RCS 22 - Antiarrhythmic Drugs 1

Question 1

Answer 1

C

Question 2

Provide a detailed summary of the myocardial action potential.

Answer 2

• Phase 0 - when the threshold potential is reached, Na+ channels open causing a large influx of Na+. Ends when Na+ channels inactivate
• Phase 1 - Na+ channels inactivate and K+ channels rapidly open and close to quickly end the rapid depolarization from Phase 0 (and slightly repolarize)
• Phase 2 - Ca++ and K+ channels open to allow a slow influx of Ca++ and efflux of K+ which almost balance each other out so that only a very slow repolarization occurs (almost a plateau)
• Phase 3 - Ca++ channels close causing a net efflux of K+ and repolarization
• Phase 4 - Na+ permeaability gradually increases until threshold is reached

Question 3

Provided a detailed summory of the cardiac pacemaker action potential.

Answer 3

• Phase 0 - when threshold is reached, Ca++ channels open causing rapid depolarization
• Phase 3 - Ca++ channels inactivate and K+ channels open to repolarize
• Phase 4 - I_f Na+ channels spontaneously open to gradually depolarize the cell to threshold

Question 4

List and describe the different EKG intervals that can be affected pharmacologically. What types of drugs will affect these intervals?

Answer 4

• PR interval - the start of atrial depolarization to the start of ventricular depolarization. Drugs that decrease/increase AV conduction velocitey will increase/decrease the PR interval
• QT interval - the start of ventricular depolarization to the end of ventricular repolarization. Drugs that increase/decrease the myocardial action potential cycle time in any way will increase/decrease the QT interval

Question 5

All arrhythmias result from either_____________, _______________, ot _________________

Answer 5

• Disturbances in impulse formation
• Disturbances in impulse conduction
• A combination of the above

Question 6

List and briefly describe the different types of cardiac arrhythmias.

Answer 6

• Ventricular Arrhythmias
  
  • bradycardia
  • tachycardia
  • Ventricular fibrillation
• Supraventricular Arhythmias
  
  • Sinus tachycardia
  • Sinus bradycardia
  • Atrial fibrillation

Question 7

List the primary factors that can precipitate arrhythmias

Answer 7

• Ischemia/Hypoxia
• Acidosis/Alkalosis
• Electrolyte abnormalities
• Excessive catecholamine exposure
• Autonomic influences
• Drug Toxicity (especially antiarrhythmics)

Question 8

List and describe the four leading direct causes of arrhythmias.

Answer 8

• Abnormal automaticity - typically automaticity is set by the SA node, however, if other cardiac sites develop enhanced automaticity, they may generate competing stimuli and cause arrhythmias
• Re-entrant circuits - cardiac depolarizations bifurcate many times in order to quickly reach all the areas of the heart. If one of the legs of a bifurcation is damaged, the depolarization may end up traveling in the reverse direction of that leg, leading to a depolarization cycle that causes an arrhythmia (see image)
• Afterdepolarizations
• Accessory tract pathways

Question 9

What is the general strategy for treating arrhythmias caused by abnormal automaticity

Answer 9

Blocking Na+ and Ca++ channels to decrease the slope of phase 4 and/or raise the threshold potential.

The goal is to reduce the frequency of discharge