Pharmacology for Dentistry

Cardiotonics (Cardiac Glycosides) 171

ECG changes: Digitalis, in therapeutic
doses causes inversion of T wave, sagging
of S-T segment and shortening of Q-T
internal (shortening of systole). In toxic dose,
it causes prolongation of P-R interval
(slowing of AV conduction), atrial
arrhythmias (atrial tachycardia and atrial
fibrillation) with AV block and ventricular
arrhythmias.

Extracardiac Actions

• Digitalis produces diuresis in CHF
  patients, it increases excretion of
  sodium and water by the kidney
  which may be due to decrease in the
  venous pressure bringing about
  shifting of edema fluid into the
  circulation and also improves the
  renal circulation.


• Digitalis can produce nausea and
  vomiting which is probably due to the
  chemoreceptor trigger zone (CTZ)
  stimulation.


• Digitalis has mild vasoconstrictor action
  increasing the peripheral resistance. But
  in CHF patients peripheral resistance
  decreases due to withdrawal of reflex
  sympathetic overactivity. Venous tone
  improves in normal as well as CHF
  patients. It has no significant effect on
  coronary circulation.

Mechanism of Action of Digitalis

Digitalis acts by interfering with the
sodium and potassium transport across the
cell membrane and by increasing the
amount of coupling calcium i.e. making
more calcium available for excitation-
contraction coupling.

Cardiac glycosides inhibit Na+ K+-
ATPase by competing with potassium and

may probably explain reversal of toxic effects

of digitalis by potassium. Inhibition of the Na+

K+-ATPase leads to increase in intracellular

sodium and decrease in potassium.

Calcium also forms a link between the

electrical events in the membrane and

contractile proteins. Digitalis makes more

calcium available for excitation-contraction

coupling and increasing cardiac contractility.

Digitalis also exerts some indirect action

on heart mainly by increase in vagal activity

which ultimately influences activity of AV

node, SA node and auricles.

Pharmacokinetics

Among the cardiac glycosides, digitoxin is

absorbed rapidly and completely from the

gastrointestinal tract with oral absorption of

approximately 90 to 100 percent with plasma

protein binding of approx. 95 percent with

plasma half life of 5 to 7 days. It enters the liver

cells where it is metabolised to epidigitoxigenin

and is excreted in bile and urine.

Adverse Effects

It includes anorexia, vomiting which

may be of central origin. Headache, visual

disturbance, xanthopsia (yellow vision),

white vision, diplopia, drowsiness,

disorientation, delirium and psychotic

behaviour. Cardiac related effects include

cardiac arrhythmias e.g. tachyarrhythmias,

ventricular arrhythmias, supraventricular

arrhythmia, AV block and bradycardia.

Treatment of Digitalis Induced

Arrhythmias

Tachyarrhythmias

K+ tends to antagonise digitalis induced

enhanced automaticity and decreases bind-