9.1 Fluid and Electrolyte Imbalances

Fluid and Electrolyte Foundations

Physiological Adaptation makes up 7–13% of the NCLEX-PN (April 2026 test plan), and fluid/electrolyte items appear throughout it. The exam is computerized adaptive testing (CAT): 85–150 questions in up to 5 hours, stopping when the engine is 95% confident you are above or below the -0.18 logit passing standard. Expect select-all-that-apply and chart/lab questions that hinge on exact numbers — memorize the ranges below.

Body Fluid Compartments

Total body water is about 60% of adult body weight, split between two compartments. Knowing where fluid sits explains why imbalances cause the signs they do.

• Intracellular fluid (ICF): ~40% of body weight (two-thirds of body water), inside cells
• Extracellular fluid (ECF): ~20% of body weight (one-third of body water)
  • Intravascular (plasma) — within blood vessels
  • Interstitial — between cells
  • Transcellular — cerebrospinal, synovial, pleural, peritoneal fluid

Daily requirement: roughly 30 mL/kg/day (≈2,000–2,500 mL for adults). Needs rise with fever (add ~13% per 1°C), vomiting, diarrhea, wound drains, and burns.

Fluid Volume Imbalances

Worked example: A patient's morning weight is 70 kg; yesterday it was 72 kg. The 2 kg loss equals ~2 L of fluid. If the patient is on furosemide for heart failure, that loss is the goal — but pair it with a potassium check, because loop diuretics waste potassium. Report a gain of more than 2–3 lb in 24 hours or 5 lb in a week.

Sodium (Normal 136–145 mEq/L)

Sodium governs water distribution and neurologic status. "Where sodium goes, water follows."

• Hyponatremia (<136): water intoxication, SIADH, diuretics, vomiting → headache, confusion, muscle cramps, seizures. Brain cells swell. Treat the cause; severe symptomatic cases get cautious hypertonic (3%) saline — correct slowly to avoid osmotic demyelination.
• Hypernatremia (>145): dehydration, diabetes insipidus, excess salt → thirst, dry sticky mucosa, restlessness, twitching. Cells shrink. Treat with hypotonic fluids and free water.

Potassium (Normal 3.5–5.0 mEq/L)

Potassium is the cardiac and neuromuscular electrolyte; small shifts kill.

Safety trap: Never give IV potassium by IV push or bolus — it causes fatal arrhythmia. Always dilute and infuse slowly (≤10 mEq/hr on a general floor) via pump. Critical values: <2.5 or >6.5 mEq/L.

Calcium (8.6–10.2 mg/dL) and Magnesium (1.5–2.5 mEq/L)

• Hypocalcemia: hypoparathyroidism, vitamin D deficiency, pancreatitis, post-thyroidectomy → positive Chvostek's (facial twitch when cheek tapped) and Trousseau's (carpal spasm with BP cuff), tetany, prolonged QT, seizures. Treat with calcium ± vitamin D; IV calcium gluconate for tetany.
• Hypercalcemia: hyperparathyroidism, cancer, immobility → weakness, lethargy, constipation, kidney stones, shortened QT. Hydrate and give loop diuretics.
• Magnesium mirrors calcium clinically. Before a magnesium sulfate infusion (e.g., preeclampsia), assess deep tendon reflexes — loss of patellar reflex, respiratory rate <12, or low urine output signals toxicity. Antidote: calcium gluconate.

Acid-Base in One Pass

ROME helps: Respiratory Opposite (pH and CO₂ move opposite), Metabolic Equal (pH and HCO₃ move the same way). Kussmaul respirations (deep, rapid) are the lungs blowing off CO₂ to compensate for metabolic acidosis.

Putting It Together: Reading a Lab Question

Most NCLEX-PN electrolyte items hand you a chart and a value, then ask for the priority. Work the problem in a fixed order so you never freeze: first identify whether the value is high or low against the normal range, then connect it to its hallmark sign, then choose the action — and remember that cardiac and respiratory effects always outrank everything else.

Consider a worked scenario. A patient on furosemide and digoxin reports leg cramps and feels "my heart is skipping." The potassium returns at 2.9 mEq/L. Low potassium both produces the cramps and U waves and dangerously potentiates digoxin toxicity, because both push the heart toward arrhythmia. The priority is not simply to give a banana — it is to place the patient on cardiac monitoring, hold the next digoxin dose pending provider notification, and prepare for cautious diluted IV potassium replacement.

This single scenario braids together three high-yield rules: loop diuretics waste potassium, hypokalemia worsens digoxin toxicity, and IV potassium is never pushed.

Foods and Teaching by Electrolyte

Nutrition and patient teaching are reliably tested, because diet is squarely within the LPN/VN scope. Match the deficiency to the food source.

• Potassium-rich: bananas, oranges and juice, potatoes, tomatoes, spinach, avocado, dried fruit
• Calcium-rich: dairy, fortified plant milks, sardines with bones, leafy greens, tofu
• Magnesium-rich: nuts, seeds, whole grains, legumes, dark leafy greens
• Sodium to limit (fluid excess, hypertension, heart failure): canned soups, deli/cured meats, processed snacks, restaurant food

When a patient takes a potassium-sparing diuretic such as spironolactone, teach them to avoid salt substitutes — most are potassium chloride and can drive hyperkalemia. Daily weights, taken at the same time on the same scale in similar clothing, remain the single most sensitive home measure of fluid balance the nurse can teach a heart-failure or renal patient to track and report.