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Overview

Class I - Na+ Channel Blockers
  • General principles 
    • slow or block sodium conduction preferentially in depolarized cells
      • selective for abnormal cardiac tissue that is frequently depolarized
      • use and state dependent
    • phase 4 depolarization in SA node 
      • (automaticity current) is dependent on Na+ channel opening
        • blockage results in ↓ slope of phase 4 depolarization and ↓ SA node rate
    • hyperkalemia causes ↑ toxicity for all class I drugs
Class IA
  • iaExamples
    • quinidine, procainamide, and disopyramide
      • remember: quin + the amides
  • Effects
    • acts on atrial and ventricular arrhythmias
      • especially reentrant and ectopic supraventricular and ventricular tachycardia
    • ↓ rate of phase 0 depolarization resulting in
      • ↑ QT interval
      • ↑ AP duration
      • ↑ effective refractory period (ERP)
  • Toxicity
    • quinidine
      • cinchonism
        • headache, tinnitus, and vertigo
      • thrombocytopenia
      • ↑ QT interval
        • can result in torsades de pointes
      • can enhance digoxin toxicity
    • procainamide
      • reversible SLE-like syndrome
        • remember: procAiNAmide (antinuclear antibody in SLE)
Class IB
  • ibExamples
    • lidocaine, mexiletine, and tocainide ("Lido's Mexican Tacos")
      • phenytoin can also fall into class IB
  • Effects 
    • acts on ischemic or depolarized Purkinje and ventricular tissue
      • of Class I, have the lowest affinity for sodium channels
      • does not act on healthy or atrial tissue
      • effective in treating structurally abnormal tissue (especially post-MI)  
      • useful in acute ventricular arrhythmias and in digitalis-induced arrhythmias
    • ↓ duration of phase 3 repolarization resulting in
      • ↓ AP duration 
  • Toxicity
    • local anesthetic
    • CNS stimulation/depression   
Class IC
  • icExamples
    • flecainide, encainide, and propafenone
  • Effects
    • acts on His-Purkinje system in cardiac tissue WITHOUT structural abnormalities
      • useful in V-tachs progressing to VF and refractory SVT
      • drugs of last resort
    • no effect on AP duration
  • Toxicity
    • proarrhythmic
    • contraindicated with structural abnormalities
      • e.g., post-MI
    • significantly prolongs refractory period in AV node
Class II - β-blockers
  • Examples
    • propranolol, esmolol, metoprolol, atenolol, and timolol
      • esmolol very short acting
  • Mechanism
    • ↓ slope of phase 4 resulting in ↓ automaticity
      • due to ↓ cAMP and ↓ Ca2+ currents
      • results in ↑ PR interval and ↓ conduction in AV node
  • Clinical use
    • ventricular tachycardia and SVT
    • slowing ventricular rate during atrial fibrillation and atrial flutter
    • post-MI arrhythmia prophylaxis (cardioprotective)
  • Toxicity
    • cardiovascular effects
      • bradycardia, AV block, and CHF
    • CNS effects
      • sedation, sleep alterations, and depression
    • may mask the signs of hypoglycemia
    • impotence
    • exacerbation of asthma
      • only for non-selective (non-β1 specific)
    • metoprolol can cause dyslipidemia
    • treat overdose with glucagon 
      • ↑ cAMP via a mechanism independent of β receptors
Class III - K+ Channel Blockers
  • iiiExamples
    • sotalol, ibutilide, bretylium, dofetilide, and amiodarone  
  • Mechanism
    • ↑ phase 3
      • due to ↓ K+ current
      • results in ↑ AP duration, ↑ ERP, and ↑ QT interval
    • effective for atrial and ventricular arrhythmias
    • antiarrhythmics of last resort
    • amiodarone has class I, II, III, and IV effects
      • due to alterations in lipid membrane
  • Toxicity
    • torsades de pointes due to ↑ QT interval
      • amiodarone is the only example of a drug that lengthens QT but does not have risk of torsades de pointes 
    • sinus bradycardia
    • sotalol
      • excessive β blockade 
    • bretylium
      • new arrhythmias
      • ↓ BP
    • amiodarone
      • pulmonary fibrosis
      • hepatotoxicity
      • hypothyroidism/hyperthyroidism
        • amiodarone is 40% iodine by weight (amIODarone)
        • must watch LFTs, TFTs, and PFTs
      • corneal deposits
      • photosensitivity
      • skin color changes (blue/gray)
      • neurologic effects
      • constipation
      • cardiovascular effects
        • bradycardia, heart block, and CHF
Class IV - Ca2+ Channel Blockers
  • Examples
    • verapamil, diltiazem
      • note: other Ca2+ channel blockers (nifedipine and amlodipine) have little action on the heart
  • Mechanism  
    • ↓ L-type Ca2+ channels current primarily in AV nodal cells
      • L-type Ca2+ channels responsible for the plateau phase
    • results in ↓ conduction velocity, ↑ ERP, and ↑ PR interval
    • used in prevention of nodal arrhythmias and AV nodal reentry
  • Toxicity
    • antimuscarinic effects
      • constipation, dizziness, and flushing
    • edema
    • CV effects
      • negative inotropy, AV block, and sinus node depression 

 

 
 

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