digitalis
Meaning of digitalis: A garden flower that became a heart drug, still in use after two centuries, with a famously narrow margin for error.
Definition of digitalis
Digitalis is the plant genus of the foxgloves and, by extension, the group of heart medicines prepared from their leaves. These compounds, known as cardiac glycosides, make heart muscle squeeze more forcefully and slow the rate at which electrical signals reach the pumping chambers. Digoxin is the member still in general use. The same chemistry makes every part of the plant poisonous.
How does it strengthen the beat?
By blocking a pump. Every heart muscle cell runs a pump that pushes sodium out and pulls potassium in, and these compounds jam it. Sodium builds up inside the cell, which slows a second exchanger that normally trades sodium coming in for calcium going out. Calcium is what makes muscle contract, so more of it left inside means a harder squeeze. A separate effect works through the vagus nerve, increasing its braking action on the relay station between the upper and lower chambers of the heart. That is why the drug slows a racing pulse in atrial fibrillation, and it is now the more common reason for prescribing it.
Which plant, and which drug?
| Source | Compound | Standing |
|---|---|---|
| Woolly foxglove | Digoxin | The preparation in general use |
| Common foxglove | Digitoxin | Longer acting, cleared by the liver, now rare |
| Whole dried leaf | Digitalis leaf | The historic preparation, of uneven strength |
Why is the margin so narrow?
The helpful dose sits close to the poisonous one. Potassium competes with the drug for the same site on the pump, so a low potassium level, often the result of a water tablet taken for the same heart condition, deepens the effect without any change in dose. The kidneys clear digoxin, so failing kidneys let it accumulate. Poisoning brings nausea, appetite loss, confusion and disturbed color vision, classically yellow-green halos around lights, along with dangerous rhythms. Antibody fragments that bind the drug are held as an antidote. Foxglove growing in a garden carries the same compounds, and swallowing the leaves produces the same illness. In heart failure the drug eases symptoms and reduces admissions to hospital without lengthening life, so it now sits behind other treatments rather than at the front, and the cardiac monitoring it requires is part of the reason.
Also known as
Used in a sentence
Digitalis toxicity was suspected when nausea and visual disturbance followed a change in kidney function.
Good to know
Digoxin, the digitalis preparation still in use, is extracted from the woolly foxglove rather than the common foxglove, and a low potassium level increases its toxicity at an unchanged dose.
Word origin
Where “digitalis” comes from
From the Latin digitus, a finger, describing the flower of the foxglove, which slips over a fingertip like a thimble.