Before We Start
What are electrolytes and why do they matter?
Electrolytes are minerals in the body that carry an electrical charge. They're found in blood, urine, and cells — and they control critical functions including nerve conduction, muscle contraction (including the heart), fluid balance, and acid-base balance.
The most important thing to understand about electrolytes is WHERE they live. The body maintains precise concentrations of each electrolyte inside and outside of cells. When those concentrations shift — due to illness, medications, or disease — serious complications occur.
💡 PISO — The Foundation
Potassium lives Inside cells (intracellular)
Sodium lives Outside cells (extracellular)
This is the single most important electrolyte fact in nursing. Everything else about electrolyte imbalances builds on knowing this.
Intracellular vs Extracellular — what these terms mean
Intracellular = inside the cell. The fluid inside cells is called intracellular fluid (ICF). It makes up about 2/3 of total body water.
Extracellular = outside the cell. The fluid outside cells includes blood plasma and interstitial fluid (fluid between cells). It makes up about 1/3 of total body water.
The cell membrane separates these two compartments. Electrolytes can move across the membrane through channels and pumps — but the body works hard to maintain the right concentration in each compartment.
Potassium — Inside
K⁺: the primary intracellular cation
Normal serum (blood) potassium: 3.5–5.0 mEq/L
Potassium is the dominant positive ion INSIDE cells. About 98% of the body's potassium is intracellular. The blood level (3.5–5.0) represents only the tiny fraction that has moved outside cells into the bloodstream.
Potassium's primary job is to maintain the electrical potential across cell membranes — which is what allows nerves to fire and muscles (including the heart) to contract.
Hypokalemia — Low Potassium (below 3.5)
Causes: vomiting, diarrhea, diuretics (especially loop and thiazide), poor intake
Signs and symptoms:
• Muscle weakness and cramps (most common)
• Fatigue
• Constipation
• Cardiac: EKG changes — flattened T waves, U waves (extra wave after T), dysrhythmias
• Severe: paralysis, respiratory failure, life-threatening dysrhythmias
Nursing priority: Monitor EKG. Never give IV potassium as a bolus — always diluted, always via infusion pump, max rate 10–20 mEq/hr. Burns the vein and can cause fatal cardiac arrhythmia if pushed fast.
💊 "Potassium kills" — IV potassium is a high-alert medication. NEVER push undiluted IV potassium. This is a cause of preventable death in hospitals.
Hyperkalemia — High Potassium (above 5.0)
Causes: renal failure, acidosis, ACE inhibitors/ARBs, potassium-sparing diuretics, hemolysis
Signs and symptoms:
• Muscle weakness (same as hypokalemia — confusing!)
• Paresthesias (tingling)
• Cardiac: peaked T waves (earliest EKG sign), widened QRS, sine wave pattern, ventricular fibrillation
• Severe: cardiac arrest
Treatment priorities:
1. Calcium gluconate IV — stabilizes the cardiac membrane (buys time — does NOT lower K⁺)
2. Sodium bicarbonate / Insulin + dextrose — shifts K⁺ into cells temporarily
3. Kayexalate (sodium polystyrene) or patiromer — removes K⁺ from the body
4. Dialysis — definitive treatment in severe cases
💊 Peaked T waves on EKG + potassium above 6.0 = cardiac emergency. Notify provider immediately. Get calcium gluconate ready.
Sodium — Outside
Na⁺: the primary extracellular cation
Normal serum sodium: 135–145 mEq/L
Sodium is the dominant positive ion OUTSIDE cells — in the blood and interstitial fluid. It's the primary determinant of blood osmolarity (the concentration of dissolved particles in the blood) and therefore controls the distribution of water between compartments.
The key rule: Water follows sodium. Where sodium goes, water goes. This is the foundation of IV fluid selection and fluid balance management.
Hyponatremia — Low Sodium (below 135)
Causes: excess water retention, vomiting/diarrhea with water replacement only, SIADH, heart failure, cirrhosis
Signs and symptoms (brain swells as water moves in):
• Mild: nausea, headache, fatigue
• Moderate: confusion, muscle cramps, lethargy
• Severe: seizures, coma, herniation
Critical caution: Correct sodium SLOWLY. Correcting too fast causes osmotic demyelination syndrome (central pontine myelinolysis) — permanent neurological damage. Max correction rate: 8–10 mEq/L per 24 hours.
