Fluid & Electrolyte Study Notes

Fluid & Electrolyte Fundamentals

• Fluid & electrolyte (F&E) balance is critical for homeostasis; children and the elderly are especially prone to imbalances.
• Total-body water varies with age; it declines in older adults, reducing reserve volume.
• Functions of body water
  • Medium for biochemical reactions
  • Transport of nutrients, waste, hormones
  • Regulation of temperature & lubrication of joints/eyes

Fluid Distribution & Movement

• Compartments
  • Intracellular fluid (ICF) – ≈ frac{2}{3} of total body water
  • Extracellular fluid (ECF) – ≈ frac{1}{3} (interstitial + intravascular + transcellular)
• Passive transport
  • Diffusion – particles move from higher to lower concentration
  • Osmosis – water moves through a semipermeable membrane from lower solute to higher solute concentration
  • Filtration – hydrostatic pressure pushes water/solutes across membrane (e.g., glomerular filtration)
• Active transport – energy-requiring movement against gradient (e.g., \text{Na}^+ / \text{K}^+ pump)
• Tonicity & living cells
  • Isotonic – equal solute; cell size unchanged
  • Hypertonic – water leaves cell → cell shrinks ("skinny")
  • Hypotonic – water enters cell → cell swells ("hypo → hippo")

Physiologic Regulators of Fluid Volume

• Kidneys – prime organ; each nephron filters, reabsorbs, secretes to adjust
• Neural sensors
  • Osmoreceptors – trigger thirst & modulate vasopressin (ADH) release
  • Baroreceptors – sense pressure/volume → alter cardiac output & SVR
• Hormonal control
  • Renin–Angiotensin–Aldosterone System (RAAS)
  • ↓ renal perfusion → renin → angiotensin II → vasoconstriction & aldosterone release
  • Aldosterone ↑ \text{Na}^+ & water reabsorption, ↓ \text{K}^+ reabsorption
  • Antidiuretic Hormone (ADH) – from posterior pituitary; ↑ water reabsorption in distal nephron when volume/pressure is low
  • Atrial Natriuretic Peptide (ANP) – released by atrial stretch; opposes RAAS → promotes \text{Na}^+/water excretion, vasodilation

Age-Related Fluid Considerations

• Slower renal compensation for acid–base & electrolyte shifts
• Polypharmacy (antihypertensives, diuretics) ↑ risk
• Blunted thirst perception; chronic low-grade dehydration common

Fluid Volume Deficit (Hypovolemia)

• Etiologies: hemorrhage, burns, diarrhea, vomiting, suction, wounds, NPO, diuretics, endocrine disorders, elderly
• Assessment cues
  • Vital signs: ↓ BP (orthostatic), ↑ HR, ↑ RR
  • Weight ↓, urine output ↓ (< 0.5\,\text{mL·kg}^{-1}\text{·h}^{-1})
  • Poor skin turgor, dry mucosa, altered LOC
  • Labs: ↑ serum urea, creatinine, osmolarity; ↑ \text{Na}^+ (hypernatremia) & urine specific gravity; BUN/Cr ratio > 80
• Management
  • Oral rehydration → isotonic/appropriate fluids & diet
  • IV isotonic crystalloid (e.g., 0.9\% NS, LR)
  • Blood products if hemorrhage
  • Monitor I/O, daily weights, hemodynamics, labs

Fluid Volume Excess (Hypervolemia / Over-hydration)

• Causes: renal/heart failure, excess intake, high \text{Na}^+, hormonal (SIADH), inadequate elimination
• Assessment cues
  • ↑ BP, ↑ HR (bounding), ↑ weight, ↑ urine (early), ↓ specific gravity (dilution)
  • JVD, S3, pulmonary crackles, edema (dependent/pitting)
  • Neurologic: confusion, seizure risk
  • Labs: ↓ hematocrit, ↓ serum \text{Na}^+ (dilutional), ↓ osmolality
• Management
  • Restrict fluids & \text{Na}^+; daily weights
  • Diuretics (loop, thiazide)
  • High-/Semi-Fowler’s position, oxygen if needed
  • Address underlying cardiac/renal etiology

Comparison Summary

• Hypovolemia – “low volume in blood” → concentrated labs, weak threads, poor turgor
• Hypervolemia – “high volume in blood” → diluted labs, edema, crackles, JVD

