688
P A R T 8
Drugs acting on the cardiovascular system
not be taken at the same time but should be adminis
tered 2 to 4 hours apart. Box 44.4 highlights important
information about the interactions between digoxin and
common herbal remedies.
Preventing digoxin toxicity in children and the
elderly
Children and older adults are at increased risk for digoxin
toxicity. Individuals in both of these groups have body
masses that are smaller than the average adult body mass,
and they may have immature or ageing kidneys. Digoxin
is excreted unchanged in the kidneys, so any change in
kidney function can result in increased serum digoxin
levels and subsequent digoxin toxicity. Extreme care
should be taken when administering digoxin to people in
either of these age groups.
Paediatric findings
Many institutions require that paediatric digoxin doses
be checked by a second healthcare professional before
administration.This practice provides an extra check to
help prevent the toxicity of this potentially dangerous
drug.The child should then be assessed before the drug
is given, including careful cardiac auscultation and apical
pulse measurement to monitor heart rate and rhythm to
detect any possible toxic effects.
Geriatric findings
Older adults may not receive the same kind of attention as
a policy, but they should be monitored for any factor that
might affect digoxin levels when the drug is administered.
Such factors may include:
• Renal function (Is the blood urea nitrogen concentration
elevated?)
• Low body mass (Is the person underweight,
undernourished, taking laxatives?)
• Current pulse, including quality and rhythm
• Hydration (Is the skin loose? Are the mucous membranes
dry?The presence of these conditions could signal
potential electrolyte disturbances.)
Many geriatric people eventually need a decrease in
dose, from 0.25 mg once a day to 0.125 mg once a day
or 0.25 mg every other day.The healthcare professional
administering the drug is often in the best position to
detect any changes in the person’s condition that might
indicate a need for further evaluation.
The evidence
BOX 44.2
Digoxin immune Fab (
Digibind
) [not available in New
Zealand] is an antigen-binding fragment (Fab) derived
from specific anti-digoxin antibodies. These antibodies
bind molecules of digoxin, making them unavailable
at their site of action. The digoxin antibody–antigen
complexes accumulate in the blood and are excreted
through the kidney. Digoxin immune Fab is used for the
treatment of life-threatening digoxin intoxication (serum
levels 0.10 ng/mL with serum potassium 0.5 mEq/L in
a setting of digoxin intoxication) and potential life-
threatening digoxin overdose.
The amount of digoxin immune Fab that is infused
intravenously is determined by the amount of digoxin
ingested or by the serum digoxin level if the ingested
amount is unknown. The person’s cardiac status should
be monitored continually while the drug is given and
for several hours after the infusion is finished. Because
there is a risk of hypersensitivity reaction to the infused
protein, life-support equipment should be on standby.
Serum digoxin levels will be very high and unreliable
for about 3 days after the digoxin immune Fab infusion
because of the high levels of digoxin in the blood. The
person should not be redigitalised for several days
to 1 week after digoxin immune Fab has been used,
because of the potential of remaining fragments in the
blood.
■■
BOX 44.3
Digoxin antidote: Digoxin immune Fab
Prototype summary: Digoxin
Indications:
Treatment of HF, atrial fibrillation.
Actions:
Increases intracellular calcium and allows
more calcium to enter the myocardial cell during
depolarisation; this causes a positive inotropic
effect (increased force of contraction), increased
renal perfusion with a diuretic effect and decrease
in renin release, a negative chronotropic effect
(slower heart rate), and slowed conduction through
the atrioventricular (AV) node.
Pharmacokinetics:
Route Onset
Peak
Duration
Oral
30–120 mins 2–6 hours 6–8 days
IV 5–30 mins
1–5 hours 4–5 days
T
1/2
:
30 to 40 hours; largely excreted unchanged in
the urine.
Adverse effects:
Headache, weakness, drowsiness,
visual disturbances, arrhythmias, GI upset.
St John’s wort and psyllium have been shown to
decrease the effectiveness of digoxin; this combination
should be avoided. Increased digoxin toxicity has been
reported with ginseng, hawthorn and licorice. People
should be advised to avoid these combinations.
Herbal and alternative therapies
BOX 44.4
