Fluid & Electrolytes – Med-Surg/Pharmacology Review

Homeostasis & Fluid Balance

• Homeostasis = body’s ability to keep an internal equilibrium despite external changes.

• Fluid balance ≈ “keeping the right amount of water + electrolytes.”

• Key electrolytes repeatedly referenced: potassium (K^+), sodium (Na^+), chloride (Cl^-), magnesium (Mg^{2+}), plus glucose & urea as solutes.

Osmolality (Concentration of Body Fluids)

• Definition: number of dissolved particles (solutes) per kilogram of water; reflects concentration.

• Serum osmolality

  • Normal: 285–295\,\text{mOsm\,kg}^{-1}.

  • >295 ⇒ “too concentrated” (dehydration; ↓H₂O, ↑solutes).

  • <285 ⇒ “too diluted” (over-hydration; ↑H₂O, ↓solutes).

  • Measures: Na^+, glucose, urea (BUN).

• Urine osmolality

  • Normal: 50–1200\,\text{mOsm\,kg}^{-1} (wide range).

  • High ⇒ kidneys holding H₂O → concentrated urine.

  • Low ⇒ kidneys excreting H₂O → diluted urine.

• Clinical links

  • Dehydration ≈ high serum osmolality + concentrated urine.

  • Over-hydration ≈ low serum osmolality + very dilute urine (or occasionally very concentrated if kidneys fail to excrete).

• Always monitor: I&O, lab values, and edema/skin turgor.

Fluid Compartments (Where the 60 % Body Water Resides)

• IV fluid moves: intravascular → interstitial → intracellular depending on tonicity & body needs.

Osmosis

• Movement of water only across a semipermeable membrane from lower solute (more water) → higher solute (less water) to equalize concentrations.

• Example:

  • Cell placed in salty solution (high solute) ⇒ water exits cell ⇒ cell shrinks.

• Clinical IV example: 0.45 % NaCl (½ NS) = hypotonic

  • Water shifts into cells → re-hydrates.

  • Monitor for edema (peripheral or cerebral → headache, confusion, seizure).

Thirst Mechanism

• Control center: lamina terminalis near hypothalamus.

• Senses serum osmolality.

  • ↑ Osmolality ⇒ stimulates thirst → drink → osmolality normalizes.

  • ↓ Osmolality ⇒ turns off thirst.

• Populations with blunted thirst: elderly, dementia → must offer fluids, monitor I&O & dehydration signs.

Antidiuretic Hormone (ADH = Vasopressin)

• Secreted by posterior pituitary; “body’s water-saver.”

• Triggered by: dehydration, ↑osmolality, vomiting, diarrhea, sweating.

• Action: travels to kidneys → ↑water reabsorption in nephron → ↓urine output → restores volume.

Kidney Function (Natural Filter)

• Two kidneys, retro-peritoneal lower back.

• Filter ≈ 50 gallons blood/day.

• Remove wastes: urea, creatinine, extra H₂O.

• Reabsorb what body needs: water, electrolytes → keeps BP, pH, nerves, muscles stable.

• Monitoring

  • Urine output (sudden ↓ = red flag).

  • Labs: BUN, creatinine, GFR.

  • Fluid imbalance signs: edema, dehydration, BP changes.

Fluid Loss Pathways

1. Sensible (measurable) – urine, vomit, liquid stool, wound drainage, visible blood loss, profuse sweat you can collect.

2. Insensible (not easily measured) – respiration, water in formed stool, skin evaporation without visible sweat.

• Dehydration signs: poor skin turgor, sunken eyes, dry mucosa/lips, dark urine, ↓urine output, low BP.

Movement of Solutes vs Water Across Membranes

• Diffusion – passive spread of particles/solutes (electrolytes) from high → low concentration. Example: O_2 diffuses from alveoli (high) → blood (low).

• Active transport – requires ATP; classic Na⁺/K⁺ pump moves 3\,Na^+ out & 2\,K^+ in per cycle (maintains excitability of nerves, heart, muscles). Drugs such as digoxin influence this pump.

Major Electrolytes (Quick Reference)

Potassium Imbalances

Hypokalemia

• Lab: <3.5\,mEq/L (critical adult <3.0; infants <2.5).

• Causes: loop/thiazide diuretics, amphotericin B, GI losses (vomit, diarrhea, laxative), ↓intake, ETOH excess.

• Mild (3–3.5): often asymptomatic; maybe fatigue, muscle weakness, constipation.

• Severe (<3): arrhythmias, respiratory paralysis, hypotension.

• Dx: serum BMP/CMP, ECG changes.

• Tx: oral/IV K^+, dietary high-potassium foods (bananas, OJ, cantaloupe, avocado, beans, lentils, coconut water, tomato juice); adjust meds.

