Fluid & Electrolyte Balance
Master fluid compartments, osmolality, IV solution types, electrolyte imbalances, and key nursing interventions.
Fluid and Electrolyte Balance
Fluid and electrolyte balance is one of the most heavily tested physiological concepts on the NCLEX. Nurses must understand how the body regulates fluid distribution, recognize imbalances quickly, and intervene appropriately. This guide covers body fluid compartments, osmolality, IV solution selection, and the major electrolyte imbalances you will encounter on the exam and in clinical practice. For a condensed reference, visit the Fluid and Electrolyte Cheat Sheet.
Body Fluid Compartments
Total body water accounts for approximately 60% of adult body weight. This fluid is distributed between two major compartments:
- Intracellular fluid (ICF): Contains about two-thirds of total body water. ICF is the fluid within cells and is rich in potassium, magnesium, and phosphate.
- Extracellular fluid (ECF): Contains about one-third of total body water. ECF includes interstitial fluid (fluid between cells), intravascular fluid (plasma within blood vessels), and transcellular fluid (cerebrospinal fluid, synovial fluid, pleural fluid). ECF is rich in sodium, chloride, and bicarbonate.
Fluid moves between compartments by osmosis, diffusion, filtration, and active transport. Understanding these mechanisms is essential for predicting how IV solutions will redistribute in the body.
Osmolality and Tonicity
Osmolality measures the concentration of solutes per kilogram of water. Normal serum osmolality is 275-295 mOsm/kg. Tonicity refers to the effect of a solution on cell volume:
- Isotonic solutions have the same osmolality as plasma and do not cause fluid shifts across cell membranes.
- Hypotonic solutions have lower osmolality than plasma, causing water to move into cells (cells swell).
- Hypertonic solutions have higher osmolality than plasma, causing water to move out of cells (cells shrink).
IV Solution Types
Selecting the correct IV solution is a critical nursing responsibility. The choice depends on the patient's fluid status and electrolyte levels:
- Isotonic solutions: 0.9% Normal Saline (NS), Lactated Ringer's (LR), D5W (isotonic in the bag but becomes hypotonic once dextrose is metabolized). Use for fluid resuscitation, blood transfusions (NS), and replacing ECF losses.
- Hypotonic solutions: 0.45% NS (half-normal saline), 0.225% NS. Use to treat cellular dehydration and hypernatremia. Avoid in patients with increased intracranial pressure (ICP) or third-spacing.
- Hypertonic solutions: 3% NS, 5% NS, D10W, D5 0.9% NS. Use to treat severe hyponatremia, shift fluid from intracellular to extracellular space. Administer via central line when concentration exceeds 3%. Monitor closely for fluid overload.
Fluid Volume Deficit (Dehydration/Hypovolemia)
Fluid volume deficit occurs when fluid output exceeds intake. Causes include hemorrhage, vomiting, diarrhea, excessive diuresis, and third-spacing. Key signs include tachycardia, hypotension, orthostatic changes, decreased urine output (less than 30 mL/hr), poor skin turgor, dry mucous membranes, elevated hematocrit, and increased urine specific gravity (greater than 1.030). Treatment involves isotonic IV fluid replacement and addressing the underlying cause.
Fluid Volume Excess (Hypervolemia)
Fluid volume excess results from excessive fluid intake or impaired fluid excretion (heart failure, renal failure, liver cirrhosis). Signs include weight gain, edema, bounding pulse, elevated blood pressure, jugular vein distention (JVD), crackles in lungs, and decreased hematocrit. Treatment includes fluid and sodium restriction, diuretics (furosemide), and monitoring daily weights. A weight gain of 1 kg equals approximately 1 liter of fluid retained.
Electrolyte Normal Values
| Electrolyte | Normal Range | Key Functions |
|---|---|---|
| Sodium (Na+) | 136-145 mEq/L | ECF osmolality, nerve impulse transmission |
| Potassium (K+) | 3.5-5.0 mEq/L | Cardiac rhythm, muscle contraction, nerve function |
| Calcium (Ca2+) | 9.0-10.5 mg/dL | Bone integrity, clotting, muscle contraction |
| Magnesium (Mg2+) | 1.3-2.1 mEq/L | Enzyme reactions, cardiac rhythm, neuromuscular function |
| Phosphorus (PO4) | 3.0-4.5 mg/dL | Bone and teeth formation, ATP production |
| Chloride (Cl-) | 98-106 mEq/L | Acid-base balance, osmolality |
Sodium Imbalances
Hyponatremia (Na+ less than 136 mEq/L): Often caused by water excess (dilutional) rather than true sodium loss. Causes include SIADH, excessive hypotonic IV fluids, water intoxication, and diuretic therapy. Symptoms include nausea, headache, confusion, seizures, and lethargy. Severe cases can lead to cerebral edema. Treatment includes fluid restriction, hypertonic saline for severe cases (correct slowly to avoid osmotic demyelination syndrome -- no more than 8-12 mEq/L in 24 hours).