Treatment: Fluid restriction for dilutional hyponatremia. Normal saline for volume-depleted hyponatremia. Hypertonic saline (3% NaCl) only for severe symptomatic hyponatremia — requires ICU monitoring.
💊 Correct sodium slowly — "too fast, the bridge collapses" (osmotic demyelination). Max 8–10 mEq/L per 24 hours in chronic hyponatremia.
Hypernatremia — High Sodium (above 145)
Causes: dehydration, inadequate fluid intake, diabetes insipidus, excessive sodium intake
Signs and symptoms (brain shrinks as water moves out):
• Thirst (earliest symptom)
• Dry mucous membranes
• Confusion, irritability, restlessness
• Muscle twitching, seizures
• Severe: coma
Critical caution: Also correct slowly. Rapid correction causes cerebral edema.
Treatment: Free water replacement — oral water or D5W IV (effectively free water once glucose metabolized). Identify and treat the underlying cause.
💊 Hypernatremia = too little water relative to sodium. Treatment = add water. But add it slowly — rapid correction = brain swelling.
Why This Matters for IV Fluids
Choosing the right IV fluid — isotonic, hypotonic, hypertonic
Isotonic fluids — same osmolarity as blood (stays in vessels)
Normal Saline (0.9% NaCl), Lactated Ringer's, D5W (isotonic in the bag — becomes hypotonic in the body).
Used for: volume replacement, dehydration, hemorrhage. Stays in the intravascular space.
Hypotonic fluids — lower osmolarity than blood (moves water INTO cells)
0.45% NaCl (half normal saline), 0.33% NaCl.
Used for: cellular dehydration, hypernatremia. Caution: can worsen cerebral edema in head injury patients.
Hypertonic fluids — higher osmolarity than blood (pulls water OUT of cells)
3% NaCl, D5 0.9% NaCl, D10W.
Used for: severe hyponatremia, cerebral edema (pulls fluid out of brain). Must be given via central line with continuous monitoring.
📌 NCLEX Application
Electrolyte imbalances are heavily tested on NCLEX. Know the normal ranges, key signs, and priority interventions cold.
Potassium priority facts:
• Never IV push potassium — always diluted, always on a pump
• Hypokalemia + digoxin = dangerous (hypokalemia potentiates digoxin toxicity)
• Check potassium before giving digoxin — hold if below 3.5
Sodium priority facts:
• SIADH (syndrome of inappropriate ADH) = low sodium from excess water retention
• Diabetes insipidus = high sodium from insufficient ADH (excessive dilute urine output)
• Both are corrected slowly — rapid correction of either causes neurological damage
✓ Quick Self-Test
Answer each question:
1. Normal potassium range? Normal sodium range?
2. A patient with renal failure has a potassium of 6.2. What EKG change do you expect first?
3. Why is IV potassium never pushed as a bolus?
4. A patient with heart failure has sodium of 128. What is the likely cause and treatment?
5. Where does potassium normally live — inside or outside the cell?
Answers:
1. K⁺: 3.5–5.0 mEq/L · Na⁺: 135–145 mEq/L
2. Peaked (tall, narrow) T waves — earliest EKG sign of hyperkalemia
3. Rapid IV potassium causes fatal cardiac dysrhythmia
4. Dilutional hyponatremia from fluid retention. Treatment: fluid restriction, treat underlying heart failure
5. Inside (intracellular) — that's the "I" in PISO
⚠️ The Trap — Confusing Hypo and Hyper Presentations
Both hypokalemia AND hyperkalemia cause muscle weakness. Both hyponatremia AND hypernatremia cause neurological symptoms. Students try to memorize unique symptoms for each — but the overlapping presentations are the trap.
The way out: always check the lab value. Clinical presentation alone cannot reliably distinguish high from low electrolytes. When you suspect an electrolyte imbalance, get a BMP. The number tells you what the symptoms cannot.
NCLEX will give you clinical signs AND a lab value. Use both together — not one without the other.
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BEEP — Bleeding Precautions
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