美西律
美西律（Mexiletine）是一種口服的 **IB類抗心律不整藥物**，其電生理特性與利多卡因（Lidocaine）非常相似。由於利多卡因具有顯著的首過代謝效應且無法口服，美西律常被視為用於長期控制心律不整的「口服版利多卡因」。 在獸醫學中，它主要用於治療犬隻的**心室性心律不整**（如心室提早收縮 PVCs 和心室性心搏過速）。它特別常用於患有致心律不整性右心室心肌病（ARVC）的拳師犬，以及患有擴張性心肌病（DCM）的杜賓犬。臨床上常與乙型交感神經阻斷劑（如 Atenolol）或 Sotalol 併用以達到協同效果。 此外，美西律也可用於治療某些罕見的遺傳性肌肉疾病，包括**先天性肌強直**（常見於鬆獅犬和迷你雪納瑞）及**肌纖維顫搐**（常見於傑克羅素㹴）。
作用機制: Mexiletine is a **Class IB antiarrhythmic**. * **Mechanism**: It binds to and inhibits the inward **fast sodium channels (Na+)** in the myocardial cell membrane. * **Electrophysiologic Effects**: By blocking these channels, it reduces the rate of rise of the action potential (**Phase 0**). In Purkinje fibers, it decreases automaticity, shortens the action potential duration, and decreases the effective refractory period. * **Selectivity**: Like lidocaine, it is highly selective for diseased, ischemic, or rapidly firing myocardial tissue, making it effective at suppressing ventricular ectopic pacemakers without significantly depressing normal sinus node function or conduction (unless pre-existing abnormalities exist).
各物種劑量
- Treating or assisting in treatment of ventricular arrhythmias · 5-8 mg/kg · PO · q8h
- Treating or assisting in treatment of ventricular arrhythmias · 4-10 mg/kg · PO · q8h
- Ventricular arrhythmias in Boxers · 5-7.5 mg/kg · PO · three times daily · Given with sotalol at 1.5-3 mg/kg twice daily; was successful in 7/8 dogs treated in study.
- Treating or assisting in treatment of ventricular arrhythmias · 4-8 mg/kg · PO · q8h · Combined with atenolol (0.5 mg/kg PO q12-24h).
- Familial arrhythmic cardiomyopathy of Boxers (ARVC) · 5-8 mg/kg · PO · q8h · Given with atenolol at 12.5 mg (total dose) q12h.
- Treating or assisting in treatment of ventricular arrhythmias · 5-6 mg/kg · PO · q8h · Always give with food to avoid nausea.
- Myotonia congenita or myokymia · 8.3 mg/kg · PO · q8h · Most studied in Chow Chows, miniature schnauzers, and Jack Russell terriers.
劑量為合格獸醫專業人員的臨床參考。請務必對照最新藥品仿單及個別病患確認。
給藥途徑
禁忌症
- Pre-existing 2nd or 3rd degree AV block (without a pacemaker)
- Cardiogenic shock
- Known hypersensitivity to mexiletine
不良反應
- Vomiting (very common)
- Nausea
- Trembling/Tremors
- Unsteadiness/Ataxia
- Dizziness
- Depression/Lethargy
- Shortness of breath
- Proarrhythmia (PVCs)
- Chest pain
- Seizures (rare)
- Agranulocytosis (rare)
- Thrombocytopenia (rare)
藥物相互作用
- Antacids (Aluminum-Magnesium) · May slow the absorption of mexiletine.
- Atropine · May reduce the rate of oral absorption.
- Cimetidine · May increase or decrease mexiletine blood levels.
- Griseofulvin · May accelerate the metabolism of mexiletine.
- Lidocaine · May cause additive adverse effects (especially CNS toxicity).
- Metoclopramide · May accelerate the absorption of mexiletine.
- Opiates · May slow the absorption of mexiletine.
- Phenobarbital · May accelerate the metabolism of mexiletine.
- Primidone · May accelerate the metabolism of mexiletine.
- Phenytoin · May accelerate the metabolism of mexiletine.
- Rifampin · May accelerate the metabolism of mexiletine.
- Theophylline · Mexiletine may reduce the metabolism of theophylline, potentially leading to theophylline toxicity.
- Urinary Acidifying Drugs (e.g., methionine, ammonium chloride) · May accelerate the renal excretion of mexiletine.
監測
- Electrocardiogram (ECG) to assess antiarrhythmic efficacy and monitor for proarrhythmia
- Clinical signs of adverse effects (especially GI and CNS signs)
- Plasma concentrations (Therapeutic range in humans: 0.5-2 mcg/mL; note that toxicity may be noted even at therapeutic levels)
- Liver enzymes (AST may transiently increase in a small percentage of patients)
過量
Toxicity associated with overdosage can be significant and life-threatening. * **Clinical Signs**: In human case reports, **CNS signs (tremors, seizures, depression) always preceded cardiovascular signs** (hypotension, bradycardia, heart block). * **Treatment**: * Perform GI tract emptying protocols (emesis/gastric lavage) if indicated and safe. * Administer supportive therapy. * Acidification of the urine may be considered to enhance urinary excretion. * **Atropine** may be useful if severe hypotension or bradycardia occurs.
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