甲苯噻嗪
甲苯噻嗪（Xylazine）是一種強效的**Alpha-2 腎上腺素受體激動劑**，在獸醫學中廣泛用作鎮靜劑、鎮痛藥和肌肉鬆弛劑。它也常被用作貓的可靠**催吐劑**。 > **臨床要點：** 不同物種對甲苯噻嗪的敏感性差異極大。**反芻動物對其極度敏感**，僅需馬匹劑量的約十分之一即可達到相同效果。相反，豬具有高度抗性，需要反芻動物劑量的20-30倍。 雖然它能提供極佳的內臟鎮痛效果（特別是在馬匹中），但由於其顯著的心血管抑制作用（包括心跳過緩、心律不整和心輸出量減少），在小動物中的使用通常保留給年輕、健康的病患。它未獲 FDA 批准用於食用動物，儘管有時會在嚴格的停藥期規範下進行標籤外使用。
作用機制: 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).
各物種劑量
- 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.
給藥途徑
禁忌症
- 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
不良反應
- 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)
藥物相互作用
- 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
監測
- 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
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.
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