Tachyarrhythmia pharmacology

Question 1

Tachyarrhythmias can lead to syncope (collapse) and death. How do they lead to this?

Answer 1

Less diastolic filling time
Decreased EDVV
Decreased SV
Decreased CO

Question 2

How does tachyarrythmia cause a vicious cycle on the heart?

Answer 2

Increased cardiac workload causes myocardial hypertrophy

Too much muscle decreases EDVV and CO

Question 3

What is the pharmacological aim for tachyarrhythmias?

Answer 3

Slow impulses or reduce firing of pacemaker cells (counteract sympathetic nervous system)
Not aiming to correct arrhythmia

Question 4

How is heart rate slowed down physiologically?

Answer 4

Blocking ion channels

Reduce nerve impulses

Question 5

There are 5 classes of negative chronotropes. What does each class do to decrease heart rate?

Answer 5

Class I - block fast Na channels
Class II - beta blockers
Class III - block K+ channels (prolong AP)
Class IV - block Ca channels
Class V - miscellaneous

Question 6

How do class I negative chronotropes work?

Answer 6

Block fast sodium channels

Affect open and refractory channels, more active channels = more binding

Question 7

What are the 3 types of class 1 negative chronotropes?

Answer 7

1a = old type, moderate Na channel blockers
1b = weak Na channel blockers
1c = strong Na channel blockers

Question 8

What don’t class I negative chronotropes affect? Why is this important

Answer 8

Don’t affect resting channels or nodal tissue

Reduces heart rate in arrhythmias but doesn’t alter normal HR

Question 9

What do class I -ve chronotropes depend on to work effectively?

Answer 9

Normal extracellular K function

Hypokalaemia decreases drug function, hyperkalaemia increases drug function

Question 10

Give an example of a class 1a -ve chronotrope. When is this drug used? How is it administered? What is its adverse effects?

Answer 10

Quinidine
Only used in atrial fibrillation in horses/cattle - never SA! Needs constant watch
Oral/parenteral route
Arrhythmia, -ve inotropy, vasodilation leadings to CHF, GI signs

Question 11

How does quinidine work?

Answer 11

Decreases contractility

Slightly vagolytic - anticholinergic so can increase HR!

Question 12

Give an example of a 1b -ve chronotrope. How is it administered? What are the side effects?

Answer 12

Lidocaine
Slow IV administration
CNS (as local anaesthetic) = excitation, seizures, nausea

Question 13

Which species is sensitive to lidocaine?

Answer 13

Cats

Question 14

What are the advantages and disadvantages of lidocaine? How is it metabolised?

Answer 14

Little haemodynamic effects
Cats sensitive
Hypotension at toxic levels
Rapid hepatic metabolism

Question 15

How do class II negative chronotropes work?

Answer 15

B1 blockers and some B2 blockade causes vasoconstriction
3rd generation has some alpha blockade which causes vasodilation
Slow calcium influx which slows conduction pathway and increases refractory period

Question 16

Class II -ve chronotropes also have what other effects?

Answer 16

Negative lusiotrope

Negative inotrope

Question 17

In what conditions might class II -ve chronotropes be useful?

Answer 17

Supraventricular and ventricular tachycardia

Hypertension

Question 18

Give an example of a class II -ve chronotrope. What type of blocker is it and what adverse effects does it have?

Answer 18

Atenolol
Selective B1 blocker
Adverse effects - CNS

Question 19

Class III -ve chronotropes have more than one MOA. What is an example of tan MOA? What effect do they have on an ECG?

Answer 19

Prolong AP by blocking K channels

Cause longer Q-T interval

Question 20

What are adverse effects of class III -ve chronotropes?

Answer 20

Hypotension

Bradycardia

Question 21

Give an example of a class III -ve chronotrope

Answer 21

Sotalol

Racemic mix of isomers - inhibits K+ channels

Question 22

How do class IV -ve chronotropes work?

Answer 22

Block calcium channels of cardiomyocytes, nodal tissue and smooth muscle
Shortens plateau phase -slows conduction in nodal tissue (remember cells with automacity have calcium dependent APs –> partial AV block)

Question 23

What other properties do class IV -ve chronotropes have?

Answer 23

Negative inotropes
Positive lusiotropes
Vasodilators

Question 24

Give an example of a class IV -ve chronotrope. What does this also do? How is it administered and what are side effects

Answer 24

Dilitiazem
Vasodiltor
Oral/parenteral route
Toxicity –> myocardial depression, hypotension and AV block

Question 25

Give an example of a class V -ve chronotrope. How does it work?

Answer 25

Digoxin
Increases vagal outflow as vagomimetic
Slows conduction and SAN AP production

Question 26

Digoxin is used to treat what conditions?

Answer 26

Supraventricular tachycardia

Atrial fibrillation

Question 27

What is unique about digoxin?

Answer 27

Only negative chronotrope that isn’t a negative inotrope

Question 28

What are the adverse effects of digoxin

Answer 28

Myocardial and GI toxicity (indicated by softening faeces)