Analgesia:** Activation of central alpha-2 receptors → decreased release of **norepinephrine** and dopamine → profound CNS depression and analgesia. * **Muscle Relaxation:** Centrally mediated inhibition of intraneuronal transmission of impulses in the CNS. * **Cardiovascular Effects (Biphasic):** Initial stimulation of peripheral alpha-1 and alpha-2 receptors → transient vasoconstriction and **hypertension** → subsequent central sympatholytic effect → **hypotension**, **bradycardia**, and potentially second-degree AV block. Cardiac output can decrease by up to 30%. * **Emesis:** Stimulates the chemoreceptor trigger zone (CRTZ), particularly in cats. * **Endocrine/Renal:** Decreases insulin release → transient **hyperglycemia**. Decreases ADH production → **polyuria** (especially in ruminants).
動物種別の用量
- Label dosage (Note: Many believe this is too high) · 1.1 mg/kg IV, 1.1-2.2 mg/kg IM or SC · IV, IM, SC · Once
- Preanesthetic · 0.2-1 mg/kg IV, IM or SC · IV, IM, SC · Once
- Emetic · 0.44 mg/kg IM · IM · Once
- Analgesic · 0.1-1 mg/kg IV, IM or SC · IV, IM, SC · Once
- Post-operative anxiety · 0.1-0.5 mg/kg IV, IM or SC · IV, IM, SC · Once
- Emesis · 0.6 mg/kg (IM) or 1 mg/kg (SC) · IM/SC · once · Effective in >75% of cats. Emesis generally occurs rapidly within 10 min. Further doses depress the vomiting centre and may not result in further vomiting.
- Growth hormone suppression test · 100 μg/kg · IV · once · Consult specialist texts for protocol.
- Sedative/analgesic · 0.5-2 mg/kg IM or SC · IM, SC · Once · Usually combined with atropine (0.05 mg/kg) or glycopyrrolate (0.01 mg/kg IM).
- Sedative/analgesic combination · Xylazine (2 mg/kg) plus butorphanol (0.2 mg/kg) IM · IM · Once
- Anesthesia combination · Telazol (1.5 mg/kg) plus xylazine (1.5 mg/kg) IM · IM · Once · May reverse xylazine with yohimbine (0.05 mg/kg IM). Can also add butorphanol 0.2 mg/kg.
投与経路
禁忌
- Animals receiving epinephrine
- Active ventricular arrhythmias
- Preexisting cardiac dysfunction
- Hypotension or shock
- Respiratory dysfunction
- Severe hepatic or renal insufficiency
- Preexisting seizure disorders
- Severely debilitated animals
- Last trimester of pregnancy (particularly in cattle)
- Dehydrated or debilitated ruminants
- Urinary tract obstruction (ruminants)
- Intra-arterial injection (may cause severe seizures and collapse)
- Cardiovascular disease
- Systemic disease affecting cardiovascular performance
- Geriatric patients
- Pregnant animals
有害事象
- Emesis (especially in cats, occasionally dogs)
- Muscle tremors
- Bradycardia with partial A-V block
- Reduced respiratory rate and tidal volume
- Sudden movement or kicking in response to sharp auditory stimuli (especially horses)
- Increased urination (cats, cattle)
- Bloat from aerophagia (dogs, cattle)
- Sweating (horses)
- Salivation, ruminal atony, regurgitation, and diarrhea (cattle)
- Premature parturition (cattle)
- Transient hypertension followed by hypotension
- Decreased mucociliary clearance (horses)
- Cardiovascular depression (bradycardia, arrhythmias)
- Vomiting (especially in cats)
- Diuresis (suppression of ADH secretion)
薬物相互作用
- Acepromazine · Potential for additive hypotensive effects; use cautiously in animals susceptible to hemodynamic complications.
- Chloramphenicol · Prolonged sedation and gastrointestinal stasis possible.
- CNS Depressants (barbiturates, narcotics, anesthetics, phenothiazines) · Additive CNS depression. Dosages of these agents may need to be reduced.
- Epinephrine · May induce the development of ventricular arrhythmias (with or without concurrent halothane).
- Reserpine · May cause colic-like clinical signs in horses; concurrent use should be avoided.
- Anaesthetic agents · Significantly reduces the dose of all other anaesthetic agents required to maintain anaesthesia. · major
- Sympathomimetic amines · Increased risk of severe cardiac arrhythmias. · major
モニタリング
- Level of anesthesia/analgesia
- Respiratory function (rate, depth, tidal volume)
- Cardiovascular status (heart rate, rhythm, blood pressure)
- Hydration status (especially if polyuria is present)
- Heart rate and rhythm (ECG)
- Blood pressure
- Respiratory rate and depth
- Blood glucose (in susceptible patients)
過量投与
In the event of an accidental overdosage, profound **CNS and respiratory depression**, **hypotension**, and **cardiac arrhythmias** may occur. Seizures have also been reported. * **Reversal Agents:** Alpha-2 antagonists such as **yohimbine**, **atipamezole**, and **tolazoline** can be used alone or in combination to reverse the effects of xylazine and speed recovery times. * **Supportive Care:** Mechanical respiratory support and respiratory stimulants (e.g., doxapram) may be required to treat severe respiratory depressant effects.
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。