Hypernatremia (Na+ greater than 145 mEq/L): Usually caused by water deficit or excess sodium intake. Causes include diabetes insipidus, dehydration, excessive sodium bicarbonate administration, and inadequate water intake. Symptoms include extreme thirst, dry mucous membranes, restlessness, irritability, and seizures. Treatment includes hypotonic IV fluids and free water replacement. Correct gradually to prevent cerebral edema.
Potassium Imbalances
Hypokalemia (K+ less than 3.5 mEq/L): Causes include diuretic therapy (loop and thiazide), vomiting, diarrhea, alkalosis, and inadequate intake. Symptoms include muscle weakness, fatigue, leg cramps, decreased bowel motility (ileus), and cardiac dysrhythmias. ECG changes include flattened T waves, prominent U waves, and ST depression. Treatment involves potassium replacement (oral preferred; IV potassium must be diluted and infused no faster than 10-20 mEq/hr via peripheral line). Never administer IV potassium by push.
Hyperkalemia (K+ greater than 5.0 mEq/L): Causes include renal failure, ACE inhibitors, potassium-sparing diuretics, tissue destruction (burns, crush injuries), and acidosis. Symptoms include muscle weakness, paresthesias, abdominal cramping, and cardiac dysrhythmias. ECG changes include tall peaked T waves, widened QRS complex, prolonged PR interval, and eventually sine wave pattern leading to cardiac arrest. Emergency treatment includes IV calcium gluconate (cardioprotective), insulin with glucose (shifts K+ intracellularly), sodium bicarbonate, kayexalate (sodium polystyrene sulfonate), and dialysis for refractory cases.
Calcium Imbalances
Hypocalcemia (Ca2+ less than 9.0 mg/dL): Causes include hypoparathyroidism, vitamin D deficiency, renal failure, pancreatitis, and alkalosis. Symptoms include numbness and tingling, muscle cramps, tetany, seizures, and prolonged QT interval. Two classic assessment signs: Chvostek's sign (tapping the facial nerve causes facial twitching) and Trousseau's sign (inflating a blood pressure cuff causes carpal spasm). Treatment includes IV calcium gluconate (administer slowly), oral calcium and vitamin D supplements.
Hypercalcemia (Ca2+ greater than 10.5 mg/dL): Causes include hyperparathyroidism, malignancy, prolonged immobilization, and excessive vitamin D. Symptoms include muscle weakness, decreased deep tendon reflexes, constipation, polyuria, kidney stones, shortened QT interval, and altered mental status. Treatment includes IV normal saline for hydration, loop diuretics (furosemide -- not thiazides), calcitonin, and bisphosphonates.
Magnesium and Phosphorus Imbalances
Hypomagnesemia (Mg2+ less than 1.3 mEq/L): Often accompanies hypokalemia and hypocalcemia. Causes include chronic alcoholism, malnutrition, diuretic therapy, and diarrhea. Symptoms mirror hypocalcemia: tremors, tetany, seizures, and cardiac dysrhythmias (torsades de pointes). Treatment includes IV magnesium sulfate with continuous cardiac monitoring.
Hypermagnesemia (Mg2+ greater than 2.1 mEq/L): Usually iatrogenic from excessive magnesium administration or renal failure. Symptoms include lethargy, hypotension, bradycardia, decreased deep tendon reflexes, and respiratory depression. Loss of DTRs is an early warning sign. Treatment includes IV calcium gluconate (antagonist), diuretics, and dialysis.
Phosphorus has an inverse relationship with calcium. Hyperphosphatemia commonly occurs in renal failure and causes reciprocal hypocalcemia. Hypophosphatemia is seen in refeeding syndrome, alcoholism, and diabetic ketoacidosis.
NCLEX Study Tips
Focus on recognizing which electrolyte imbalance a patient is presenting with based on clinical signs. Remember key ECG changes for potassium and calcium imbalances. Know which IV solutions are appropriate for specific conditions. For more practice, explore the Physiological Study Guide and the Physiological Flashcards.