Fluid & Electrolytes – NCLEX-RN Study Guide

Overview

Fluid and electrolyte balance is a foundational concept for NCLEX-RN success, as imbalances affect virtually every body system. This guide covers the major fluid imbalances, electrolyte disorders (sodium, potassium, calcium, magnesium, and phosphate), and the nursing assessments and interventions required to manage them safely. Mastery of normal lab values, clinical signs, and priority nursing actions is essential for both the exam and clinical practice.

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Quick Reference: Normal Lab Values

| Electrolyte | Normal Range |

|---|---|

| Sodium (Na⁺) | 135–145 mEq/L |

| Potassium (K⁺) | 3.5–5.0 mEq/L |

| Calcium (Ca²⁺) | 8.5–10.5 mg/dL |

| Magnesium (Mg²⁺) | 1.5–2.5 mEq/L |

| Serum Osmolarity | 275–295 mOsm/kg |

| Urine Specific Gravity | 1.001–1.029 |

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Fluid Imbalances

Overview

Body fluid exists in two main compartments: intravascular (plasma) and intracellular. Fluid movement between compartments is driven by osmosis (water follows solutes) and hydrostatic pressure. Imbalances manifest in every body system, and daily weight is the single most accurate monitoring tool.

Fluid Volume Deficit (Hypovolemia)

Definition: Insufficient circulating fluid volume, most commonly from blood loss, vomiting, diarrhea, or inadequate intake.

Key Assessment Findings:

Priority Nursing Interventions:

Fluid Volume Excess (Hypervolemia)

Definition: Excess fluid accumulation in the intravascular and interstitial spaces, often from heart failure, renal failure, or excessive IV fluid administration.

Key Assessment Findings:

Priority Nursing Interventions:

IV Fluid Tonicity

| Type | Example | Effect | Use |

|---|---|---|---|

| Isotonic | 0.9% NS, LR | Stays in intravascular space | Hypovolemia, resuscitation |

| Hypotonic | 0.45% NaCl | Shifts fluid INTO cells | Cellular dehydration |

| Hypertonic | 3% NaCl, D10W | Pulls fluid OUT of cells | Severe hyponatremia, cerebral edema |

> ⚠️ Watch Out For: Hypotonic solutions are contraindicated in patients with increased intracranial pressure (ICP) or head injuries — they shift fluid into brain cells and worsen cerebral edema.

Key Terms:

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Sodium (Na⁺) Imbalances

Hyponatremia (Na⁺ < 135 mEq/L)

Pathophysiology: Low sodium → decreased plasma osmolarity → water shifts INTO cells → cells swell (especially brain cells).

Causes: SIADH, excessive water intake, heart failure, cirrhosis, excessive hypotonic IV fluids

Signs & Symptoms (severity correlates with rate of drop):

Treatment:

> ⚠️ Watch Out For: Correcting hyponatremia too rapidly can cause osmotic demyelination syndrome (ODS/central pontine myelinolysis) — a devastating neurological complication. Correction should not exceed 10–12 mEq/L in 24 hours.

Hypernatremia (Na⁺ > 145 mEq/L)

Pathophysiology: High sodium → increased plasma osmolarity → water shifts OUT of cells → cells shrink.

Causes: Dehydration, diabetes insipidus, excessive sodium intake, inadequate water intake

Signs & Symptoms:

Treatment:

Key Terms:

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Potassium (K⁺) Imbalances

> 🔑 Critical Concept: Potassium is the primary intracellular cation. Even small changes in serum levels cause significant cardiac and neuromuscular effects. Potassium imbalances are among the most tested and most dangerous electrolyte disorders on NCLEX.

Hypokalemia (K⁺ < 3.5 mEq/L)

Causes: Diuretics (thiazides, loop diuretics), vomiting, diarrhea, NG suction, steroids

Signs & Symptoms:

Treatment:

> ⚠️ Watch Out For — IV Potassium Safety Rules:

> - NEVER give IV potassium as an IV push or bolus (can cause cardiac arrest)

> - Maximum peripheral IV rate: 10 mEq/hour

> - Maximum central IV rate: 20 mEq/hour

> - Always dilute — never give undiluted

> - Must be administered via an infusion pump

> - Monitor the IV site — potassium is vesicant (causes tissue damage if it infiltrates)

Hyperkalemia (K⁺ > 5.0 mEq/L)

Causes: Renal failure (CKD most common), acidosis, cell destruction (burns, rhabdomyolysis), ACE inhibitors, potassium-sparing diuretics

Signs & Symptoms:

