Phase O

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The Action Potential of the Myocardial Cell
Phase 1
Phase 2
Phase O
Phase 3
Phase 4
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Explanation of the Phases

• Phase 0 -- sudden and rapid depolarization and
  Na influx
• Phase 1 -- early incomplete repolarization
  slowed by
• Phase 2 -- the depolarizing effects of slow Ca
  overlaps the repolarization of the Na pump
  causing the plateau on this phase
• Phase 3 -- rapid repolarization leading to
• Phase 4 -- resting state

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Inotropic, dromotropic and chronotropic effects

• Inotropic -- the force of myocardial contraction
• Dromotropic -- an alteration in conduction
  velocity
• Chronotropic -- improving cardiac output at a
  known stroke volume

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Class I Drugs The Sodium Channel
Blockers depress the fast response or fast
channel sodium ion influx (Phase O of cardiac
action potential).
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Subclass IA

• Moderately inhibits fast inward Na movement,
  inhibiting upstroke of action potential (Phase O)
• Also slows repolarization
• Examples---Procainamide or Quinidine

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Subclass IB

• Slow Na influx but lt IA subclass
• Shorten the action potential duration
• Example--- lidocaine

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Subclass IC

• Strongly inhibits fast Na inward current
• Inhibits His-Purkinje conduction widening the QRS
  interval
• Shortens the action potential duration of the
  Purkinje fibers
• Example flecainide

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Class II Drugs
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Non-selective Beta Blockers
Generic Name Trade Name Indications (All Class
II) Non-selective _ blockers Propanolol,
Nadolol Inderal, Corgard Tachyarrythmias--
sympathetic
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Generic Trade name Metoprolol Lopressor
Atenolol Tenormin Acebutolol Sectral Esm
olol Brevibloc
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Class III Drugs Prolong The Action Potential
(Phase 3)
Bretylium Bretylol Prevention and treatment of
ventricular fibrillation
sympathetic blockade by preventing
norepinephrine release Amiodirone Cordaron
e Recurrent ventricular fibrillation
unstable ventricular tachycardia
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Class IV Drugs Calcium Channel Blocking Agents
(Calcium Antagonists)
Tx of Atrial tachyarrythmias Generic Trade
name Verapimil Isoptin Nifedipine Adalat,
Procardia Diltiazem Cardizem Nicardipine
Cardene Verapimil is a negative inotrope,
dromotrope and chronotrope suppressing SA node
and AV node response