Fluid & Electrolyte Imbalances

Electrolyte Relationships

Ranges vary by lab, facility, and textbook — for exams, use the range given in the question. See the full NCLEX-PN lab-value reference & sources →

Know which electrolytes are "frenemies" (opposites) and which are "besties" (same direction).

Inversely Proportional (Opposites)

• Sodium ↔ Potassium — "frenemies." If one goes up, the other tends to go down. Aldosterone retains sodium and excretes potassium.
• Calcium ↔ Phosphorus — inverse relationship. When calcium is high, phosphorus is low (and vice versa). Critical in CKD.

Work Together (Same Direction)

• Magnesium + Calcium — "besties." Both rise and fall together. You often can't correct low calcium until you correct low magnesium first.
• Calcium + Vitamin D — absorption partners. Vitamin D is needed for calcium absorption from the gut.
• Sodium + Chloride — "boxy twins." Always move together. Hyperchloremia mirrors hypernatremia.

The Potassium-Digoxin Connection

Hypokalemia increases the risk of digoxin toxicity. Potassium and digoxin compete for the same binding sites on the heart. When K⁺ is low, more digoxin binds → toxicity. Always check K⁺ in patients on digoxin.

Sodium (Na⁺)

Normal: 135–145 mEq/L. "Where sodium goes, water follows."

Sodium is the primary extracellular electrolyte. It regulates fluid balance, blood pressure, blood volume, and nerve/muscle function. The key concept: sodium and water move together. High sodium = water retention. Low sodium = water loss.

Think

Hyponatremia (<136): Diluted blood (too much water OR too little sodium). Hypernatremia (>145): Concentrated blood (too little water OR too much sodium).

Common causes

Hypo: SIADH, water intoxication, diuretics, vomiting/diarrhea, heart failure, excessive hypotonic IV fluids. Hyper: Dehydration, diabetes insipidus, excessive sodium intake, fever, burns, Cushing's.

Neuro signs

Hypo: Confusion, headache, seizures, lethargy → coma. Hyper: Confusion, seizures, restlessness, irritability.

GI signs

Hypo: Nausea, vomiting, abdominal cramps. Hyper: Thick/swollen tongue, dry/sticky mucous membranes, extreme thirst.

Cardiovascular

Hypo: Tachycardia, hypotension. Hyper: Hypertension (fluid retention), edema, tachycardia.

Urine

Hypo: Dilute (low specific gravity). Hyper: Concentrated, small volume (high specific gravity).

Treatment

Hypo: Fluid restriction (if dilutional), hypertonic saline (3% NS) for severe — correct SLOWLY to prevent cerebral edema. Hyper: Hypotonic IV fluids (0.45% NS), increase oral water intake, treat underlying cause.

Potassium (K⁺)

Normal: 3.5–5.0 mEq/L. The most dangerous electrolyte imbalance — directly affects the heart.

Potassium is the primary intracellular electrolyte. It is critical for cardiac conduction, nerve impulse transmission, and muscle contraction. Even small changes outside the normal range can cause life-threatening arrhythmias.

Common causes

Hypo (<3.5): Loop/thiazide diuretics, vomiting, diarrhea, NG suction, alkalosis, not eating. Hyper (>5.0): Renal failure (#1), potassium-sparing diuretics, tissue destruction (burns, crush injuries), acidosis, excessive K⁺ supplements.

Cardiac/EKG

Hypo: Flat/inverted T waves, prominent U wave, ST depression, arrhythmias. Hyper: Tall peaked T waves ("tombstone T waves"), widened QRS, bradycardia → V-fib → cardiac arrest.

GI

Hypo: Decreased bowel sounds, constipation, paralytic ileus ("silent bowel"). Hyper: Hyperactive bowel sounds, diarrhea, nausea, cramping ("boom boom boom").

Neuromuscular

Hypo: Muscle weakness, leg cramps, fatigue, diminished reflexes. Hyper: Muscle weakness (progressing to flaccid paralysis), numbness, tingling.

