Cardiac glycosides

Question 1

What are the generic and brand names of cardiac glycosides?

Answer 1

• Digoxin (Lanoxin)

Question 2

What is the method of action of cardiac glycosides?

Answer 2

• Negatively chronotropic – reduces heart rate
• Positively inotropic – increases force of contractions
• AF/atrial flutter: Works via increasing vagal tone = parasympathetic tone
• Heart failure: Inhibits Na+/K+-ATPase pumps in myocytes = ↑ concentration of intracellular Na+
• Myocytes also exchange Na+ into the cell for Ca2+ out normally
• Increase in Na+ intracellularly slows down Ca2+ leaving the cell = increased contraction and slows down heart rate as concentration gradients are less steep

Question 3

What are the indications for cardiac glycosides?

Answer 3

• AF – reduced ventricular rate
• Atrial flutter – reduces ventricular rate
• Severe heart failure – 3rd line treatment when already taking a antihypertensive e.g. beta blocker, ACE inhibitor

Question 4

What are the contraindications of cardiac glycosides?

Answer 4

• 2nd degree heart block
• Intermittent complete heart block
• With or at risk of ventricular arrhythmias
• Renal failure – reduced dose
• Hypokalaemia – digoxin competes with K+ so effect is enhanced if ↓ K+
• Hypomagnesaemia
• Hypcalcaemia

Question 5

What are the side effects of cardiac glycosides?

Answer 5

• Bradycardia
• GI disturbance – sickness/diarrhea
• Headache
• Rash
• Dizziness
• Visual disturbances – blurred/yellow vision
• Arrhythmias – due to digoxin toxicity

Question 6

What are the interactiond with cardiac glycosides?

Answer 6

• Loop diuretics – due to hypokalaemia
• Thaizide diuretics – due to hypokalaemia
• Amiodorone
• Ca2+ channel blockers – increase plasma concentration = toxicity
• Spironlactone – increase plasma concentration = toxicity
• Quinine – increase plasma concentration = toxicity

Question 7

Which organ eliminates cardiac glycosides?

Answer 7

Kidneys

Question 8

What patient information should be given to patients taking cardiac glycosides?

Answer 8

• Low therapeutic index – cardiac monitoring regularly

* Effects of digoxin can be lost during stress and exercise (against parasympathetic tone)