Fluid & Electrolyte Disorders — Core Concepts
- Rois Narvaez
- Oct 6, 2025
- 5 min read
Fluid and electrolyte balance forms the physiologic foundation for perfusion, cardiac rhythm, and homeostasis. Even small shifts in water or ions alter cell excitability and acid–base equilibrium.Nurses interpret subtle changes in I&O, lab trends, and patient presentation to prevent complications ranging from hypovolemic shock to arrhythmias.
Safety anchors:
Monitor I&O, daily weights, vitals, neuro & cardiac signs
Trend serum electrolytes (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, PO₄³⁻)
Use infusion pumps for all hypertonic or potassium-containing IVs
Reassess fluid therapy frequently in older adults and renal/hepatic patients
Clinical judgment:Pattern → loss, retention, or shift?Priority → stabilize circulation, correct cause, prevent recurrence.
Use this pack to:
Scan high-yield tables
Memorize hallmark findings
Apply fluid principles clinically
Practice NGN-style reasoning for safe interventions.
⚗️ 1) Fluid Balance Disorders
Conceptual Focus
Fluid homeostasis depends on intake, renal output, ADH, aldosterone, and capillary dynamics.Nurses interpret changes in weight, urine output, mucosa, BP, and osmolality to identify imbalances early.
Clinical Judgment Focus
Evaluate volume status: look for tissue perfusion changes, not just numbers.
Identify cause: renal vs. extrarenal loss, hormonal, third-spacing.
Implement safe correction: isotonic for deficit, restriction/diuretics for excess.
Disorder | Causes / Risk Factors | Diagnostic Findings / Labs | Nursing Management / Interventions |
Fluid Volume Deficit (FVD) (Hypovolemia, Dehydration) | Vomiting, diarrhea, hemorrhage, fever, burns, diuretics, NPO, third spacing | ↓ BP, ↑ HR, weak pulses, poor turgor, dry mucosa, ↑ Hct, ↑ BUN, ↑ urine specific gravity | Replace fluids (oral/IV isotonic NS or LR), monitor I&O & weights, assess orthostatic BP, treat underlying cause, watch for hypovolemic shock |
Fluid Volume Excess (FVE) (Hypervolemia, Overload) | Heart failure, renal failure, SIADH, corticosteroids, excess IV fluids | ↑ BP, bounding pulses, crackles, edema, ↓ Hct, ↓ BUN, pulmonary congestion | Restrict Na⁺/fluids, give diuretics, elevate HOB, monitor lungs, daily weights, evaluate response to therapy |
Third-Spacing (Fluid Shift) | Burns, sepsis, hypoalbuminemia, post-op stress | Edema, ↓ BP, ↓ UO despite normal intake, ↑ weight | Correct cause, albumin or colloid therapy, monitor perfusion, assess for fluid mobilization phase |
SIADH (Syndrome of Inappropriate ADH) | CNS lesions, head trauma, lung cancer, SSRIs, carbamazepine | ↓ Na⁺, ↓ osmolality, concentrated urine, weight gain | Fluid restriction, hypertonic saline via pump, seizure precautions, monitor neurostatus, treat cause |
Diabetes Insipidus (DI) | ↓ ADH (neurogenic) or renal unresponsiveness (nephrogenic) | Polyuria, polydipsia, ↑ Na⁺, ↑ osmolality, ↓ urine specific gravity | Administer desmopressin (DDAVP), monitor Na⁺ and osmolality, encourage fluids, assess hydration |
Acid–Base Connection:FVD → metabolic acidosis risk (poor perfusion/lactic acid)FVE → possible respiratory alkalosis (pulmonary congestion)
Memory Aid — F.L.U.I.D.S.Find cause • Labs trend • Urine check • Intake/output • Daily weight • Safety first
🧂 2) Sodium (Na⁺) Imbalances
Normal: 135–145 mEq/LControls water distribution and nerve conduction. Closely linked to ADH and osmolality.