Hyperkalemia

• Lab: >5.0\,mEq/L (critical adult >6.1; newborn >8).

• Causes: renal failure (#1), K-sparing diuretics, dehydration, diabetes, burns/trauma, transfusions, excess intake.

• Mild: often none; Moderate–severe: muscle weakness → paralysis, dangerous ECG changes → arrhythmias → cardiac arrest.

• Tx: calcium gluconate (stabilize heart), loop diuretics, possible insulin + glucose, dialysis, dietary K restriction (avoid salt substitutes containing KCl).

Sodium Imbalances

Hyponatremia

• Lab: <135\,mEq/L (critical <120).

• Common cause: excess water intake (dilutional). Others: thiazide diuretics, GI losses, ETOH, burns, CHF/CKD/CLD.

• Mild: nausea, general malaise.

• Moderate: lethargy, confusion, HA, irritability.

• Severe: muscle twitching, seizures, death.

• Dx: BMP/CMP, urine Na⁺, serum osmolality.

• Tx: fluid restriction; IV isotonic or hypertonic saline for severe; educate moderate water intake, monitor urine color, consider sports drinks during intense exercise.

Hypernatremia

• Lab: >145\,mEq/L (critical >160).

• Causes: dehydration (water loss > Na loss), GI losses, burns, sweating, CKD, diabetes, impaired thirst (elderly, infants).

• Manifestations mirror hypo but from neuronal dehydration: confusion → seizures → death.

• Dx: BMP/CMP, plasma osmolality.

• Tx: gradual IV fluid replacement (D5W or 0.45 % NS); correct slowly to avoid cerebral edema.

• Patient education: limit processed meats (deli, bacon, hot dogs), canned soup/veg, breads, cheeses, condiments (ketchup, BBQ, soy sauce) & sports drinks.

Calcium Imbalances

Hypocalcemia

• Lab: <8.5\,mg/dL.

• Causes: laxatives, long-term steroids, loop diuretics, PPIs, vitamin D deficiency, menopause (hormonal), multiple transfusions (citrate binding), low Mg^{2+}/PO_4^{3-}.

• S/S: numb/tingling fingers/toes, muscle cramps, Chvostek’s sign (facial twitch when tapping cheek), Trousseau’s sign (carpal spasm with BP cuff), possible laryngospasm/arrhythmia if severe.

• Tx: dietary Ca + vit D, supplements; severe → IV calcium gluconate/chloride.

Hypercalcemia

• Lab: >10.5\,mg/dL.

• Causes: cancers, vitamin D toxicity, lithium, thiazides, renal failure, prolonged immobility (bone resorption).

• S/S: “moans, stones, bones, groans” → constipation, abd pain, N/V, anorexia; bone pain, muscle weakness, neuro changes.

• Mild asymptomatic → encourage hydration; symptomatic mild → oral phosphates (↓ Ca absorption).

• Moderate → IV isotonic saline + loop diuretics.

• Severe → hemodialysis.

Magnesium, Phosphate, Chloride, Bicarbonate (Mentioned Briefly)

• Imbalances influence or are influenced by Ca^{2+} & K^+; assess concurrently in complex cases.

Ethical / Practical Nursing Implications

• IV fluid type & rate matter; re-assess patient frequently (neuro status, lung sounds, edema, I&O, labs, vitals).

• Elderly/demented: proactively offer fluids; do not rely on thirst.

• Kidney / heart patients: sodium & fluid restriction education essential.

• Digoxin, diuretics, lithium, amphotericin B, PPIs, steroids all alter electrolytes; anticipate, monitor, teach.

• Always correct sodium slowly to prevent cerebral edema; always place hyper/hypo-kalemic pts on ECG.

• Recognize insensible losses in fever or ventilated pts; adjust replacement accordingly.

Quick Reference – High-Yield Numbers

• Serum Osmolality 285–295 mOsm/kg

• Urine Osmolality 50–1200 mOsm/kg

• K^+ 3.5–5.0 mEq/L (crit <3 or >6.1)

• Na^+ 135–145 mEq/L (crit <120 or >160)

• Ca^{2+} 8.5–10.5 mg/dL

• Key pump: 3\,Na^+{out} : 2\,K^+{in} : 1\,ATP

Study Tips / Integration for Med-Surg I & Pharmacology

• Link IV fluid tonicity to expected fluid shifts (e.g., 0.9 % NS vs 0.45 % NS vs D5W).

• Memorize normal ranges & critical values; practice interpreting lab panels.

• Draw concept maps: cause → pathophysiology → signs/symptoms → diagnostics → interventions.

• Anticipate drug–electrolyte interactions (diuretics, digoxin, lithium, Ca gluconate, insulin w/ glucose, etc.).

• Practice scenario questions: “Elderly pt on thiazide arrives confused with Na 118…” identify priority actions.