Treatment

Hypo: K⁺-rich foods (bananas, oranges, potatoes, spinach), IV potassium (ALWAYS diluted, NEVER push, via pump), K⁺ supplements PO. Hyper: Calcium gluconate IV (protect heart FIRST), insulin + D50 (shift K⁺ into cells), Kayexalate (excrete K⁺), dialysis if severe.

Critical Potassium Rules

• IV potassium must ALWAYS be diluted and given via infusion pump — NEVER given IV push (causes fatal cardiac arrest)
• Maximum peripheral rate: usually 10 mEq/hr — faster rates require a central line with continuous cardiac monitoring and a provider order (may burn — patient may report pain at IV site)
• Never add potassium to a hanging IV bag (uneven distribution)
• Check urine output BEFORE giving potassium — if <30 mL/hr, HOLD (kidneys excrete K⁺; impaired kidneys → hyperkalemia)
• Monitor EKG continuously when giving IV potassium

Calcium (Ca²⁺)

Normal: 9.0–11.0 mg/dL (reference ranges vary by lab; use the value a question provides). Know Chvostek's and Trousseau's — tested on every exam.

Calcium is essential for bone formation, muscle contraction, blood clotting, and nerve transmission. Parathyroid hormone (PTH) regulates calcium levels. Vitamin D is required for calcium absorption.

Common causes

Hypo (<9.0): Hypoparathyroidism (post-thyroidectomy), CKD (can't activate Vit D), pancreatitis, inadequate intake, low albumin. Hyper (>11.0): Hyperparathyroidism, cancer (bone metastasis), prolonged immobilization, excessive Vit D/Ca²⁺ intake, thiazide diuretics.

Neuromuscular

Hypo: INCREASED excitability — muscle cramps, spasms, tetany, seizures, laryngospasm. Hyper: DECREASED excitability — muscle weakness, fatigue, diminished reflexes, lethargy.

Special signs

Hypo: Chvostek's sign (facial twitch when cheek tapped) and Trousseau's sign (carpopedal spasm with BP cuff inflated 3 min). Hyper: "Bones, stones, groans, and moans" — bone pain, kidney stones, abdominal pain, confusion/depression.

Cardiac

Hypo: Prolonged QT interval, arrhythmias. Hyper: Shortened QT interval, heart block, cardiac arrest.

Treatment

Hypo: IV calcium gluconate (slowly), oral calcium + Vit D, correct magnesium first if low. Hyper: IV NS hydration (dilutes calcium), loop diuretics (furosemide — promotes Ca²⁺ excretion), calcitonin, bisphosphonates.

Magnesium (Mg²⁺)

Normal: 1.7–2.2 mg/dL (some references teach 1.5–2.5 mEq/L — the units differ, so don't mix them). "Magnesium is the sheriff in town."

Magnesium plays a critical role in nerve and muscle function, cardiac rhythm, and enzyme activation. It is closely linked to calcium — both are "besties" that rise and fall together. You often cannot correct hypocalcemia until hypomagnesemia is corrected first.

Think

Hypo: "Sheriff left town — all hell broke loose" (everything speeds up and gets chaotic). Hyper: "Sheriff in town — everyone slows down" (everything is suppressed).

Common causes

Hypo (<1.7): Chronic alcoholism (#1), malnutrition, loop diuretics, diarrhea, NG suction. Hyper (>2.2): Renal failure (#1 — can't excrete Mg), excessive Mg intake (antacids, laxatives), Mg sulfate overdose (OB).

Neuromuscular

Hypo: Hyperexcitability — tremors, hyperactive reflexes, muscle cramps, seizures, positive Chvostek's/Trousseau's. Hyper: Depression — decreased reflexes (first sign of toxicity), lethargy, drowsiness, muscle weakness, respiratory depression.

Cardiac

Hypo: Torsades de Pointes — potentially fatal "twisting" arrhythmia. Also: flat T waves, PVCs. Hyper: Bradycardia, hypotension, heart block, cardiac arrest (at very high levels).