Disorder | Causes / Risk Factors | Diagnostics / Labs | Nursing Management / Interventions |
Hyponatremia (<135) | SIADH, diuretics, GI losses, CHF, renal failure, excess hypotonic fluids | ↓ Na⁺, ↓ osmolality, confusion, seizures, edema | Fluid restriction, isotonic or hypertonic (3%) saline via pump, seizure precautions, monitor neurostatus |
Hypernatremia (>145) | Water deprivation, DI, hypertonic feedings, excessive Na⁺ intake | ↑ Na⁺, ↑ osmolality, dry mucosa, thirst, restlessness | Gradual rehydration (D5W or 0.45% NS), monitor Na⁺ and neuro signs, avoid rapid correction |
Special Nursing Focus:Rapid Na⁺ correction → central pontine myelinolysis risk (never >10–12 mEq/L per 24 hr).
⚡ 3) Potassium (K⁺) Imbalances
Normal: 3.5–5.0 mEq/LPrimary intracellular cation essential for neuromuscular and cardiac activity.Directly influenced by acid–base balance and renal excretion.
Disorder | Causes / Risk Factors | Diagnostics / Labs | Nursing Management / Interventions |
Hypokalemia (<3.5) | Diuretics, vomiting, diarrhea, NG suction, steroids, alkalosis | Flat T/U waves, weakness, ↑ digoxin toxicity | Administer oral/IV K⁺ (≤10 mEq/hr), ECG monitor, high-K⁺ foods, assess Mg²⁺ (low Mg = low K) |
Hyperkalemia (>5.0) | Renal failure, K⁺-sparing diuretics, acidosis, trauma, transfusion | Peaked T waves, wide QRS, bradycardia | Stop K⁺ sources, give Ca gluconate, insulin + D50, sodium bicarb, Kayexalate, dialysis if severe |
Memory Aids:
K+ killers: ACEI, ARBs, spironolactone
Low K+ = low Mg²⁺ correction first!
K.I.D.N.E.Y. = Keep ECG • Investigate cause • Don’t push KCl • Note diet • Evaluate muscles • Yield nephrotoxins
🦴 4) Calcium (Ca²⁺) Imbalances
Normal: 8.5–10.5 mg/dLVital for bones, clotting, nerve transmission, and muscle tone.
Disorder | Causes / Risk Factors | Diagnostics / Labs | Nursing Management / Interventions |
Hypocalcemia (<8.5) | Hypoparathyroidism, vit D deficiency, pancreatitis, renal failure | +Chvostek, +Trousseau, tetany, prolonged QT | Administer Ca gluconate, vit D, seizure precautions, monitor airway and ECG |
Hypercalcemia (>10.5) | Hyperparathyroidism, malignancy, immobility | Short QT, weakness, kidney stones | IV fluids (3–4 L/day), loop diuretics, calcitonin, mobility, avoid Ca foods/supplements |
Memory Aids:C.A.L.C.I.U.M. — Cramps low, Arrhythmia, Loop diuretics, Calcitonin, IV fluids, Urine flush, Monitor ECGInverse relation: Ca²⁺ ↓ → PO₄³⁻ ↑
🧲 5) Magnesium (Mg²⁺) Imbalances
Normal: 1.3–2.1 mEq/LStabilizes cardiac rhythm and neuromuscular excitability.
Disorder | Causes / Risk Factors | Diagnostics / Labs | Nursing Management / Interventions |
Hypomagnesemia (<1.3) | Alcoholism, malabsorption, diuretics, DKA | Tremors, ↑ DTRs, seizures, dysrhythmias | IV MgSO₄ (slow), monitor DTRs and RR, seizure precautions, Mg-rich foods |
Hypermagnesemia (>2.1) | Renal failure, antacids/laxatives | ↓ DTR, hypotension, bradycardia | Stop Mg, give Ca gluconate, support airway, dialysis if severe |
Memory Aid — M.A.G.Monitor DTRs • Airway support • Gluconate if high
🧪 6) Chloride (Cl⁻) Imbalances
Normal: 98–106 mEq/LTracks with sodium; influences acid–base balance (forms HCl).
