682 Cardiovascular and Renal System Drugs: Cardiac Glycosides

This lecture specifically focuses on cardiac glycosides, with a particular emphasis on digoxin's pharmacological effects, mechanisms, and clinical considerations.

Inotropic Effects: Relate to the force of contraction of the heart muscle.
- Positive Inotropes: Increase the force of contraction (e.g., cardiac glycosides like digoxin).
- Negative Inotropes: Decrease the force of contraction (e.g., beta blockers).

Chronotropic Effects: Relate to the heart rate.
- Positive Chronotropes: Increase the heart rate (e.g., dopamine).
- Negative Chronotropes: Decrease the heart rate (e.g., beta blockers).

Dromotropic Effects: Relate to impulse conduction through the heart.
- Positive Dromotropes: Increase impulse conduction (e.g., phenytoin).
- Negative Dromotropes: Decrease impulse conduction (e.g., verapamil).

Digoxin primarily works by inhibiting the sodium-potassium ATPase pump.
Key action points include:
- Normally, this enzyme regulates the influx of sodium (Na⁺) and efflux of potassium (K⁺) after action potentials.
- Inhibition by digoxin results in:
- Increased Sodium Influx: Na⁺ enters the cell while potassium is exchanged out.
- Reduced Calcium Efflux: The excess sodium retention reduces the exit of calcium (Ca²⁺), leading to:
  - Calcium Accumulation: This accumulation within the cardiac myocytes increases contractile forces, enhancing cardiac contractility.
Competition with potassium ions:
- Hypokalemia: Increases the risk of digoxin binding and toxicity.
- Hyperkalemia: Reduces the effectiveness of digoxin.

Increased contractility leads to elevated cardiac output.
Additional effects include:
- Decreased sympathetic tone promoting arterial pressure reduction, enhancing:
- Ventricular filling.
- Decreased afterload for better ventricular emptying.
- Reduced edema.
Increased urinary output results from enhanced cardiac output, improving renal blood flow.
Decreased release of renin due to elevated arterial pressure leads to:
- Reduced production of angiotensin II, lowering afterload.
- Reduced production of aldosterone, leading to decreased blood volume.

Digoxin enhances vagal effects on the heart by:
- Facilitating greater firing and responsiveness of the SA node to acetylcholine.
Resulting effects:
- Promotion of impulse formation and conduction.

Monitoring potassium levels is critical to minimize the risk of arrhythmias.
Therapeutic drug levels for digoxin are between 0.5 to 0.8 ng/mL.
Overdose Management:
- Antidote: Digibind (DigiFab).
- Caution in using antiarrhythmics like quinidine which can increase digoxin levels, while phenytoin and lidocaine are considered acceptable.

Dietary Impact: High fiber diets can reduce digoxin absorption.
Overall food intake delays the rate of absorption but does not change total absorption extent.
Variability in Bioavailability: Patients should remain consistent with their specific digoxin brand (brand vs. generic).
Protein Binding: 23% of digoxin binds to albumin; attention to nutritional status is crucial.
Hepatic Metabolism: Minimal hepatic metabolism, requiring dose adjustments in renal impairment.
- Half-life: Approximately 1.5 days.

Past practices favored digitalization (loading doses); however, current guidelines do not recommend it.
Patients can achieve therapeutic levels without loading doses within one week.
After the last dose, complete elimination takes around seven days.

There is significant inter-patient variability; thus, drug levels are not always reliable indicators of efficacy or adverse effects.

Contraindications for using digoxin:
- Patients with AV block.
- Uncontrolled ventricular arrhythmias.
- Severe renal impairment.
Cautionary use in:
- Patients with chronic kidney disease and hypothyroidism (requires dose reduction due to decreased volume of distribution).
- Malnourished patients with lower albumin levels (leads to increased drug effects).
- Electrolyte abnormalities: Monitoring potassium levels is essential for both hypokalemia and hyperkalemia.
- Pregnancy and lactation: Use extremely cautiously.

Common adverse effects include:
- Gastrointestinal: Diarrhea, anorexia, nausea, vomiting (due to drug's effect on the chemoreceptor trigger zone).
- CNS effects: Fatigue, disorientation, visual disturbances, hallucinations.
- Toxicity Indicator: Presence of yellow halos around lights.
- Gynecomastia: Rare potential side effect.

Weight Monitoring: Patients should report if they gain over 2 lbs/day or 5 lbs/week.
Heart Rate Monitoring: If heart rate <60 or >100, patients should consult their provider.
Drug Levels: Essential for titrating doses; measured after 4-5 half-lives (approximately 1-2 weeks post-therapy).

Toxicity manifestations in pediatric patients may include:
- Atrial arrhythmias or tachycardia.
In adults, toxicity presents significantly in patients with renal impairment, with symptoms emerging when levels exceed 2 ng/mL.

Hypokalemia-inducing drugs: Diuretics (thiazides, loops) increase the risk of digoxin toxicity and should be monitored.
Hyperkalemia-inducing agents: ACE inhibitors and ARBs may reduce digoxin efficacy.
Use of sympathomimetics: May complement digoxin effects but also pose risks for arrhythmias.
Avoid: Key antiarrhythmics such as quinidine that can displace digoxin and increase toxicity risks.
Caution with calcium channel blockers (like verapamil, diltiazem) as they can elevate digoxin levels, requiring dosage adjustments.

Digoxin plays a crucial role in managing heart conditions but requires meticulous monitoring of patient status and interactions for safe and effective use. Supportive dietary and lifestyle education is vital for optimal outcomes as well.