PHARMACOLOGY - Antiarrhythmic Drugs Flashcards
What is a supraventricular arrhythmia?
A supraventricular arrhythmia is an arrhythmia which originates in the atria
What is a ventricular arrhythmia?
A ventricular arrhythmia is an arrhythmia which originates in the ventricles
What are the two main classifications of drugs used to treat tachyarrythmias?
Vaughan Williams antiarrhythmic drugs
Digoxin
What are the four classifications of Vaughan Williams antiarrhythmic drugs?
Class 1 - Sodium channel blockers
Class 2 - β blockers
Class 3 - Potassium channel blockers
Class 4 - Calcium channel blockers
What is the mechanism of action of Vaughan Williams class 1 antiarryhthmics?
Vaughan Williams class 1 antiarryhthmics block sodium channels in cardiac muscle cells which inhibits the influx of sodium into the cells and thus slows the rate of depolarisation and contraction
What are the two main subclasses of class 1 antiarrhythmic drugs?
Class 1a: Immediate dissociation
Class 1b: Fast dissociation
When is the use of class 1a antiarrhythmics indicated?
Class 1a antiarrythmics are used for haemodynamically significant or life-threatening arrhythmias
Give an example of a class 1a antiarrythmic
Quinidine
When specifically is quinidine used for treating horses?
Quinidine is used to treat atrial fibrillation in horses
What are the three potential side effects of class 1a antiarrhythmics?
Hypotension
Increased QRS duration
Ventricular tachycardia
When is the use of class 1b antiarrhythmics indicated?
Class 1b antiarrythmics are used for haemodynamically significant or life-threatening arrhythmias
Give an example of a class 1b antiarrhythmic
Lidocaine
What is lidocaine used for other than as an antiarrhythmic?
Local anaesthetic
What is the mechanism of action for Vaughan Williams class 2 antiarryhthmics?
Vaughan Williams class 2 antiarryhthmics block β receptors in cardiac muscle cells which reduces sympathetic activity which will decrease myocardial contractility and slow atrioventricular (AV) node conduction which will decrease heart rate
When is the use of Vaughan Williams class 2 antiarryhthmics indicated?
Vaughan Williams class 2 antiarryhthmics are used for the the treatment of supraventricular and ventricular arrhythmias and to treat hypertrophic cardiomyopathy
What are the five potential side effects of Vaughan Williams class 2 antiarryhthmics?
Worsen congestive heart failure
Lethargy
Depression
Bradycardia
Bronchospasm
What is the mechanism of action for Vaughan Williams class 3 antiarryhthmics?
Vaughan Williams class 3 antiarryhthmics block potassium channels in cardiac muscle cells which will inhibit potassium efflux out of the cell and thus slow down repolarisation and prolong action potential duration, which will prolong myocardial contraction
Give an example of a Vaughan Williams class 3 antiarryhthmic
Amiodarone
What are the four potential side affects of Vaughan Williams class 3 antiarryhthmics?
Elevated liver enzymes
Gastrointestinal (GI) disturbances
Pulmonary fibrosis
Thyroid effects
What is the mechanism of action for Vaughan Williams class 4 antiarryhthmics?
Vaughan Williams class 4 antiarryhthmics block L-type calcium channels on pacemaker cells in nodal tissue and cardiac muscle cells. In pacemaker cells inhibiting the influx of calcium will slow the rate in which the cells depolarise and prolong the action potential duration, decreasing the heart rate. In cardiomyocytes, this will inhibit the influx of calcium into the cell during the plateau phase, which will shorten the plateau phase, decreasing the action potential duration which will decrease myocardial contractility
When is the use of Vaughan Williams class 4 antiarryhthmics indicated?
Vaughan Williams class 4 antiarryhthmics are used to treat supraventricular arrhythmias and hypertrophic cardiomyopathy
Give an example of a Vaughan Williams class 4 antiarryhthmic drug
Verapamil
What are the four pharmacodynamic effects of digoxin?