1. Peaked (tall, narrow) T waves ← earliest ECG change

2. Prolonged PR interval

3. Widened QRS

4. Sine wave pattern

5. Ventricular fibrillation / cardiac arrest

Emergency Treatment (in order):

| Drug | Action | Purpose |

|---|---|---|

| Calcium gluconate | Stabilizes cardiac membrane | Protects heart (does NOT lower K⁺) — given first |

| Sodium bicarbonate | Drives K⁺ into cells | Temporary shift |

| Regular insulin + dextrose | Drives K⁺ into cells | Temporary shift |

| Kayexalate (sodium polystyrene) | Binds K⁺ in GI tract | Removes K⁺ from body |

| Dialysis | Removes K⁺ from blood | Definitive removal |

> ⚠️ Watch Out For: Calcium gluconate protects the heart but does NOT lower serum potassium. Additional treatments are required to actually eliminate or shift potassium.

Dietary Teaching for Hyperkalemia (CKD patients):

Avoid high-potassium foods: bananas, oranges, potatoes, tomatoes, avocados, dairy, nuts, whole grains

Key Terms:

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Calcium (Ca²⁺) Imbalances

Hypocalcemia (Ca²⁺ < 8.5 mg/dL)

Causes: Hypoparathyroidism (especially post-thyroid/parathyroid surgery), vitamin D deficiency, pancreatitis, massive blood transfusions (citrate binds calcium), hypomagnesemia

Signs & Symptoms — increased neuromuscular excitability:

Treatment: IV calcium gluconate (emergency), oral calcium + vitamin D supplements

Hypercalcemia (Ca²⁺ > 10.5 mg/dL)

Causes: Hyperparathyroidism, malignancy (bone metastases), prolonged immobility, thiazide diuretics, excess vitamin D

Classic Signs — "Groans, Moans, Stones, and Bones":

Treatment: IV normal saline hydration + furosemide, bisphosphonates, calcitonin

> ⚠️ Watch Out For: Hypocalcemia decreases DTRs, while hypERcalcemia also decreases DTRs — both can present with diminished reflexes. However, the mechanism differs: hypocalcemia increases neuromuscular irritability (leading to tetany), while hypercalcemia depresses neuromuscular function.

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Magnesium (Mg²⁺) Imbalances

Hypomagnesemia (Mg²⁺ < 1.5 mEq/L)

Highest Risk Population: Chronic alcoholics — alcohol impairs renal Mg²⁺ reabsorption, causes poor dietary intake, and increases GI losses.

Other Causes: Loop/thiazide diuretics, diarrhea, malabsorption, prolonged NPO/NG suction

Signs & Symptoms (similar to hypocalcemia — both cause increased neuromuscular excitability):

> 🔑 Key Connection: Hypomagnesemia makes it nearly impossible to correct hypokalemia and hypocalcemia — always check and replace magnesium first.

Hypermagnesemia (Mg²⁺ > 2.5 mEq/L)

Causes: Renal failure, excessive antacid or laxative use (Mg-containing products), excessive magnesium sulfate infusion

Signs & Symptoms — progressive loss of neuromuscular function:

1. Nausea, flushing, warmth

2. Loss of deep tendon reflexes (DTRs) ← earliest sign of toxicity

3. Respiratory depression (rate < 12/min)

4. Cardiac arrest

Magnesium Sulfate Toxicity (Eclampsia Patient):

| Assessment Finding | Significance |

|---|---|

| Absent DTRs (patellar reflex) | Stop infusion — early toxicity |

| RR < 12 breaths/min | Respiratory depression — emergency |

| Urine output < 30 mL/hr | Reduced excretion → accumulation |

Antidote: Calcium gluconate 1 g IV — keep at bedside for all patients on magnesium infusion

> ⚠️ Watch Out For: Always check the patellar reflex before each dose/during infusion of magnesium sulfate. Absent DTRs = Stop the magnesium infusion immediately.

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Phosphate & Refeeding Syndrome

Hypophosphatemia

Refeeding Syndrome:

Key Clinical Manifestation: Respiratory muscle weakness → failure to wean from mechanical ventilator ← classic NCLEX scenario

Prevention: Start nutritional support slowly with careful electrolyte monitoring

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Nursing Assessment & Interventions

Fluid Balance Monitoring

#### Daily Weight — Gold Standard

#### Urine Specific Gravity

#### Serum Osmolarity

Skin Turgor Assessment

Key Nursing Priorities by Imbalance

| Imbalance | Priority Assessment | Priority Action |

|---|---|---|

| Hypovolemia | VS, urine output, skin turgor | Isotonic IV fluids |

| Hypervolemia | Lung sounds, weight, edema | Fluid restriction, diuretics