Intravenous Solution Guide

• Isotonic – stays where you put it (vascular): 0.9\% NS, LR, 5\% Dextrose in Water (D5W initially)
  • Uses: blood loss, dehydration, maintenance
  • Risk: fluid overload; LR not for liver disease
• Hypotonic – goes out of vessel into cell: 0.45\% NS, 0.33\% NS, D5W (after dextrose use)
  • Uses: DKA, hypernatremia
  • Risk: cell swelling → cerebral edema
• Hypertonic – enters vessel from cell: 3\% NS, 5\% NS, D50.9\% NS, D10W
  • Uses: cerebral edema, severe hyponatremia, maintenance when restricted volume
  • Contra: burns, trauma, ↑ ICP; risk of cell dehydration/shrinkage & pulmonary edema

Electrolyte Overviews

Sodium (\text{Na}^+ 135–145\,\text{mEq/L})

• Function: nerve impulse, muscle contraction, water balance
• Hyponatremia (< 135)
  • S/S: HA, lethargy, seizures, cramps; “SALTy” mnemonic – Stupor, Anorexia, Lethargy, Tendon reflex ↓
  • Causes: excess water, diuretics, SIADH, GI losses
  • Care: fluid restriction, IV \text{Na}^+, seizure precautions
• Hypernatremia (> 145)
  • S/S: thirst, flushed skin, irritability, seizures, intracranial bleed, low-grade fever
  • Causes: water loss, \text{Na}^+ gain, DI, hypertonic feeds
  • Care: hypotonic/\text{Na}^+-free fluids, limit \text{Na}^+, monitor neuro

Potassium (\text{K}^+ 3.5–5.0\,\text{mEq/L})

• Function: intracellular osmolarity, nerve/muscle (heart) action potentials
• Hypokalemia (< 3.5)
  • S/S: fatigue, leg cramps, ↓ bowel sounds, U-wave ECG, dysrhythmia
  • Causes: diuretics (non-K-sparing), GI loss, insulin, alkalosis
  • Care: oral/IV \text{K}^+ (never IV push), K-sparing diuretic, monitor ECG
• Hyperkalemia (> 5.0)
  • S/S: muscle cramps, paresthesia, peaked T, widened QRS, cardiac arrest
  • Causes: renal failure, ACE-I, spironolactone, cell lysis, acidosis
  • Care: calcium gluconate (cardiac membrane), insulin + glucose, sodium bicarb, Kayexalate, dialysis; restrict K

Calcium (\text{Ca}^{2+} 9–10.5\,\text{mg/dL})

• Function: bone/teeth, clotting, neuro-muscular & cardiac conduction
• Hypocalcemia (< 9)
  • S/S: tetany, cramps, Trousseau’s, Chvostek’s, dysrhythmia, numb lips/fingers
  • Causes: hypoparathyroid, pancreatitis, vit D deficit, CKD, phosphate excess
  • Care: calcium gluconate/chloride IV (warm, slow), vit D, seizure precautions, dietary ↑
• Hypercalcemia (> 10.5)
  • S/S: lethargy, bone pain, kidney stones, ↓ DTR, arrhythmia
  • Causes: hyperparathyroid, malignancy, thiazides
  • Care: IVF + loop diuretic, calcitonin, bisphosphonate, phosphate, mobilize safely
• Inverse relationship with phosphate (\text{PO}_4^{3-})

Magnesium (\text{Mg}^{2+} 1.5–2.5\,\text{mEq/L})

• Function: enzyme cofactor, neuromuscular stability, blood sugar/BP regulation
• Hypomagnesemia (< 1.5)
  • S/S: ↑ DTR, tremor, tachycardia, seizures, Trousseau/Chvostek, dysphagia
  • Causes: malnutrition, alcoholism, diarrhea, diuretics, insulin
  • Care: Mg sulfate IV/PO (monitor deep tendon reflex), diet ↑ Mg, seizure precautions
• Hypermagnesemia (> 2.5)
  • S/S: hypotension, bradycardia, ↓ DTR, respiratory depression, lethargy
  • Causes: renal failure, Mg antacids/laxatives, DKA
  • Care: calcium gluconate IV, diuretics, dialysis, Mg-restricted diet
• Mg parallels Ca levels (both sedative-like)

Nursing & NCLEX Focus

• Recognize cues: compare current vitals/labs to baseline; note meds impacting F&E (diuretics, ACE-I, laxatives, antacids, steroids).
• Monitor interactions between prescribed fluids & existing imbalances (e.g., avoid hypotonic fluids in ↑ ICP).
• Evaluate responses: I/O trends, daily weights > 1\,\text{kg} change = \approx 1\,\text{L} fluid.
• Patient teaching
  • Diet: high/low \text{Na}^+, \text{K}^+, \text{Ca}^{2+} depending on condition
  • Fluid restriction or encouragement
  • Medication adherence & side-effects
• Safety: orthostatic hypotension (hypovolemia), seizure precautions (Na/Ca/Mg), fall risk (diuretics), airway (swallow eval in hypo-Mg/Ca)