Electrophysiology

Question 1

What causes EADs?

Answer 1

Excessive AP prolongation, allows the Ca channels to recover. Often the result of block K channels

Question 2

What are the four phases of an AP?

Answer 2

Phase 0 - Na channel, large energy conduction
Phase 1 - K + channel
Phase 2 - Ca entry –> contraction, causes plateau
Phase 3 - K+ channel, delayed rectifier, governs APD (refractoriness)
Phase 4 - background K+; maintains resting state

Question 3

What controls AV node refractory period?

Answer 3

time-dependent Ca channel recovery, allowing it to act as a filter

Question 4

What is enhanced automaticity?

Answer 4

Enhanced phase 4 spontaneous depolorization… essentially the line between the two APs has a steeper slope

Question 5

What are DADs?

Answer 5

Caused by spontaneous diastolic Ca release, favoured by excess Ca load, as well as abnormal function of Ca release mechanism; too much Ca, is pushed out in exchange for Na and leads to a net inward movement of positive ions (depolarization)

Question 6

Describe the cardiac CA handling apparatus

Answer 6

Systole:
Ca enters through channel during phase 2; triggers more release via RyR2… it’s this free intracellular Ca that causes contraction
Diastole:
Ca removed into sarcoplasmic reticulum and across cell membrane

Question 7

What are the different degrees of AV block?

Answer 7

First: slowing of AV conduction/long PR interval, no blocked beats
Second: Mobitz type 1/Wenkebach –> progressive PR lengthening leading to blocked P wave
Type 2 –> no progressive lengthening, QRS complex usually long
Third: complete AV block; no sinus impulses get to ventricles

Question 8

What are the intrinsic rates for the AV node and His-Purkinje?

Answer 8

AV node - 50

His-Purkinje - 30

Question 9

What is treatment for AV block?

Answer 9

Atropine - remove cholinergic tone
Isoproterenol - stimulate beta-adrenergic receptors
Temporary transvenous pacemaker

Question 10

How would you prolong circuit time to prevent reentry?

Answer 10

• Block Ca channels mediating AV node conduction

- Hyperpolarize AV node cells by enhancing K current, either pharmacologically or by enhancing vagal tone

Question 11

What can cause ventricular tachycardia?

Answer 11

Enhanced automaticity, EADs, DADs, re-entry (often MI)

Question 12

How do drugs depress slow responses APs?

Answer 12

• Sympathetic antagonism
• Vagal enhancement
• Calcium channel blockers
• Purinergic agonists

Question 13

What are the 5 different classes?

Answer 13

Class 1 - sodium channel blockers (fast channel - decreases automaticity)
Class 2 - beta adrenergic receptor blockers (slow channel)
Class 3 - K channel blockers; increase APD (fast channel - can prevent or terminate re-entry)
Class 4 - Ca channel blockers (slow channel)