Comprehensive Study Notes on Digoxin and Cardiac Glycosides

Digoxin is categorized as a cardiac glycoside.
It is historically obtained from digitalis, a chemical compound derived from a plant referred to as the Volksgloff.
Traditionally, this plant was used to strengthen the heart.
Therapeutic applications include the treatment of supraventricular tachycardia ( $SVT$ ), which encompasses conditions such as atrial fibrillation ( $AF$ ).

Digoxin is described as a unique agent that targets the central nervous system ( $CNS$ ) while simultaneously increasing parasympathetic activity at the level of the heart.
Direct cardiac effects: - It decreases cardiac conduction specifically through the AP node. - This causes a reduction in heart rate. - It acts as a negative chromatropic agent.
Contractile effects: - It increases the force of cardiac contraction. - It acts as a positive iodotropic agent.

Normal physiological function of the cardiomyocyte: - The Sodium Potassium ( $Na+/K+$ ) ATPase pump facilitates the inward movement of $K^+$ ions in exchange for outward movement of $Na^+$ ions to maintain membrane potential. - The membrane potential results in a more positive charge at the outer membrane compared to the interior of the cardiomyocyte. - An exchange mechanism allows $3\,Na^+$ ions to re-enter the cardiomyocyte in exchange for the exit of $1\,Ca^{2+}$ ion.
Pharmacological action of Digoxin: - Digoxin inhibits the Sodium Potassium ( $Na+/K+$ ) ATPase pump. - This inhibition prevents $K^+$ ions from entering the cardiac myocytes. - Consequently, $Na^+$ ions become trapped within the cardiac myocytes. - Because the membrane potential does not fluctuate as usual, the exchange between $Na^+$ and $Ca^{2+}$ ions does not take place. - This leads to the increased retention of $Ca^{2+}$ ions within the cardiomyocytes.
Resultant impact on contractility: - When an action potential occurs, the elevated levels of intracellular $Ca^{2+}$ stimulate the sarcoplasmic reticulum. - This process increases cardiac contractility, finalizing the positive iodotropic effect.

Administration and Bioavailability: - The drug is administered orally and demonstrates good bioavailability. - Dose adjustment is necessary for a specific sub-population because up to $50\,\%$ of the drug is metabolized into an inactive metabolite within the gut.
Elimination and Half-life: - Digoxin possesses a long half-life, comparable to the antiarrhythmic agent amiodarone. - It is excreted by the kidneys via the P-glycoprotein ( $PGP$ ) efflux pump.
Therapeutic Index: - Digoxin has a very narrow therapeutic index. - Small fluctuations in plasma concentration can make patients highly susceptible to significant adverse effects.

Cardiovascular Effects: - Altered cardiac conduction leads to an increased risk of arrhythmia or cardiac arrest.
Central Nervous System ( $CNS$ ) Effects: - Headache and drowsiness. - Fatigue and confusion. - Blurred vision and altered color vision. - Documentation through an anecdote: A painting of a doctor was created by a patient being treated with the Volksgloff plant; the patient suffered from altered color vision (xanthopsia), resulting in a drawing with a distinctly yellow hue.
Gastrointestinal Effects: - Vomiting. - Diarrhea.

Heart Rate Modulators: Interaction with other drugs that decrease heart rate can precipitate severe bradycardia.
Hypokalemia-inducing Drugs: Significant toxicities can occur if Digoxin is combined with any medication that causes low potassium ( $K^+$ ) levels.
Cholestyramine: This agent can bind to Digoxin, which increases its elimination from the body.
Verapamil: This drug inhibits the P-glycoprotein ( $PGP$ ) pump in the kidneys, leading to decreased excretion and increased plasma concentrations of Digoxin.
Calcium Supplements: - Digoxin increases $Ca^{2+}$ levels within cardiac myocytes by inhibiting the Sodium ATPase pump. - Administering calcium supplements increases plasma $Ca^{2+}$ , enhancing the positive iodotropic effect excessively. - This interaction can result in heart damage and should be avoided unless strictly supervised and calcium levels are monitored.

The Function of the Sodium Potassium ATPase Pump: - $Na^+$ ions bind to the inner side of the pump, stimulating phosphorylation. - Phosphorylation alters the pump's structure: the inner section closes, and the exterior opens to release $Na^+$ icons. - Subsequently, $2\,K^+$ ions bind to the exterior side, which triggers the removal of the phosphate group. - This dephosphorylation cause the pump to revert to its original shape, moving $K^+$ into the cell.
The Role of Digoxin and Potassium: - Digoxin binds preferentially to the pump when it is in its phosphorylated state. - $K^+$ ions normally inhibit phosphorylation and promote the removal of the phosphate group. - In states of hypokalemia (insufficient $K^+$ ), the pump remains stuck in the phosphorylated state. - This increases the number of binding sites for Digoxin, promoting Digoxin toxicity.