Disorder | Causes / Risk Factors | Diagnostics / Labs | Nursing Management / Interventions |
Hypochloremia (<98) | Vomiting, NG suction, metabolic alkalosis | ↓ Cl⁻, shallow breathing | Replace with NS, correct alkalosis |
Hyperchloremia (>106) | Dehydration, metabolic acidosis, NS overload | Kussmaul respirations, weakness | Correct acidosis, switch fluids, monitor ABG |
🧫 7) Phosphate (PO₄³⁻) Imbalances
Normal: 2.5–4.5 mg/dLInterlinked with calcium metabolism; key in ATP and bone structure.
Disorder | Causes / Risk Factors | Diagnostics / Labs | Nursing Management / Interventions |
Hypophosphatemia (<2.5) | Malnutrition, alcoholism, DKA recovery | Weakness, ↓ reflexes | Replace phosphate (oral/IV), monitor Ca²⁺ |
Hyperphosphatemia (>4.5) | CKD, hypoparathyroidism, tumor lysis | Tetany, calcification | Phosphate binders (sevelamer), low-PO₄ diet, dialysis |
🧠 Quick Checks (Retrieval)
1️⃣ Identify a fluid balance disorder with low urine output but rising weight.2️⃣ What Na⁺ level and symptom combination indicates severe hyponatremia?3️⃣ Which electrolyte imbalance shows prolonged QT?4️⃣ How fast can IV K⁺ be infused safely?5️⃣ A patient on furosemide and digoxin presents with muscle weakness — which imbalance do you suspect?
Answers:
SIADH or third-spacing
Na⁺ <120 + seizures/confusion
Hypocalcemia
≤10 mEq/hr via pump
Hypokalemia
🧩 NGN-Style Mini-Item #1
Scenario:An elderly client presents with diarrhea for 4 days, BP 88/56, HR 122, dry tongue, and Na⁺ 150 mEq/L.
Select all that apply:☐ Start NS bolus rapidly☐ Monitor daily weight☐ Start hypotonic IV after initial stabilization☐ Restrict fluids☐ Assess for confusion and seizures
Correct: ✅ NS bolus → stabilize • ✅ Daily weight • ✅ Hypotonic IV (gradual correction) • ✅ Monitor neurostatusWhy: Restore volume first, then correct Na⁺ slowly to prevent cerebral edema.
🧩 NGN-Style Mini-Item #2
Scenario:A patient on hemodialysis presents with peaked T-waves and K⁺ 6.8 mEq/L.
Priority actions:☐ Administer Ca gluconate IV☐ Give insulin with D50☐ Restrict oral K⁺ foods☐ Prepare for dialysis☐ Push IV KCl for muscle weakness
Correct: ✅ Ca gluconate • ✅ Insulin/D50 • ✅ Prepare dialysis • ✅ Restrict K⁺Rationale: Protect myocardium first, shift K⁺ intracellularly, then remove it.
✨ Closing Practice Pearls
1 kg = 1 L fluid. Daily weight is the most sensitive volume indicator.
Correct slowly. Rapid shifts cause brain or cardiac injury.
Trends > snapshots. Look for progressive lab or clinical changes.
Pairings: Na⁺↔Cl⁻, Ca²⁺↔Phosphate, Mg²⁺↔K⁺.
Cardiac trio: K⁺, Ca²⁺, Mg²⁺ control rhythm stability.
Old rule still true: “If urine output falls, listen to the lungs before you push fluids.”
Educate: hydration cues, diet (Na⁺, K⁺, Mg²⁺), medication side effects, when to seek care.
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