Digitalis Leaf Powder (Digitalis purpurea)

Definition

Digitalis is the dried leaf of Digitalis purpurea Linn. (Purple Foxglove), family Scrophulariaceae (Plantaginaceae). It is the classical source of cardiac glycosides (digitoxin, digoxin from D. lanata), used for centuries in heart failure and atrial arrhythmias.

General / Essential Features

Anomocytic stomata (irregular cell type): most common type — surrounded by irregular cells, not in any defined arrangement — key feature
Non-glandular trichomes: uniseriate (multi-celled), covering and glandular types; covering trichomes with warty cuticle
Prismatic calcium oxalate crystals: in mesophyll parenchyma cells
Sinuous-walled epidermal cells in surface view — irregular, wavy cell outlines
Spiral and annular vessels from vascular bundles
Lignified fibres: from vascular and mechanical tissue
Palisade ratio: used in quantitative leaf microscopy (Digitalis: 7–10)

Sites

Digitalis purpurea: native to Western Europe — now widely cultivated
Digitalis lanata (Woolly Foxglove): principal source of digoxin — commercially important
Active constituents: digitoxin (D. purpurea), digoxin (D. lanata), gitoxin, lanatoside C
Official in BP and USP as a source of cardiac glycosides

Pathophysiology

Cardiac glycosides inhibit Na+/K+ ATPase in myocardial cells. This increases intracellular Na+ which reduces Na+/Ca2+ exchanger activity, increasing intracellular Ca2+. Enhanced Ca2+ availability increases myocardial contractility (positive inotropy). Simultaneously, vagal stimulation slows the AV node conduction (negative chronotropy and dromotropy) — useful in AF rate control.

Etiology

Heart failure with reduced ejection fraction (HFrEF): positive inotropy increases cardiac output
Atrial fibrillation: rate control via increased vagal tone at AV node
Narrow therapeutic index: digoxin toxicity at serum levels >2 ng/mL
Toxicity risk increased by: hypokalaemia, hypomagnesaemia, renal impairment, hypothyroidism, amiodarone
Herbal poisoning: accidental ingestion of foxglove leaves or lily-of-the-valley

Clinical Features

Therapeutic: improved symptoms in HF, controlled ventricular rate in AF
Digitalis toxicity: nausea, vomiting, xanthopsia (yellow-green colour vision), bradycardia, arrhythmias
ECG: 'reverse tick' or 'Salvador Dalí moustache' ST depression — toxicity sign
Hyperkalaemia: from Na+/K+ ATPase inhibition — marker of severe toxicity
Toxicity triggers: hypokalaemia (increases receptor affinity), verapamil, amiodarone interaction

Diagnosis

Microscopy: anomocytic stomata, uniseriate trichomes, prismatic crystals, sinuous-walled epidermis
Keller-Kiliani test: glacial acetic acid + FeCl3 → blue-green (specific for 2-deoxy sugar of cardiac glycosides)
Serum digoxin level: therapeutic range 0.5–2 ng/mL
ECG: scooped ST segment depression, PR prolongation
Digitalis-specific Fab antibody fragments (Digibind/DigiFab): specific treatment for severe toxicity

Treatment

HF: digoxin 0.125–0.25 mg/day orally; target serum level 0.5–0.9 ng/mL
AF rate control: loading dose followed by maintenance (0.125–0.25 mg/day)
Digitalis toxicity: withhold drug, correct hypokalaemia, anti-arrhythmics for arrhythmias
Severe toxicity: Digibind (digoxin-specific Fab antibody) — each vial binds ~0.5 mg digoxin
Avoid in: hypertrophic obstructive cardiomyopathy (HOCM), Wolff-Parkinson-White syndrome

References

Trease, G.E. & Evans, W.C. (2009). Pharmacognosy (16th ed.). Elsevier Saunders.
Ponikowski, P., et al. (2016). 2016 ESC Guidelines for heart failure. European Heart Journal, 37(27), 2129–2200.
Indian Pharmacopoeia Commission. (2018). Indian Pharmacopoeia 2018 (8th ed.). Government of India.