Fluid, Electrolyte, and Acid-Base Balance

Homeostasis & Body Water

  • Homeostasis
    • Natural physiologic equilibrium.
    • Maintained through multiple adaptive responses that keep fluid & electrolyte composition/volume within narrow limits.
  • Body-Water Content
    • Varies by age, biologic sex, adiposity.
    • Approximate % of total body weight that is water:
    • Preterm neonate ≈ 80\%
    • Term neonate/Infant ≈ 70\%
    • Child ≈ 60\%
    • Healthy adult ≈ 50\text{–}60\%
    • Older adult ≈ 45\text{–}50\%
    • ↑ fat = ↓ water (adipose contains little water).

Fluid Compartments & Distribution

  • Intracellular Fluid (ICF)
    • ≈ 40\% body weight ((\approx\tfrac{2}{3}) total body water).
  • Extracellular Fluid (ECF)
    • Sub-compartments:
    • Interstitial (between cells)
    • Intravascular (plasma)
    • Transcellular (CSF, pleural, synovial, GI, etc.).
  • Distribution Terms
    • First spacing = normal distribution in ICF & ECF.
    • Second spacing = excess interstitial fluid (edema).
    • Third spacing = fluid trapped in a potential/"non-functional" space (ascites, pleural effusion); unavailable for circulation.
  • Fluid Gain/Loss Indicator
    • 1\text{ L water}=1\text{ kg}=2.2\text{ lb} → daily weight is the single best indicator.

Osmotic Concepts

  • Osmotic Movement
    • Water moves down its concentration (↑→↓ water) gradient across semi-permeable membranes.
    • Isotonic: equal osmolality inside & outside cells; no net shift.
    • Hypotonic medium: ECF ↓ solute → water enters cells (cell swells).
    • Hypertonic medium: ECF ↑ solute → water leaves cells (cell shrinks).
  • Osmolarity vs. Osmolality
    • Osmolarity = total mOsm per L of solution.
    • Osmolality = total mOsm per kg of water (clinical standard).
    • Normal serum osmolality 280\text{–}295\,\text{mOsm/kg}
    • >295 → water deficit (hypertonic).
    • <275 → water excess (hypotonic).

Geriatric Considerations

  • ↓ renal concentrating & conserving ability (nephron loss, ↓ GFR).
  • Hormonal changes: ↓ renin & aldosterone, ↑ ADH & ANP.
  • ↓ subcutaneous tissue → ↑ insensible water loss & impaired thirst perception.

Fluid Balance Regulation Mechanisms

  • Antidiuretic Hormone (ADH): released from posterior pituitary; kidneys reabsorb free water.
  • Thirst Mechanism (hypothalamus): stimulates fluid intake.
  • Aldosterone (RAAS): Na⁺ & water reabsorption, K⁺ excretion.
  • Atrial/Brain Natriuretic Peptides (ANP, BNP): released when atria/ventricles stretch; oppose RAAS & ADH → natriuresis + diuresis.
  • GI Tract absorbs ~8000\,\text{mL} water daily (can also lose via vomiting/diarrhea).

Fluid-Volume Deficit (Hypovolemia)

  • Definition: ECF volume depletion from abnormal loss, inadequate intake, or plasma → interstitial shift.
  • Common causes: hemorrhage, burns, vomiting, diarrhea, overdiuresis, fever, third-spacing, inadequate PO.
  • Manifestations: restlessness, lethargy, thirst, dry mucous membranes, ↓ skin turgor, postural hypotension, ↑ HR, ↓ CVP, ↓ urine output, concentrated urine, weight loss, ↑ respiratory rate, cold/clammy skin.
  • Complications: hypovolemic shock, renal impairment, electrolyte imbalances.
  • Treatment / Interprofessional care:
    • Encourage PO fluids if able.
    • Replace volume with balanced isotonic IV solutions (e.g., 0.9\% NS, Lactated Ringer’s).
    • Possible blood products for hemorrhage.
    • Treat underlying cause.

Fluid-Volume Excess (Hypervolemia)

  • Definition: Excess fluid intake/retention or interstitial → plasma shift.
  • Causes: renal failure, heart failure, Cushing’s, SIADH, excessive isotonic/hypotonic fluids, long-term corticosteroids.
  • Manifestations: bounding pulses, JVD, ↑ BP, crackles, dyspnea, weight gain, edema, polyuria, S₃ heart sound.
  • Complications: pulmonary edema, cerebral edema, skin breakdown, impaired gas exchange.
  • Treatment / Interprofessional care:
    • Diuretics (loop, thiazide).
    • Fluid & Na⁺ restrictions.
    • Remove fluid directly when indicated (paracentesis for ascites, thoracentesis for pleural effusion).
    • Monitor weights, I&O, electrolytes.
  • Priority Order Example: Patient w/ edema, crackles → Administer furosemide 40\,\text{mg} IV push takes priority (rapid removal of fluid).

