ハロタン
ハロタンは古典的な**ハロゲン化吸入全身麻酔薬**であり、その強力な作用、制御のしやすさ、および不燃性から、かつては獣医療における主要な麻酔薬でした。 現代の獣医麻酔の基礎を築きましたが、現在では多くの国で**ほぼ時代遅れとなり、市販されていません**。心血管抑制や肝毒性の観点からより安全性の高い新しい薬剤（**イソフルラン**、**セボフルラン**、**デスフルラン**など）に広く取って代わられています。 **臨床のポイント:** * 現代の吸入麻酔薬と比較して血液ガス分配係数が高いため、導入および覚醒に時間がかかります。 * 心筋をカテコールアミンに対して感作させるという特有の性質があり、心室性不整脈のリスクを著しく高めます。 * 薬物の約12%が肝臓で代謝されるため（イソフルランは1%未満）、免疫介在性肝毒性（ハロタン肝炎）を引き起こすリスクがあります。
作用機序: The exact mechanism of inhalant anesthetics remains incompletely understood, but involves multiple central nervous system targets: * **Lipid Matrix Interaction:** Historically explained by the Meyer-Overton rule, halothane interacts with the lipid bilayer of neuronal membranes, disrupting normal ion channel function. * **Receptor Modulation:** Modern evidence suggests halothane acts via **positive allosteric modulation of inhibitory receptors** (such as **GABA_A** and **glycine** receptors) → enhancing CNS depression. * **Excitatory Inhibition:** It simultaneously inhibits excitatory receptors (such as **NMDA** and **nicotinic acetylcholine** receptors) → dampening neuronal excitability. **Systemic Effects:** * Profound CNS depression and blunting of thermoregulatory centers. * Increased cerebral blood flow. * Dose-dependent respiratory depression (especially pronounced in ruminants). * Significant myocardial depression, vasodilation, and hypotension.
動物種別の用量
- General Anesthesia · 3% (induction); 0.5-1.5% (maintenance) · Inhalation · Continuous · As needed for procedure · Concentrations are dependent upon fresh gas flow rate.
- General Anesthesia · 0.5-3.5%, inhaled · Inhalation · Continuous · As needed for procedure
- Induction of anaesthesia (unpremedicated) · 3-4% concentration · Inhalational · Continuous · Until induced · Delivered via calibrated vaporizer
- Maintenance of anaesthesia · 0.8-1.0% (MAC) · Inhalational · Continuous · Duration of procedure · Adjust according to individual requirement and concurrent drugs
- General Anesthesia (Draft Horses) · 4%-5% initially, maintenance 2.5%-3% · Inhalation · Continuous · As needed for procedure · Administered in an oxygen-enriched semi-closed large animal circle system. Reduce as indicated by physical monitoring.
- General Anesthesia · 2% (small birds) or 2.5%-3% (large birds) for induction; 0.5%-1.5% for maintenance · Inhalation · Continuous · As needed for procedure · Induction usually occurs in 2-4 minutes. Recovery takes 3-5 minutes. Bradycardia, hypotension, and hypothermia may occur but rapidly resolve upon discontinuation.
- General Anesthesia (Mice, Rats, Gerbils, Hamsters, Guinea pigs, Chinchillas) · Induction: 2%-4%, maintenance: 0.25%-2% · Inhalation · Continuous · As needed for procedure · Using a non-rebreathing system.
投与経路
禁忌
- History or predilection towards malignant hyperthermia
- Significant hepatotoxicity or unexplained fever/jaundice after previous halothane exposure
- Liver disease
- Patients with a history of malignant hyperthermia
有害事象
- Dose-related hypotension
- Cardiac depression and decreased cardiac output
- Cardiac dysrhythmias (sensitizes myocardium to catecholamines)
- Malignant hyperthermia-stress syndrome (reported in pigs, horses, dogs, cats)
- Hepatotoxicity (jaundice, fatal liver necrosis)
- Respiratory depression
- Hypothermia
- Dose-dependent hypotension
- Bradycardia
- Decreased myocardial contractility
- Reduced hepatic blood flow
- Ventricular arrhythmias
- Neonatal depression (crosses placental barrier)
薬物相互作用
- Acetaminophen · Not recommended for post-operative analgesia in patients that have received halothane anesthesia due to increased risk of hepatotoxicity.
- Acepromazine · Can decrease requirements of halothane by up to 40%.
- Aminoglycosides · Use with caution; additive neuromuscular blockade may occur.
- Lincosamides · Use with caution; additive neuromuscular blockade may occur.
- Non-depolarizing neuromuscular blocking agents · Additive neuromuscular blockade may occur.
- Succinylcholine · May induce increased incidences of cardiac effects (bradycardia, arrhythmias, sinus arrest, apnea) and malignant hyperthermia in susceptible patients.
- Sympathomimetics (dopamine, epinephrine, norepinephrine, ephedrine, metaraminol) · Halothane sensitizes the myocardium to catecholamines; severe ventricular arrhythmias may result. Use with extreme caution at reduced doses.
- d-Tubocurarine · May cause significant hypotension if used with halothane.
- Sedatives · Reduces the concentration of halothane required to achieve surgical anaesthesia · moderate
- Opioid agonists · Reduces the concentration of halothane required to achieve surgical anaesthesia · moderate
- Nitrous oxide (N2O) · Reduces the concentration of halothane required to achieve surgical anaesthesia · moderate
モニタリング
- Respiratory and ventilatory status
- Cardiac rate and rhythm (ECG is highly recommended due to arrhythmogenic potential)
- Blood pressure (especially in 'at risk' patients)
- Level of anesthetic depth
- Body temperature (monitor for hypothermia or malignant hyperthermia)
- Heart rate and rhythm (ECG)
- Blood pressure
- Respiratory rate and depth
- Capnography (ETCO2)
- Anaesthetic depth (jaw tone, palpebral reflex)
- Body temperature
過量投与
Overdosage of halothane results in profound **cardiopulmonary depression**, characterized by severe hypotension, bradycardia, arrhythmias, and apnea. **Treatment:** 1. Immediately turn off the anesthetic vaporizer. 2. Flush the breathing circuit with 100% oxygen. 3. Institute positive pressure ventilation. 4. Provide cardiovascular support (e.g., IV fluid volume expansion, positive inotropes like dobutamine, and anticholinergics for bradycardia). 5. Lidocaine may be required to treat halothane-induced ventricular dysrhythmias.
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。