Section 3 Lecture 3 Antiarrhythmic Drugs Heart beats (HB) originate from AV 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 and do not require treatment others interfere with cardiac output or BP or can cause more severe or lethal rhythm disturbances.

2 Resting membrane potential of a cardiac cell Resting membrane potential (RMP): A net negative charge relative to the outside of a cardiac cell Action potential: Change in the distribution of ions across the cell membrane and propagation of an electrical impulse and the subsequent contraction of myocardial cells. AP consist of 4 phases Spontaneous depolarization is referred to automaticity or pacemaker activity

3 P-wave: atrial depolarization PR: Time it takes for the action potential to conduct through the atria and the AV node QRS wave: Ventricular depolarization T wave: Ventricular repolarization ECG –is a summation of the action potentials generated by the heart QT: time between ventricular depolarization and repolarization

4 Pacemaker cells 3 distinct phases: 0- rapid depolarization 3- repolarization 4- resting potential (unstable) Non pacemaker cells 5 distinct phases: 0- rapid depolarization 1-initial repolarization 2- action potential plateau 3- repolarization 4- resting potential (stable)

5 ionOutside Cell Inside Cell Equilibrium Potential mV Na mM10 mM+50 K+K+ 4 mM150 mM-90 Ca 2+ 2 mM mM+140 Threshold potential (TP)

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7 P-wave: atrial depolarization PR: Time it takes for the action potential to conduct through the atria and the AV node QRS wave: Ventricular depolarization T wave: Ventricular repolarization ECG –is a summation of the action potentials generated by the heart QT: time between ventricular depolarization and repolarization

8 Pathophysiology of Arrhythmias Coronary ischemia and hypoxia Electrolyte imbalance Overstimulation of sympathetic NS General anesthetics Other conditions that perturb cardiac conduction

9 Abnormal impulse formation Increased automaticity Decreased the required time for depolarization from the maximal diastolic potential (MDP) to the threshold potential

10 Afterdepolarizations Abnormal calcium influx into cardiac cells during or immediately after phase 3 of ventricular action potential