Treatment

Hypo: IV magnesium sulfate (slowly — too fast causes flushing, hypotension), oral Mg supplements, increase dietary Mg. Hyper: Stop Mg sources, IV calcium gluconate (antidote), IV fluids, loop diuretics (if kidneys work), dialysis if severe.

Phosphorus

Normal: 2.5–4.5 mg/dL. Inverse relationship with calcium.

Phosphorus works with calcium for bone formation and energy metabolism (ATP). When calcium goes up, phosphorus goes down — and vice versa. This is especially important in CKD patients.

Common causes

Hypo (<3.0): Refeeding syndrome (#1 — malnourished patients started on nutrition too fast), alcoholism, antacids (bind phosphorus), DKA treatment. Hyper (>4.5): Renal failure (#1 — can't excrete phosphorus), hypoparathyroidism, excessive intake, chemo (tumor lysis syndrome).

Effect on calcium

Hypo: Calcium rises (inverse). Hyper: Calcium drops (inverse) → can cause hypocalcemic symptoms (tetany, Chvostek's, Trousseau's).

Signs

Hypo: Muscle weakness, confusion, respiratory failure (diaphragm weakness), bone pain. Hyper: Usually asymptomatic until calcium drops → then tetany, muscle cramps, numbness/tingling.

Treatment

Hypo: Oral or IV phosphorus replacement, treat underlying cause. Hyper: Phosphate binders (calcium carbonate, sevelamer) — take WITH meals, restrict dietary phosphorus (dairy, cola, processed meats), dialysis.

Dehydration vs Fluid Overload

Know the signs of too little fluid vs too much — and the nursing priorities for each.

Causes

Dehydration: Vomiting, diarrhea, hemorrhage, burns, fever, diuretics, inadequate intake, DI. Overload: Heart failure, renal failure, excessive IV fluids, SIADH, cirrhosis, high sodium intake.

Vital signs

Dehydration: Tachycardia, hypotension, orthostatic hypotension, weak/thready pulse. Overload: Hypertension, bounding pulse, tachycardia (heart is overworked).

Skin/mucous membranes

Dehydration: Dry mucous membranes, poor skin turgor (tenting), dry/cracked lips, sunken eyes. Overload: Edema (peripheral, periorbital), taut/shiny skin, moist crackles in lungs.

Urine

Dehydration: Decreased output, concentrated (dark, high specific gravity >1.030). Overload: May be decreased (heart/kidney failure) or increased (kidneys trying to excrete).

Weight

Dehydration: Weight loss (1 kg = 1 L of fluid lost). Overload: Weight gain (1 kg = 1 L of fluid retained). 2 lbs/day or 5 lbs/week = report.

Neck veins

Dehydration: Flat neck veins (low volume). Overload: JVD — jugular vein distension (fluid overload).

Labs

Dehydration: Elevated Hct, BUN (concentrated blood), elevated specific gravity. Overload: Decreased Hct, BUN (diluted blood), decreased specific gravity.

Treatment

Dehydration: IV isotonic fluids (0.9% NS, LR), oral rehydration, treat underlying cause. Overload: Fluid restriction, diuretics (furosemide), low-sodium diet, elevate HOB, I&O monitoring, daily weights.

IV Fluid Types

Three categories — know what each one does to the cells and when to use it.

Isotonicstays in vessels

How it works: Same concentration as blood → stays in vascular space (no fluid shift). Examples: 0.9% NS (Normal Saline), Lactated Ringer's (LR), D5W*. Used for: Dehydration, shock, hemorrhage, DKA. Only NS is compatible with blood products. Caution: Can cause fluid overload. Use cautiously in HF and renal failure.

Hypotonicfluid → into cells

How it works: Lower concentration → fluid moves INTO cells (cells swell). "Hippo-tonic = cells swell like hippos." Examples: 0.45% NS (half-normal saline), 0.33% NS. Used for: Cellular dehydration, DKA (when blood glucose <250), hypernatremia. Caution: AVOID in increased ICP (causes cerebral edema). Can cause cardiovascular collapse if given too fast.

