氟烷
氟烷是一種經典的**鹵素類吸入性全身麻醉藥**，過去因其效力強、易於控制且不可燃，在獸醫學中曾是主要的麻醉劑。 雖然它為現代獸醫麻醉奠定了基礎，但目前在許多國家**已基本被淘汰且無商業供應**。它已被安全性更高（特別是在心血管抑制和肝毒性方面）的新型藥物（如**異氟醚 (isoflurane)**、**七氟醚 (sevoflurane)** 和 **地氟醚 (desflurane)**）所廣泛取代。 **臨床要點：** * 與現代吸入性麻醉劑相比，氟烷具有較高的血氣分配係數，導致誘導和甦醒時間較慢。 * 它會使心肌對兒茶酚胺敏感，顯著增加心室心律不整的風險。 * 約有12%的藥物經由肝臟代謝（異氟醚則小於1%），這增加了其引發免疫介導性肝毒性（氟烷性肝炎）的潛在風險。
作用機制: 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.
各物種劑量
- 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.
給藥途徑
禁忌症
- 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
不良反應
- 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)
藥物相互作用
- 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
監測
- 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
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.
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