Digoxin is a cardiac glycoside that is mainly used to treat heart failure and superventricular tachycardias (such as atrial fibrilation).
Its main route of excretion is via the kidneys. Poisoning with digoxin involves (as is broadly true for all overdoses) either; (i) an excess of digoxin coming into the body; (ii) an increased sensitivity to the effects of digoxin; (iii) a reduction in the elimination of digoxin. Its theraputic effects are mainly on the heart, so it is unsurprising that overdoses produce cardiac (heart) symptoms. Overdoses also cause systemic (body-wide, and more general) symptoms. Digoxin poisoning, more correctly called Digoxin toxicity can be fatal.
Causes of overdose:
(i) Overdosing
Many people taking digoxin are elderly. The theraputic range of digoxin is quite small, meaning that small overdoes can have toxic effects. Forgetting that a morning's dose has been taken, with accidental repeat dosing, or misunderstanding of the dosing regime can result in overdose. More tragically, Digoxin has been the source of drug errors. It is dosed in micrograms rather than milligrams as the majority of medicines are. There are cases in which prescriptions erroneously written as milligrams have resulted in inexperienced nurses giving patients hundreds of tablets in a day to fulfill the prescription. This is very rare, but there are documented cases around the world. Unfortunately for the nurses concerned, their punishment for accurately obeying the drug charts is much more severe than that accorded the prescribing physician.
(ii) Increased sensitivity
Digoxin works by changing the way potassium moves across the membranes of heart muscle cells. Its effects, and toxicity are potentiated by low potassium levels (hypokalaemia). Thus, a normal dose can become toxic with a low potassium.
(iii) Reduced excretion
As mentioned, the body gets rid of digoxin via the kidneys. If there is any degree of renal failure the level of digoxin in the blood will rise, even if the daily dose is the same.
Effects of overdose:
Digoxin is a proarrhythmic agent. This means that it can cause disorders of electrical activity in the heart, which can lead to ventricular tachycardias or fibrilation, which can rapidly lead to death. It produces characteristic changes in the ECG (EKG for silly foreigners) which include the much loved reverse tick sign of a back sloping ST segment. In laymen terms this is just evidence from an electrical heart trace that the heart's activity is being affected by the digoxin - a worrying sign. More non-specific symptoms of overdose include nausea, vomitting, confusion, coma and death. Diagnosis is either clinical, or digoxin blood levels can be measured in the lab - this can take some time.
Treatment:
The majority of cases seen in medical practice are a result of long term changes in renal function. This means very slow increases in digoxin levels, and is treated by simply reducing the dose of digoxin. More rarely, but more seriously is toxicity brought about by changes in potassium level. These are treated by restoring potassium to its correct level.
Rarer still are the large overdoses. These can be very serious. Patients are often very unwell, and may have cardiac arrests, or need ITU management. In these situations the level of active digoxin in the blood must be rapidly decreased. There are two main methods of achieving this. The first is using FAB - fragments of antibodies which bind to digoxin and render it inactive. One such preparation is called Digibind, and is very expensive. The other option is dialysis. This involves direct filtering of the blood. This is an expensive and time consuming procedure and would be used as a last resort.