リドカイン
リドカインは、多用途の**アミド系局所麻酔薬**および**クラスIB抗不整脈薬**です。獣医療においては、局所・区域麻酔だけでなく、心室性不整脈、神経障害性疼痛、消化管運動障害の管理のために全身投与としても広く利用されています。 主な臨床応用： * **抗不整脈治療：** 犬の急性で生命を脅かす心室頻拍（VT）および心室期外収縮（VPC）に対する第一選択薬です。 * **全身性鎮痛：** 定速点滴（CRI）として投与され、オピオイドやケタミン（例：**MLK CRI**）と併用されることが多く、強力な補助鎮痛を提供し、吸入麻酔薬の最小肺胞濃度（MAC）を低下させ、神経障害性疼痛や痛覚過敏を軽減します。 * **消化管運動促進および抗炎症作用：** 馬では、術後イレウスの治療や予防、および活性酸素種（ROS）の消去による虚血再灌流障害の軽減のために全身性リドカインが頻繁に使用されます。 > **臨床上のポイント：** 猫はリドカインの心機能抑制および中枢神経系（CNS）の毒性作用に対して非常に敏感です。猫への全身投与には議論があり、極めて慎重な取り扱いと正確な用量設定が必要です。
作用機序: Lidocaine exerts its effects through multiple mechanisms depending on the target tissue: * **Antiarrhythmic Action (Class IB):** Lidocaine binds to and blocks **fast voltage-gated sodium channels (Nav1.5)** in the myocardium. It exhibits *use-dependent* and *state-dependent* blockade, meaning it preferentially binds to sodium channels in their **inactive state** (which occurs during depolarization). This action → attenuates phase 4 diastolic depolarization → decreases automaticity → suppresses ectopic ventricular pacemakers without significantly affecting the SA or AV nodes at therapeutic levels. * **Local Anesthetic/Analgesic Action:** Blocks **neuronal voltage-gated sodium channels**, preventing the influx of sodium required for the initiation and conduction of action potentials in peripheral nerves. Systemically, it reduces ectopic firing from damaged afferent neurons, providing relief from neuropathic pain. * **Prokinetic & Cytoprotective Action:** The exact mechanism for enhancing intestinal motility (especially in equine ileus) is multifactorial, likely involving suppression of sympathetic inhibitory reflexes, direct anti-inflammatory effects, and acting as a **scavenger of reactive oxygen species (ROS)** to prevent lipid peroxidation and reperfusion injury.
動物種別の用量
- Antiarrhythmic · Initially, IV bolus of 0.25-0.5 mg/kg given slowly; can repeat at 0.15-0.25 mg/kg in 5-20 minutes; if effective, 10-20 micro-grams/kg/minute (0.01-0.02 mg/kg/min) as a constant rate IV infusion · IV · Bolus then CRI · Caution: Cats are reportedly very sensitive to the CNS effects.
- Antiarrhythmic · 0.25-0.5 mg/kg slow IV, with the possibility of repeating up to twice more if needed. · IV · PRN · If diluting for accurate dosing, use an insulin/tuberculin syringe.
- Epidural · 4-5 mg/kg epidurally. · Epidural · Single dose · Duration 1.5 hours · Onset <10 minutes.
- Ventricular tachyarrhythmias · Initially IV bolus of 1-1.5 mg/kg. To maintain effect, a constant IV infusion will be required. · IV · Bolus then CRI · Will generally distinguish between ventricular tachyarrhythmias (effective) and supraventricular tachyarrhythmias (no effect).
- Ventricular tachyarrhythmias · 0.25-0.5 mg/kg IV (slowly) every 5-10 minutes up to a total dose of 1.5 mg/kg · IV · q5-10m
- Postoperative ileus · Initially, IV bolus of 1.3 mg/kg followed by a IV infusion of 0.05 mg/kg/minute for 24 hours · IV · Bolus then CRI · 24 hours
- Colic patients · Initial IV bolus at 1.4 mg/kg, then as a CRI at 0.03-0.05 mg/kg/min (1.8-3 mg/kg/hr). · IV · Bolus then CRI · Lidocaine has anti-endotoxic, analgesic and anti-ileus properties.
投与経路
禁忌
- Known hypersensitivity to amide-class local anesthetics
- Severe SA, AV, or intraventricular heart block (unless artificially paced)
- Adams-Stokes syndrome
- Intravenous use of lidocaine products containing epinephrine
- Continuous rate infusion (CRI) in cats during the perioperative period (due to negative haemodynamic effects)
- Intravenous administration of lidocaine solutions containing adrenaline
- Use of adrenaline-containing solutions for complete ring block of an extremity (danger of ischaemic necrosis)
有害事象
- CNS toxicity (dose-related): drowsiness, depression, ataxia, nystagmus, muscle tremors, seizures
- Gastrointestinal: nausea and vomiting (usually transient)
- Cardiovascular: hypotension (especially with rapid IV bolus), bradycardia, PR and QRS interval prolongation, circulatory collapse at toxic doses
- Cats: heightened risk of severe cardiodepression and CNS signs
- Depression
- Seizures
- Muscle fasciculations
- Vomiting
- Bradycardia
- Hypotension
- Laryngeal oedema (in cats, associated with CFC propellants in unlicensed aerosols)
薬物相互作用
- Gas Anesthetics (Isoflurane, Sevoflurane) · Lidocaine infusions reduce MAC requirements. Additive cardiodepression may occur, especially in cats.
- Other Antiarrhythmics (Procainamide, Quinidine, Propranolol) · May cause additive or antagonistic cardiac effects; enhanced risk of toxicity.
- Cimetidine · May decrease lidocaine clearance, increasing lidocaine levels and effects. · moderate
- Furosemide · Diuretic-induced hypokalemia may reduce the antiarrhythmic efficacy of lidocaine.
- Phenobarbital / Phenytoin · May induce hepatic enzymes, increasing lidocaine metabolism and decreasing its serum levels.
- Propranolol · May decrease hepatic blood flow and lidocaine clearance, increasing lidocaine levels. · moderate
- Succinylcholine · Large doses of lidocaine may prolong succinylcholine-induced apnea.
- Other antiarrhythmics · May cause increased myocardial depression · major
モニタリング
- Continuous ECG monitoring (especially during IV bolus or CRI)
- Signs of CNS toxicity (ataxia, tremors, seizures)
- Blood pressure (monitor for hypotension)
- Serum lidocaine levels if available (Therapeutic range: 1-6 micrograms/mL)
- ECG (especially during IV therapy for arrhythmias)
- Blood pressure
- Heart rate
- CNS signs (monitor for fasciculations or seizures)
過量投与
In dogs, toxicity may result if serum levels exceed **8 micrograms/mL**. * **Clinical Signs:** Ataxia, nystagmus, depression, seizures, bradycardia, hypotension, and at very high levels, circulatory collapse. * **Management:** Because lidocaine is rapidly metabolized, simply stopping the infusion or reducing the rate (with close monitoring) is often sufficient for minor signs. * **Seizure Control:** Treat seizures or excitement with **diazepam** or a short/ultrashort-acting barbiturate. > **Warning:** Longer-acting barbiturates (e.g., pentobarbital) should be avoided. * **Cardiovascular Support:** Treat circulatory depression with IV fluids, pressor agents, and initiate CPR if necessary.
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