Hypertonicfluid ← out of cells

How it works: Higher concentration → pulls fluid OUT of cells (cells shrink). "Hyper about pulling fluid out." Examples: 3% NS, 5% NS, D10W, D5NS, D5LR, TPN. Used for: Severe hyponatremia, cerebral edema, increased ICP. Caution: Must be given slowly via central line (3% NS). Can cause fluid overload. Irritating to peripheral veins.

*D5W is technically isotonic in the bag but becomes hypotonic once infused (dextrose is metabolized, leaving free water).

Special IV Fluid Notes

• Lactated Ringer's (LR): Contains lactate (processed by liver). Contraindicated in liver failure and renal failure. Used commonly in burns, trauma, and surgery.
• Normal Saline (0.9% NS): The ONLY IV fluid compatible with blood products. Most commonly used isotonic fluid.
• 3% Saline: Hypertonic — same effect as mannitol but cheaper. Used for severe hyponatremia and increased ICP. Must give slowly via pump. Risk: fluid overload.
• TPN (Total Parenteral Nutrition): Very hypertonic — must be given through a central line. Monitor blood glucose (high dextrose content).

Colloids vs Crystalloids

• Crystalloids: All the above solutions (isotonic, hypotonic, hypertonic). Smaller molecules, don't stay in vascular space as long. Less expensive.
• Colloids: Larger molecules that STAY in the vascular space longer (albumin, dextran, hetastarch). More expensive. Albumin is a blood product — LVNs cannot administer. Dextran/hetastarch used for volume expansion (e.g., Jehovah's Witnesses who refuse blood).

Acid-Base Balance & ABGs

The ROME method makes ABG interpretation simple.

Normal ABG Values

pH7.35–7.45

<7.35 = acidosis. >7.45 = alkalosis.

PaCO₂35–45 mmHg

Respiratory component (controlled by lungs). ↑CO₂ = acid (respiratory acidosis). ↓CO₂ = alkalosis.

HCO₃⁻22–26 mEq/L

Metabolic component (controlled by kidneys). ↑HCO₃ = alkalosis. ↓HCO₃ = acidosis.

PaO₂80–100 mmHg

Oxygenation (not acid-base). <80 = hypoxemia.

3-Step ABG Interpretation

1. Look at pH: Is it acidosis (<7.35) or alkalosis (>7.45)?
2. Check CO₂ (respiratory): Does it match the pH? CO₂ and pH move in OPPOSITE directions for respiratory problems.
3. Check HCO₃ (metabolic): Does it match? HCO₃ and pH move in the SAME direction for metabolic problems.

The 4 Primary Imbalances

Respiratory Acidosis

pH ↓ · CO₂ ↑ · HCO₃ normal (or ↑ if compensating). Common causes: COPD, respiratory depression (opioids), pneumonia, airway obstruction.

Respiratory Alkalosis

pH ↑ · CO₂ ↓ · HCO₃ normal (or ↓ if compensating). Common causes: hyperventilation, anxiety, pain, PE, fever, high altitude.

Metabolic Acidosis

pH ↓ · CO₂ normal (or ↓ if compensating) · HCO₃ ↓. Common causes: DKA, renal failure, diarrhea, lactic acidosis, aspirin OD.

Metabolic Alkalosis

pH ↑ · CO₂ normal (or ↑ if compensating) · HCO₃ ↑. Common causes: vomiting, NG suction (losing acid), excessive antacids, loop diuretics.

Compensation

The body tries to fix acid-base imbalances using the opposite system:

• Respiratory problem → kidneys compensate (slow — takes hours to days)
• Metabolic problem → lungs compensate (fast — takes minutes)
• Example: Metabolic acidosis → lungs hyperventilate to blow off CO₂ (Kussmaul respirations in DKA)
• If BOTH CO₂ and HCO₃ are abnormal → the body is compensating. Look at pH: if still abnormal → partial compensation. If pH is normal → full compensation.

Frequently Asked Questions

Common student questions on this topic.

Quick-Reference Cheatsheet

Every key point from this guide on a single printable page.