Tachyarrhythmia pharmacology Flashcards
Tachyarrhythmias can lead to syncope (collapse) and death. How do they lead to this?
Less diastolic filling time
Decreased EDVV
Decreased SV
Decreased CO
How does tachyarrythmia cause a vicious cycle on the heart?
Increased cardiac workload causes myocardial hypertrophy
Too much muscle decreases EDVV and CO
What is the pharmacological aim for tachyarrhythmias?
Slow impulses or reduce firing of pacemaker cells (counteract sympathetic nervous system)
Not aiming to correct arrhythmia
How is heart rate slowed down physiologically?
Blocking ion channels
Reduce nerve impulses
There are 5 classes of negative chronotropes. What does each class do to decrease heart rate?
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
How do class I negative chronotropes work?
Block fast sodium channels
Affect open and refractory channels, more active channels = more binding
What are the 3 types of class 1 negative chronotropes?
1a = old type, moderate Na channel blockers 1b = weak Na channel blockers 1c = strong Na channel blockers
What don’t class I negative chronotropes affect? Why is this important
Don’t affect resting channels or nodal tissue
Reduces heart rate in arrhythmias but doesn’t alter normal HR
What do class I -ve chronotropes depend on to work effectively?
Normal extracellular K function
Hypokalaemia decreases drug function, hyperkalaemia increases drug function
Give an example of a class 1a -ve chronotrope. When is this drug used? How is it administered? What is its adverse effects?
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
How does quinidine work?
Decreases contractility
Slightly vagolytic - anticholinergic so can increase HR!
Give an example of a 1b -ve chronotrope. How is it administered? What are the side effects?
Lidocaine
Slow IV administration
CNS (as local anaesthetic) = excitation, seizures, nausea
Which species is sensitive to lidocaine?
Cats
What are the advantages and disadvantages of lidocaine? How is it metabolised?
Little haemodynamic effects
Cats sensitive
Hypotension at toxic levels
Rapid hepatic metabolism
How do class II negative chronotropes work?
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
Class II -ve chronotropes also have what other effects?
Negative lusiotrope
Negative inotrope
In what conditions might class II -ve chronotropes be useful?
Supraventricular and ventricular tachycardia
Hypertension
Give an example of a class II -ve chronotrope. What type of blocker is it and what adverse effects does it have?
Atenolol
Selective B1 blocker
Adverse effects - CNS
Class III -ve chronotropes have more than one MOA. What is an example of tan MOA? What effect do they have on an ECG?
Prolong AP by blocking K channels
Cause longer Q-T interval
What are adverse effects of class III -ve chronotropes?
Hypotension
Bradycardia
Give an example of a class III -ve chronotrope
Sotalol
Racemic mix of isomers - inhibits K+ channels
How do class IV -ve chronotropes work?
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)
What other properties do class IV -ve chronotropes have?
Negative inotropes
Positive lusiotropes
Vasodilators
Give an example of a class IV -ve chronotrope. What does this also do? How is it administered and what are side effects
Dilitiazem
Vasodiltor
Oral/parenteral route
Toxicity –> myocardial depression, hypotension and AV block
Give an example of a class V -ve chronotrope. How does it work?
Digoxin
Increases vagal outflow as vagomimetic
Slows conduction and SAN AP production
Digoxin is used to treat what conditions?
Supraventricular tachycardia
Atrial fibrillation
What is unique about digoxin?
Only negative chronotrope that isn’t a negative inotrope
What are the adverse effects of digoxin
Myocardial and GI toxicity (indicated by softening faeces)
