Digoxin

Main Actions

Cardiac slowing and recuced rate of conduction throught the AV node
Increased contractility
Rhythm disturbances (heart block or increase ectopic pacemaker activity)

Mode of action

Indirect

Digoxin has a central effect which results in increased vagal activity. The increase in AV conductin is manifested as an increase on PR interval.

Direct

Digoxin blocks the Na/K ATP-ase, competitively binding to the K⁺ binding site. This has several effects:

A rise in [Na]_i and decrease in K⁺ which results in a slowing of AV conduction (Potassium efflux essential for repolarisation).
Increased [Na]_i slows extrusion of Ca²⁺ which depends on a Na/Ca antiporter
Increased [Ca²⁺]_i results in an increase in SR calcium content and increased release during an action potential. This increases contractility.

Route of administration / Dose

10-20µg/kg 6 hourly enterally or parenterally until desired effect achieved. Must be given slowly, if parenterally (25 µg/min). Peak effects 2 hours after administration.

Effects

Increased force of contraction
Heart rate slows - depression of SA node discharge, AV conduction, AV node refractry period and indirect vagotonic activity.

Frequent. Anorexia, nasuea, vomiting, diarrhoea and abdo pain. Arrythmias. Exacerbated by hypokalaemia, hypernatraemia, hypercalcaemia, hypomagnesaemia, acid-base disturbance, hypoxaemia and renal failure.