Antiarrhythmic
Increases baroreceptor activity
Positive inotropic effect
Diuretic
What is the mechanism of action of digoxin as an antiarrhythmic?
Digoxin stimulates parasympathetic activity which will slow atrioventricular (AV) node conduction and will thus decease heart rate
When is the use of digoxin as an antiarryhthmic indicated?
Digoxin is used to treat supraventricular arrhythmias
How does digoxin increase the activity of baroreceptors?
Digoxin stimulates the parasympathetic nervous system and thus there will be decreased sympathetic activity and circulating catecholamines which will increase baroreceptor activity
How does digoxin have a positive inotropic effect?
Digoxin inhibits the Na+/K+ ATPase pump on cardiac muscle cells, which will increase intracellular sodium levels which will disrupt the normal Na+/Ca2+ counter-transport mechanism leading to an increase in intracellular calcium which will enhance myocardial contractility
How does digoxin have a diuretic effect?
Digoxin inhibits the Na+/K+ ATPase pump on the renal tubular epithelial cells which will inhibit the reabsorption of sodium and subsequently water into the bloodstream, resulting in diuresis
(T/F) Digoxin has a wide therapeutic index
FALSE. Digoxin has a narrow therapeutic index
Why should digoxin be given orally?
Digoxin has a 60-75% bioavailability when given orally
(T/F) Digoxin is plasma protein bound
TRUE.
Why does digoxin have such a long half life?
Digoxin has a high volume of distribution and will accumulate within skeletal muscle
What is the half life of digoxin in dogs?
24 - 30 hours
What is the half life of digoxin in cats?
36 hours
How long does it take for digoxin to reach steady state?
Seven days
How is digoxin eliminated?
Digoxin is eliminated by the kidneys
What are the six signs of digoxin toxicity?
Enhanced borborygmi
Depression
Anorexia
Vomiting
Diarrhoea
Fatal arrhythmia
Which three factors can predispose a patient to digoxin toxicity?
Hypokalaemia
Cachexia
Impaired renal function
Which breed of dog is predisposed to digoxin toxicity?
Dobermans
Why does hypokalaemia increase the risk of digoxin toxicity?
Hypokalaemia increases the risk of digoxin toxicity as digoxin binds to same the binding side as potassium on the Na+/K+ ATPase pump so when there is low potassium in the blood, this will create more binding sites for digoxin which will increase the risk of digoxin toxicity
How can cachexia increase the risk of digoxin toxicity?
Cachexia reduces skeletal muscle mass and thus if there is reduced skeletal muscle there will be more circulating digoxin as it cannot accumulate as well in skeletal muscle
Which five methods can be used to prevent digoxin toxicity?
Start on a low dose
Dose based on body surface area rather than weight
Avoid loading doses
Reduce starting dose if patient is predisposed to toxicity
Monitor digoxin serum levels
How should you treat an acute overdose of digoxin?
Activated charcoal
Digibind
What is digibind?
Digibind is an antibody which will directly chelate digoxin
What are the three main classifications of drugs used to treat bradyarrythmias?
Anticholinergic drugs
β-agonists
Methylxanthines
What is the mechanism of action for anticholinergic drugs as antiarrhythmics?
Anticholinergics inhibit the binding of acetylcholine to muscarinic receptors on cardiac muscle cells, which will thus decrease the parasympathetic activity and increase myocardial contractility
What are the four potential side effects of anticholinergic drugs?
Constipation
Tachycardia
Urinary retention
Dry mucous membranes
What is the mechanism of action for β-agonists as antiarrhythmics?
β-agonists stimulate β adrenergic receptors on cardiac muscle cells which will increase sympathetic activity and thus increase myocardial contractility
What is the mechanism of action for methylxanthines as antiarrhythmics?
Methylxanthines are phosphodiesterase (PDE) inhibitors. Phosphodiesterases (PDE) are enzymes which catalyse the hydrolysis of cAMP and thus inhibiting this action, there will be increased and prolonged cAMP, increasing calcium influx and myocardial contractility
