Foxglove

Foxgloves are a genus of flowering plants with eponymous tubular, finger/glove-like flowers. While native to Europe and western Asia, foxgloves can now also be found across the northeast and northwest regions of the USA. It is occasionally mistaken for the harmless comfrey, which is consumed as a tea in alternative medicine.

All parts of foxgloves — but especially the leaves — contain several structurally-related cardiac glycosides, with digoxin being the compound of primary interest. A mainstay of poisoners fictional and near-modern, and one of the oldest heart medications, digoxin acts by inhibiting the sodium-potassium-ATPase, resulting in a buildup of intracellular sodium and extracellular potassium. This, in turn, inhibits the action of the sodium-calcium exchanger and leads to increased intracellular calcium concentration. This potentiates greater contraction by cardiac myocytes as well as increased delay between AV node firings, making it useful for the treatment of heart failure and arrhythmias.

Presentation:

After ingestion of foxglove, patients initially present with undifferentiated gastrointestinal symptoms — abdominal pain, nausea, and vomiting. They later progress to symptoms of other systems including dizziness, altered mentation, arrhythmias, and heart block. Patients who overdose on pharmacologically-prepared digoxin tend to present with cardiac arrhythmias earlier. Visual disturbances — blurred vision, haloes, and xanthopsia — are rarer but if present can be useful in narrowing down the diagnosis.

Classically, the primary ECG findings are supraventricular tachycardia (including atrial fibrillation and atrial flutter) with premature ventricular complexes and varying degrees of heart block. Bradycardia may also be observed. A "sagging" ST-segment and inverted or biphasic T-waves may also be observed at therapeutic (and higher) levels of digoxin.

If the patient is known to have been prescribed digoxin, a digoxin blood level should be drawn. The therapeutic window falls between 0.5-2.0 ng/mL, while toxicity is observed as soon as 2.4 ng/mL. Hyperkalemia of varying severity will also be seen on a basic metabolic panel.

Management:

GI decontamination can be considered, but most patients will present well after the window of effectiveness.

Initial treatment should comprise stabilization of the cardiac membrane (e.g. magnesium sulfate infusion) and termination of arrhythmias. Cardiac pacing may be necessary for severe or refractory arrhythmias.

The efficacy of digoxin-specific immunoglobulin fragments in reducing mortality from digoxin toxicity has not been established in large clinical trials, but numerous case series have demonstrated resolution of symptoms after digoxin immune fab administration. It should be strongly considered in cases with life-threatening arrhythmias, cardiac arrest, or hyperkalemia.

Digoxin immune fab may be less useful when toxicity results from foxglove ingestion, potentially due to the presence of other types of cardiac glycosides.