Inotropic drugs Lecture 8

Question 1

What are inotropic drugs?

Answer 1

Can increase/decrease contractiltiy

Can act directly (affecting force of contraction) or indirectly (affects vasculature or blood volume/composition)

+ve inotropic drugs increase intracellular Ca2+

Use:

• Increase contractility and CO
• Heart failure

Types:

• Cardiac glycosides
• Sympathomimetics
• Phosphodiesterase Inhibitors (PDE)

Question 2

Describe cardiac glycosides

Answer 2

Eg. Digoxin

Partial inhibition of Na/K ATPase

Loss of the Na conc grad so the Na/Ca antiporter cannot work and Ca is no longer transported out of the cell

Side effects: Increase the resting membrane potential,causing depolarisation, increases excitability and can lead to dysrhythmias

GIT, neurological disturbances, gynecomastia

Low therapeutic index - dose at which they are useful is close to where they are toxic

Used in CHF (and dysrhythmias) Affected by diuretics- hypokalaemia- less competition for Na/K ATPase

Toxicity treated with increase K, and antibodies

Question 3

Describe sympathomimetic inotropes

Answer 3

Target contractility controlled by SNS (B1 receptor)

Beta receptor agonist

Mixed beta1&2- isoprenaline

Beta1- dobutamine

Increase intracellular cAMP to open Ca channels and increases Ca infflux

Used in emergencies

Has short half-life and problems with desensitisation

Question 4

Describe phosphodiesterase inhibitors

Answer 4

Stops cAMP breakdown so more action of Ca channels and increase in contractility Eg. Milrinone, enoximone

PDE type 3 - heart specific

No desensitisation

Increases excitability

Used in emergencies only - Has short half-life