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Cardiac Glycosides

NCLEX Review Guide: Cardiac Glycosides

Cardiac Glycosides Overview

Mechanism of Action

  • Cardiac glycosides inhibit the sodium-potassium ATPase pump, leading to increased intracellular calcium and enhanced myocardial contractility. This positive inotropic effect strengthens heart contractions while simultaneously slowing heart rate through vagal stimulation.
  • The primary medication is digoxin (Lanoxin), which has a narrow therapeutic window requiring careful monitoring. It also produces negative chronotropic and dromotropic effects, slowing conduction through the AV node.

Memory Aid: "DIG-IT"

  • Digoxin toxicity signs
  • Inotropic (positive) - stronger contractions
  • GI symptoms first (nausea, vomiting)
  • Interactions with diuretics
  • Therapeutic level: 0.5-2.0 ng/mL

Key Points

  • Therapeutic range: 0.5-2.0 ng/mL (toxicity >2.0 ng/mL)
  • Half-life: 36-48 hours in patients with normal kidney function
  • Primarily excreted by kidneys - dose adjustment needed in renal impairment

Clinical Applications & Administration

Indications

  • Heart failure with reduced ejection fraction - improves cardiac output and reduces symptoms of fluid retention. Digoxin is particularly beneficial in patients who remain symptomatic despite optimal therapy with ACE inhibitors and beta-blockers.
  • Atrial fibrillation rate control - slows ventricular response by increasing AV node refractoriness. Most effective when combined with beta-blockers or calcium channel blockers for optimal rate control.

    Administration Guidelines

  1. Check apical pulse for 1 full minute before administration
  2. Hold medication if pulse <60 bpm in adults or <90 bpm in infants
  3. Monitor serum digoxin levels 6-8 hours post-dose
  4. Assess for signs of toxicity before each dose
  5. Evaluate electrolyte levels, especially potassium and magnesium

Toxicity & Adverse Effects

Signs of Digoxin Toxicity

  • Early GI symptoms: anorexia, nausea, vomiting, diarrhea - these are often the first indicators of toxicity and should prompt immediate evaluation. Patients may also report abdominal pain and unusual fatigue.
  • Cardiac arrhythmias: PVCs, bigeminy, AV blocks, atrial tachycardia with block - these potentially life-threatening effects require immediate intervention and continuous cardiac monitoring.
  • Visual disturbances: yellow-green halos, blurred vision, photophobia - classic but late signs that indicate significant toxicity requiring immediate medical attention.

Toxicity vs. Therapeutic Effects

TherapeuticToxic
Improved exercise toleranceFatigue, weakness
Controlled heart rate (60-100)Bradycardia (<60) or tachyarrhythmias
Reduced edemaGI upset, visual changes

Key Points

  • Hypokalemia increases digoxin toxicity risk
  • Elderly patients more susceptible due to decreased renal function
  • Antidote: Digoxin immune FAB (Digibind)

Drug Interactions & Monitoring

Significant Interactions

  • Diuretics (especially loop and thiazide) increase digoxin toxicity risk by causing hypokalemia and hypomagnesemia, which enhance digoxin's effects on the heart. Regular electrolyte monitoring is essential when these medications are used concurrently.
  • Quinidine, verapamil, and amiodarone increase digoxin levels by inhibiting renal clearance and displacing digoxin from tissue binding sites. Digoxin dose reduction of 25-50% may be necessary.

Clinical Scenario

Mrs. Johnson, 78 years old, takes digoxin 0.25 mg daily for heart failure. She's prescribed furosemide 40 mg daily for increasing edema. After one week, she reports nausea and sees yellow halos around lights. Her digoxin level is 2.8 ng/mL.

Nursing Actions: Hold digoxin, notify physician, check electrolytes (especially K+ and Mg2+), monitor cardiac rhythm, assess for other toxicity signs.

Key Points

  • Monitor digoxin levels, renal function, and electrolytes regularly
  • Drug interactions can double or triple digoxin levels
  • Patient education on recognizing early toxicity symptoms is crucial

Commonly Confused Points

ConceptCorrect InformationCommon Misconception
Pulse ParametersHold if <60 bpm (adults)Hold if <70 bpm
Therapeutic Level0.5-2.0 ng/mL1.0-2.5 ng/mL
First Toxicity SignGI symptoms (nausea)Visual changes
Electrolyte EffectLow K+ increases toxicityHigh K+ increases toxicity

Study Tips

  • Remember "60 and Go" - if pulse ≥60, medication can go
  • Think "Banana Split" - low potassium (banana) splits the heart rhythm
  • "See Yellow, Think Toxic" - visual changes indicate serious toxicity

Quick Check

Self-Assessment

  • I can identify the therapeutic range for digoxin
  • I know when to hold digoxin based on pulse rate
  • I can recognize early signs of digoxin toxicity
  • I understand why hypokalemia increases toxicity risk
  • I know the antidote for digoxin toxicity

Common Pitfalls

  • Forgetting to check apical pulse before administration
  • Confusing therapeutic levels with toxic levels
  • Missing the connection between diuretic use and increased toxicity risk
  • Not recognizing GI symptoms as early toxicity indicators

Remember: Cardiac glycosides require vigilant monitoring and patient education. Your attention to detail in assessing pulse, recognizing toxicity signs, and understanding drug interactions will ensure safe patient care. You've got this - stay focused on the key concepts and trust your nursing knowledge!

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