Nursing Care for Fluid Volume Disturbances

  • Daily weights (same scale, time, clothing).
  • Strict I&O; note urine specific gravity.
  • Laboratory monitoring (Hct, BUN, Na⁺, serum osmolality).
  • Cardiopulmonary assessment (BP, HR, CVP, lung sounds, JVD).
  • Safety: fall risk if orthostatic, skin care for edema/fragility.

Intravenous (IV) Fluids: Purpose & Overview

  • Maintenance: when PO intake insufficient.
  • Replacement: correct deficits/losses.
  • Selection based on tonicity, electrolyte content, patient labs, comorbidities.

Isotonic Solutions

  • General features: expand ECF only; no net ICF shift. Ideal for hypovolemia.
  • Normal Saline (NS; 0.9\%\,NaCl)
    • No calories; slightly ↑ NaCl vs. plasma.
    • Preferred for immediate IV volume expansion & compatible w/ blood products/meds.
    • Risk: fluid overload, hyperchloremic acidosis (large volumes).
  • Lactated Ringer’s (LR)
    • Electrolyte composition closer to plasma (contains K⁺, Ca²⁺, PO₄³⁻, lactate → metabolized to HCO_3^-).
    • Good for burns, GI losses, surgical patients.
    • Contra-indicated with liver failure (can’t metabolize lactate), hyperkalemia, severe hypovolemia.
  • D5W (Dextrose 5\% in water)
    • Isotonic in bag; dextrose quickly metabolized → leaves free water (acts hypotonic).
    • Provides 170\,\text{kcal/L}; used for water replacement, hypernatremia prevention of ketosis.
    • No electrolytes.

Hypotonic Solutions

  • ½ NS (0.45\%\,NaCl), D5W (after metabolism).
  • Water > solute: moves water from ECF → ICF. Useful for intracellular dehydration (DKA after initial volume resuscitation).
  • Risks: cellular swelling, cerebral edema, hypotension; avoid in ↑ICP, burns, trauma.

Hypertonic Solutions

  • Examples: 3\% NaCl, 5\% NaCl, D5 ½ NS, D10W.
  • Draw water from ICF → ECF/vascular (volume expander, correct severe hyponatremia).
  • Nursing focus: frequent BP, lung sounds, serum Na⁺ monitoring; risk of intravascular fluid overload & pulmonary edema.
  • D10W: provides 340\,\text{kcal/L}; highest dextrose ‎conc. safe via peripheral IV.
  • Case: During 3\% NaCl infusion for hyponatremia, highest-priority assessment = shortness of breath & ↑ RR (sign of pulmonary overload).

Colloids (Plasma Expanders)

  • Large molecules/proteins stay intravascular → ↑ oncotic pressure.
  • Types: Albumin, Fresh Frozen Plasma, Packed RBCs, synthetic starches.
  • Indications: hypovolemia unresponsive to crystalloids, hemorrhagic shock, major surgeries/trauma.
  • Provide volume with less total fluid volume compared to crystalloids.

Summary Table

  • Hypotonic: water ↓ tonicity; e.g., 0.45\% NS.
  • Isotonic: equal; e.g., NS, LR, D5W (initial).
  • Hypertonic: higher solute; e.g., 3\% NaCl, D10W.
  • Colloids: PRBCs, albumin; ↑ oncotic pressure.

Fluid & Electrolyte NCLEX-Type Questions (Selected Examples)

  1. Post-op patient on 3\% NaCl → monitor SOB / ↑ RR first.
  2. Older adult w/ overload: expected findings include tachycardia, 2+ pitting edema, JVD, bilateral rhonchi.
  3. Serum K^+ 5.6\,\text{mEq/L} is most concerning vs. slightly low/high others.
  4. Na^+ 133 & K^+ 3.4 after 3 tap-water enemas explain dilutional hyponatremia & hypokalemia.

Electrolyte Relationships & Overview

  • Inverse relationships:
    • Na^+ ↔ K^+
    • Ca^{2+} ↔ PO_4^{3-}
    • Mg^{2+} ↔ PO_4^{3-}
  • Similar/Direct relationships:
    • Ca^{2+} & Vitamin D (absorption).
    • Ca^{2+} & Mg^{2+}
    • Mg^{2+} & K^+ (often rise/fall together).

