Fluid & Electrolytes / Acid-Base Balance
✅ Reviewed by NCLEXDeck Clinical Review Board — Updated June 2026
Comprehensive NCLEX-RN Review — Volume Status, Electrolyte Imbalances, ABG Interpretation, IV Fluids
1. Fluid Volume Imbalances
Fluid Volume Deficit (Hypovolemia / Dehydration)
- Causes: hemorrhage, vomiting, diarrhea, diuretics, poor intake, third-spacing
- S/S: ↑HR, ↓BP, orthostatic hypotension, poor skin turgor, dry mucous membranes, ↑urine specific gravity, ↓urine output, weight loss, ↑hematocrit/BUN
- Care: replace fluids (oral/IV isotonic), monitor I&O, daily weights, safety (fall risk)
Fluid Volume Excess (Hypervolemia)
- Causes: heart failure, renal failure, excess sodium/IV fluids, SIADH
- S/S: edema, weight gain, crackles, JVD, ↑BP, bounding pulse, dyspnea, ↓hematocrit (dilutional)
- Care: restrict fluids/sodium, diuretics, monitor daily weight, semi-Fowler's for dyspnea
⭐ NCLEX TIP
Daily weight is the most reliable indicator of fluid gain/loss. A 1 kg (2.2 lb) change ≈ 1 L of fluid.
2. Electrolytes — Normal Values & Imbalances
Sodium (135–145 mEq/L)
- Hyponatremia (<135): confusion, headache, seizures, muscle cramps, N/V. Correct slowly (rapid correction → osmotic demyelination).
- Hypernatremia (>145): thirst, dry mucous membranes, restlessness/agitation, seizures. Water problem.
Potassium (3.5–5.0 mEq/L) — Most Cardiac-Critical
| Hypokalemia (<3.5) | Hyperkalemia (>5.0) |
|---|
| S/S | Muscle weakness, flat/inverted T waves, U waves, dysrhythmias, ileus | Peaked T waves, wide QRS, muscle weakness, dysrhythmias/cardiac arrest |
| Causes | Diuretics, vomiting, diarrhea | Renal failure, K-sparing diuretics, tissue breakdown |
| Treatment | Replace PO/IV (never push IV K+ — always dilute via pump) | Calcium gluconate (protects heart), insulin + D50, albuterol, kayexalate, dialysis |
⭐ NCLEX TIP
Potassium extremes both cause fatal dysrhythmias. Peaked T = hyper (Points up = Plus); flat T + U wave = hypo. Cardiac monitoring is priority for either.
Calcium (9–10.5 mg/dL)
- Hypocalcemia (<9): tetany, positive Chvostek's and Trousseau's signs, paresthesias, laryngospasm. Causes: hypoparathyroidism, thyroidectomy.
- Hypercalcemia (>10.5): "bones, stones, groans, moans" — weakness, kidney stones, constipation, confusion. Causes: hyperparathyroidism, malignancy, immobility.
Magnesium (1.5–2.5 mEq/L)
- Hypomagnesemia (<1.5): tremors, hyperreflexia, torsades de pointes. Common in alcohol use disorder.
- Hypermagnesemia (>2.5): ↓DTRs, hypotension, bradycardia, respiratory depression. Antidote = calcium gluconate (seen with mag sulfate in OB).
3. Acid-Base Balance & ABG Interpretation
Normal Values
pH 7.35–7.45 | PaCO₂ 35–45 mmHg | HCO₃⁻ 22–26 mEq/L | PaO₂ 80–100
ROME Method
Respiratory Opposite (pH and CO₂ move opposite directions), Metabolic Equal (pH and HCO₃ move same direction).
Step-by-Step ABG Analysis
- Look at pH: <7.35 = acidosis; >7.45 = alkalosis
- Look at CO₂ (respiratory) and HCO₃ (metabolic) — which matches the pH problem?
- Determine compensation: if the opposite system is shifting toward normalizing pH → partial/full compensation
| Disorder | pH | CO₂ | HCO₃ | Common Causes |
|---|
| Respiratory acidosis | ↓ | ↑ | (↑ if compensating) | Hypoventilation, COPD, opioid overdose, atelectasis |
| Respiratory alkalosis | ↑ | ↓ | (↓ if compensating) | Hyperventilation, anxiety, pain, high altitude |
| Metabolic acidosis | ↓ | (↓ if compensating) | ↓ | DKA, diarrhea, renal failure, lactic acidosis, shock |
| Metabolic alkalosis | ↑ | (↑ if compensating) | ↑ | Vomiting, NG suction, antacid/diuretic overuse |
Worked example: pH 7.30, CO₂ 50, HCO₃ 24 → pH low (acidosis), CO₂ high (matches, respiratory), HCO₃ normal → uncompensated respiratory acidosis.
⭐ NCLEX TIP
"Hyperventilation blows off CO₂" → respiratory alkalosis. "Vomiting loses acid" → metabolic alkalosis. "Diarrhea loses bicarb" → metabolic acidosis. Tie the clinical scenario to the imbalance.
4. IV Fluids (Tonicity)
| Type | Examples | Use | Caution |
|---|
| Isotonic | 0.9% NS, LR, D5W (briefly) | Expand intravascular volume — shock, hypovolemia, blood loss | Fluid overload in HF/renal |
| Hypotonic | 0.45% NS | Cellular dehydration, hypernatremia | Watch for cerebral edema; not for at-risk of ↑ICP |
| Hypertonic | 3% NS, D10W | Severe hyponatremia, cerebral edema | Give slowly, monitor closely (fluid overload risk) |