Cardiac Glycosides

Question 1

Give an example of a cardiac glycoside

Answer 1

Digoxin

Question 2

How does digoxin work?

Answer 2

Inhibits the Na/K ATPase pump causing an increase in intracellular Na
To reduce this intracellular Na the Na/Ca pump begins to pump Na out and Ca in
Increase in intracellular Ca causes increase in mycoardial contractility = Positively inotropic

At the same time digoxin also increases sympathetic (vagal) tone of the heart reducing AV node conduction = negatively chonotropic (decreases HR)

Question 3

What are the main uses for digoxin?

Answer 3

1. Atrial Fibrillation/Flutter - Reduces ventricular rate but CCB and Beta blockers usually work better (does not work in paroxysmal AF)
2. Severe heart failure - Third line use after Ace inhibitors, beta blockers and ARBS/aldosterone agonists have been used

Question 4

What type of AF does digoxin not work in?

Answer 4

Paroxysmal AF

Question 5

When should digoxin not be used?

Answer 5

Heart block (intermittent complete or second degree)
Ventricular arrhythmias
Wolff Parkinson white syndrome (digoxin doesn’t block accessory pathways; if AV node is blocked then atria contract at alternative pathway rate with catastrophic consequences)

Question 6

When should digoxin be used with caution?

Answer 6

Renal failure (excreted by kidneys)
Electrolyte disturbances e.g. hypokalaemia, hypomagnesiamia an hypocalcemia (increase the risk of digoxin toxicity)

Question 7

What drugs does digoxin usually interact with?

Answer 7

• Loop and thiazide diuretics (causes hypokalaemia)
• Amiodarone, CCBs, Spironolactone and quinine all increase the plasma conc of digoxin increasing the risk of digoxin toxicity
• Any other anti arrhythmic drugs e.g. Beta blockers, CCBs, amiodarone

Question 8

What are the side effects of digoxin?

Answer 8

Bradycardia
Digoxin toxicity: nausea, vom, GI upset, dizziness, Rash, visual disturbances (yellow and blurry)
Heart block and Hypertension if given rapid IV

Question 9

How is digoxin excreted?

Answer 9

Metabolised by the liver

Excreted by the kidneys

Question 10

Does digoxin have a high or low therapeutic window?

Answer 10

Low

The effective dose can be close to the toxic dose so close monitoring is needed.

Question 11

How is digoxin monitored?

Answer 11

The best guide to the effectiveness of digoxin is the patient’s symptoms and heart rate. Check their ECG, electrolytes and renal function periodically