Specific Electrolyte Imbalances (Framework)

For each imbalance consider Causes, Findings, Labs, Nursing Interventions, Collaborative Care. (Lecture left blanks for group discussion; fill with common points.)

Hyponatremia (< 135\,\text{mEq/L})

  • Causes: GI losses, diuretics, SIADH, adrenal insufficiency, excessive hypotonic fluids, polydipsia.
  • Findings: headache, irritability, confusion, seizures, N/V, coma.
  • Interventions: fluid restriction, hypertonic saline (severe), loop diuretics, seizure precautions.

Hypernatremia (> 145\,\text{mEq/L})

  • Causes: lack of water access, diabetes insipidus, osmotic diuresis, hypertonic tube feeds/IVs, burns.
  • Findings: thirst, agitation, restlessness, lethargy → seizures, coma; dry mucosa, orthostatic hypotension, ↑ osmolality.
  • Interventions: treat cause; oral fluids, IV hypotonic/isotonic fluids, monitor neuro status.

Hypokalemia (< 3.5\,\text{mEq/L})

  • Causes: GI loss, diuretics, laxative abuse, insulin therapy, alkalosis.
  • Findings: fatigue, muscle weakness, leg cramps, polyuria, hyperglycemia, U wave on ECG.
  • Interventions: oral/IV KCl (never IV push), cardiac monitoring, treat underlying.

Hyperkalemia (> 5.0\,\text{mEq/L})

  • Causes: renal failure, ACE-I/ARB, K-sparers, acidosis, tissue catabolism.
  • Findings: cramping leg pain, paresthesia, diarrhea, tall peaked T, widened QRS, loss of P.
  • Interventions: stop K intake, diuretics, insulin-glucose, calcium gluconate, dialysis, kayexalate.
  • Sample NCLEX: K^+ 5.4 → assess ECG changes & paresthesia.

Hypocalcemia (< 8.6\,\text{mg/dL})

  • Causes: CKD, pancreatitis, ↓ vit D, hypoparathyroidism, loop diuretics.
  • Findings: tetany, Chvostek/Trousseau signs, laryngeal spasm, numbness/tingling, seizures, prolonged QT.
  • Interventions: IV or PO calcium, vitamin D, seizure precautions.

Hypercalcemia (> 10.2\,\text{mg/dL})

  • Causes: hyperparathyroidism, malignancy, vitamin D overdose, thiazides.
  • Findings: lethargy, weakness, decreased reflexes, polyuria, kidney stones, shortened QT.
  • Interventions: loop diuretics w/ isotonic saline, calcitonin, bisphosphonates, ambulation.

Hypomagnesemia (< 1.5\,\text{mEq/L})

  • Causes: alcoholism, malabsorption, diuretics, DKA, proton-pump inhibitors.
  • Findings: hyperactive reflexes, tremors, seizures, dysrhythmias, resembles hypocalcemia.
  • Interventions: oral/IV MgSO_4 (monitor for hypotension, flushing), seizure precautions.

Hypermagnesemia (> 2.5\,\text{mEq/L})

  • Causes: renal failure, excessive Mg antacids/laxatives, adrenal insufficiency.
  • Findings: lethargy, ↓ deep tendon reflexes, flushed warm skin, ↓ pulse & BP, respiratory depression, cardiac arrest.
  • Interventions: restrict Mg, IV calcium gluconate antidote, dialysis.

Hypophosphatemia (< 2.4\,\text{mg/dL})

  • Causes: malnourishment, alcohol withdrawal, diabetic ketoacidosis, respiratory alkalosis.
  • Findings: weakness, bone pain, Rhabdomyolysis, seizures.
  • Interventions: oral/IV phosphate, correct Ca/Mg concurrently.

Hyperphosphatemia (> 4.4\,\text{mg/dL})

  • Causes: AKI/CKD, tumor lysis, excessive intake, hypoparathyroidism.
  • Findings: hypocalcemia signs (tetany), calcified tissue deposits.
  • Interventions: phosphate binders (Sevelamer), restrict dairy, dialysis, correct Ca²⁺.

ECG Changes with Potassium

  • Normokalemia: normal P, PR 0.12\text{–}0.20 s, rounded T, normal QRS.
  • Hypokalemia: slight PR prolongation, flattened T, presence of U-wave, ST depression.
  • Hyperkalemia: tall peaked T, wide flat P, prolonged PR, widened QRS, depressed ST, ↓ R amplitude.

