Key Takeaways
- Digoxin therapeutic range is narrow: 0.5-2.0 ng/mL - toxicity can occur even within "normal" range
- Assess apical pulse for 60 seconds before digoxin; hold if pulse < 60 bpm in adults or < 100 bpm in infants
- Hypokalemia increases digoxin toxicity risk - monitor potassium closely (normal: 3.5-5.0 mEq/L)
- IV Potassium Chloride must NEVER be given IV push - maximum rate is 10-20 mEq/hour via pump
- Magnesium Sulfate toxicity manifests as loss of deep tendon reflexes and respiratory depression; antidote is Calcium Gluconate
High-Alert Medications: Digoxin and Electrolytes
Digoxin and concentrated electrolyte solutions are high-alert medications with narrow margins of safety. Understanding their pharmacology, monitoring parameters, and toxicity signs is essential for safe practice.
Digoxin (Lanoxin)
Mechanism of Action:
- Positive inotropic - Increases force of cardiac contraction
- Negative chronotropic - Decreases heart rate
- Negative dromotropic - Slows conduction through AV node
Indications:
- Heart failure (improves contractility)
- Atrial fibrillation (controls ventricular rate)
Digoxin Monitoring
| Parameter | Normal/Target | Action |
|---|---|---|
| Serum digoxin level | 0.5-2.0 ng/mL | Draw 6-8 hours after dose |
| Apical pulse | > 60 bpm adult | Hold if < 60 bpm |
| Apical pulse | > 100 bpm infant | Hold if < 100 bpm |
| Potassium | 3.5-5.0 mEq/L | Hypokalemia ↑ toxicity risk |
| Creatinine | 0.6-1.2 mg/dL | Renal impairment ↓ clearance |
Critical Concept: Always assess apical pulse for 60 seconds before administration.
Digoxin Toxicity
Digoxin has a narrow therapeutic index - toxicity can occur even with levels within normal range, especially in:
- Elderly patients
- Patients with renal impairment
- Patients with hypokalemia
Early Signs of Toxicity:
| System | Signs |
|---|---|
| GI | Anorexia, nausea, vomiting (often first signs) |
| Visual | Yellow-green halos around lights, blurred vision |
| CNS | Fatigue, weakness, confusion |
| Cardiac | Bradycardia, dysrhythmias |
Cardiac Manifestations:
- Bradycardia
- Heart block
- PVCs (premature ventricular contractions)
- Ventricular tachycardia/fibrillation
Digoxin-Electrolyte Interactions
| Electrolyte Imbalance | Effect on Digoxin |
|---|---|
| Hypokalemia | ↑↑ Toxicity risk (K+ and digoxin compete for same binding sites) |
| Hyperkalemia | ↓ Digoxin effectiveness |
| Hypercalcemia | ↑ Toxicity risk |
| Hypomagnesemia | ↑ Toxicity risk |
Key Point: Patients on digoxin who are also taking diuretics (which deplete potassium) are at high risk for toxicity.
Treatment of Digoxin Toxicity
- Hold digoxin immediately
- Correct electrolyte imbalances (especially potassium)
- Monitor cardiac rhythm continuously
- Administer Digoxin Immune Fab (Digibind) for severe toxicity
- Avoid cardioversion if possible (can trigger fatal arrhythmias)
Potassium Chloride (KCl)
Normal Serum Potassium: 3.5-5.0 mEq/L
Critical Administration Rules
| Rule | Rationale |
|---|---|
| NEVER give IV push | Causes fatal cardiac arrest |
| Maximum rate: 10-20 mEq/hour | Faster rates cause cardiac arrhythmias |
| Maximum concentration: 40 mEq/L peripheral, 80 mEq/L central | Higher concentrations cause phlebitis |
| Always use infusion pump | Prevents accidental bolus |
| Cardiac monitoring required for high-dose infusions | Detect arrhythmias early |
Hypokalemia (K+ < 3.5 mEq/L)
Causes:
- Diuretic therapy (loop, thiazide)
- Vomiting, diarrhea, NG suctioning
- Inadequate intake
Signs and Symptoms:
- Muscle weakness, cramps
- Cardiac arrhythmias (U waves, flat T waves)
- Decreased bowel sounds, ileus
- Polyuria
Hyperkalemia (K+ > 5.0 mEq/L)
Causes:
- Renal failure (cannot excrete K+)
- Potassium-sparing diuretics
- ACE inhibitors/ARBs
- Tissue damage (burns, crush injuries)
Signs and Symptoms:
- Muscle weakness (different mechanism than hypokalemia)
- Cardiac arrhythmias (peaked T waves, widened QRS)
- Paresthesias
Treatment of Severe Hyperkalemia (K+ > 6.5 mEq/L):
| Treatment | Mechanism | Onset |
|---|---|---|
| Calcium gluconate | Stabilizes cardiac membrane | Immediate |
| Regular insulin + D50W | Drives K+ into cells | 15-30 min |
| Sodium bicarbonate | Drives K+ into cells | 15-30 min |
| Kayexalate | Removes K+ from body | Hours |
| Dialysis | Removes K+ from body | Immediate |
Magnesium Sulfate
Normal Serum Magnesium: 1.5-2.5 mEq/L
Indications:
- Pre-eclampsia/eclampsia prophylaxis and treatment
- Torsades de pointes (cardiac arrhythmia)
- Severe hypomagnesemia
Magnesium Sulfate Toxicity
Monitor for signs of hypermagnesemia:
| Magnesium Level | Clinical Effects |
|---|---|
| 4-7 mEq/L | Loss of deep tendon reflexes (DTRs) |
| 5-10 mEq/L | Respiratory depression |
| > 10 mEq/L | Cardiac arrest |
Before Each Dose, Assess:
- Deep tendon reflexes (DTRs) - Hold if absent
- Respiratory rate - Hold if < 12/min
- Urine output - Should be > 30 mL/hour (magnesium excreted by kidneys)
Antidote: Calcium Gluconate 1-2 g IV over 3 minutes
Nursing Considerations for Magnesium Sulfate
| Parameter | Acceptable Range | Action if Outside Range |
|---|---|---|
| DTRs | Present (1-4+) | Hold if absent (0) |
| Respiratory rate | ≥ 12/min | Hold if < 12/min |
| Urine output | ≥ 30 mL/hour | Notify provider if oliguric |
| Serum magnesium | 4-8 mEq/L (therapeutic for pre-eclampsia) | Adjust infusion rate |
Electrolyte Quick Reference
| Electrolyte | Normal Range | Critical Low | Critical High |
|---|---|---|---|
| Potassium | 3.5-5.0 mEq/L | < 2.5 | > 6.5 |
| Sodium | 135-145 mEq/L | < 120 | > 160 |
| Calcium | 8.5-10.5 mg/dL | < 6.0 | > 13.0 |
| Magnesium | 1.5-2.5 mEq/L | < 1.0 | > 5.0 |
On the Exam
- Know digoxin therapeutic range (0.5-2.0 ng/mL)
- Assess apical pulse for 60 seconds before digoxin
- Hypokalemia increases digoxin toxicity
- NEVER give IV potassium push
- Check DTRs before magnesium administration
Before administering digoxin, the nurse assesses the patient's apical pulse and obtains a rate of 54 bpm. The nurse should:
A patient taking digoxin has a potassium level of 3.1 mEq/L. The nurse recognizes that this patient is at increased risk for:
Which sign would the nurse recognize as indicative of digoxin toxicity?
A patient receiving IV magnesium sulfate for pre-eclampsia has absent deep tendon reflexes. The nurse's priority action is: