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59-291 Section 3 Lecture 3

Antiarrhythmic Drugs

• Heart beats (HB) originate fromAV node

• Normal 70 beats/min at rest• Arrhythmia (dysrhythmia):

Abnormal origin, rhythm, or rate• Supraventricular arrhythmia:

arise in the atria or AV node• Ventricular arrhythmia: arise in

the ventricles• Tachyarrhythmia: HB too rapid• Bradyarrhythmia: HB too slow

Some arrhythmias are benign anddo not require treatment othersinterfere with cardiac output or BPor can cause more severe or lethalrhythm disturbances.

2Resting membrane potential of a cardiac cell

Resting membrane potential (RMP): A net negative charge relative to the outside of acardiac cell

Action potential: Change in the distribution of ions across the cell membrane andpropagation of an electrical impulse and the subsequent contraction of myocardialcells. AP consist of 4 phases

Spontaneous depolarization is referred to automaticity or pacemaker activity

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P-wave:atrialdepolarization

PR: Time it takes for theaction potential toconduct through the atriaand the AV node

QRS wave: Ventricular depolarization

T wave: Ventricular repolarization

ECG –is a summation of theaction potentials generated bythe heartQT: time between

ventriculardepolarization andrepolarization

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Pacemaker cells

3 distinct phases:

0- rapid depolarization

3- repolarization

4- resting potential (unstable)

Non pacemaker cells5 distinct phases:

0- rapid depolarization

1-initial repolarization

2- action potential plateau

3- repolarization

4- resting potential (stable)

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+1400.0001 mM2 mMCa2+

-90150 mM4 mMK+

+5010 mM145 mMNa+

EquilibriumPotentialmV

InsideCell

Outside Cell

ion

Threshold potential (TP)

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P-wave:atrialdepolarization

PR: Time it takes for theaction potential toconduct through the atriaand the AV node

QRS wave: Ventricular depolarization

T wave: Ventricular repolarization

ECG –is a summation of theaction potentials generated bythe heartQT: time between

ventriculardepolarization andrepolarization

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Pathophysiology of Arrhythmias

• Coronary ischemia and hypoxia• Electrolyte imbalance• Overstimulation of sympathetic NS• General anesthetics• Other conditions that perturb cardiac

conduction

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Abnormal impulse formation• Increased automaticity

• Decreased the required time for depolarization from themaximal diastolic potential (MDP) to the threshold potential

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• Afterdepolarizations• Abnormal calcium influx into cardiac cells during or immediately after phase 3

of ventricular action potential