Acid–Base Balance Basics

  • pH determined by [H^+]:
    • Normal pH 7.35\text{–}7.45.
  • Regulatory Systems:
    • Chemical Buffers (bicarb, phosphate, proteins) react immediately.
    • Respiratory: minutes → hours; CO₂ (carbonic acid) via rate/depth changes.
    • Renal: hours → days; reabsorb/secrete HCO_3^- & H^+.

Specific Acid–Base Disorders

Metabolic Acidosis

  • Low pH, low HCO_3^-.
  • Causes: DKA, severe diarrhea (bicarb loss), renal failure, shock (lactic acidosis).
  • Clinical picture: headache, ↓ BP, Kussmaul respirations (compensatory), hyperkalemia, warm flushed skin, N/V, ↓ LOC.

Metabolic Alkalosis

  • High pH, high HCO_3^-.
  • Causes: vomiting, NG suction, diuretics, excess NaHCO₃, hypokalemia.
  • S/S: restlessness → lethargy, tachycardia, compensatory hypoventilation, tremors, tingling fingers, muscle cramps.

Respiratory Acidosis

  • Low pH, high PaCO_2 (hypoventilation).
  • Causes: COPD, over-sedation, brainstem trauma, pneumonia, pulmonary edema, respiratory muscle paralysis.
  • S/S: hypoventilation → hypoxia, rapid shallow respirations, ↓ BP, headache, hyperkalemia, dysrhythmias, drowsiness.

Respiratory Alkalosis

  • High pH, low PaCO_2 (hyperventilation).
  • Causes: anxiety, pain, PE, fever, mechanical ventilation.
  • S/S: seizures, deep rapid breathing, tachycardia, ↓ or normal BP, hypokalemia, numbness/tingling, light-headedness.

GI Mnemonic

  • Vomiting = acid loss → metabolic alkalosis.
  • Diarrhea = base loss → metabolic acidosis.

Arterial Blood Gas (ABG) Essentials

  • Provides objective acid-base, ventilation, oxygenation data.
  • Normal Values:
    • pH 7.35\text{–}7.45
    • PaCO_2 35\text{–}45\,\text{mmHg}
    • HCO_3^- 21\text{–}28\,\text{mEq/L}
    • PaO_2 80\text{–}100\,\text{mmHg}
  • ROME Mnemonic
    • Respiratory Opposite: pH ↑ → PaCO2 ↓ (alkalosis); pH ↓ → PaCO2 ↑ (acidosis).
    • Metabolic Equal: pH & HCO_3^- move same direction.
  • Tic-Tac-Toe Method: place pH, PaCO2, HCO3^- under columns (acid/normal/base) to identify disorder quickly.

Practice / Critical Thinking Scenarios

  • Compare hypovolemia from hemorrhage (loss of whole blood; may need PRBCs) vs. gastroenteritis (loss of water + electrolytes; isotonic fluids + electrolyte replacement).
  • Pre-op pt w/ normal Na⁺/Cl⁻/K⁺ needs 0.45 % NaCl + D5 ± KCl (common maintenance) unless contraindicated.
  • Long-term care resident w/ fever, weakness: monitor orthostatic BP priority (fluid deficit risk).

Quick Reference Numeric Highlights

  • 1\text{ kg}=1\text{ L} fluid.
  • Serum osmolality normal 280\text{–}295\,\text{mOsm/kg}.
  • Hypertonic saline: 3\text{–}5\% NaCl; administer via central line for >3\%.
  • Dextrose limits peripheral infusion: ≤ 10\%.

Ethical & Practical Implications

  • Over-correction of chronic hyponatremia can cause osmotic demyelination syndrome.
  • Rapid shifts with hypertonic/hypotonic fluids demand consent & close monitoring.
  • In older adults, polypharmacy (diuretics, laxatives) heightens electrolyte risk; require interdisciplinary review.

Connections to Foundational Physiology

  • Starling forces (hydrostatic vs. oncotic) underpin first/second/third spacing.
  • RAAS, ANP, and ADH interplay links cardiovascular & renal systems.
  • Henderson-Hasselbalch concept explains combined respiratory & metabolic regulation of pH.

Study Tips

  • Memorize ROME, body-water percentages, IV fluid tonicity categories.
  • Link clinical signs to lab patterns (e.g., muscle cramps → Ca²⁺/Mg²⁺; neuro changes → Na⁺).
  • Practice ABG tic-tac-toe until automatic.