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)
- Post-op patient on 3\% NaCl → monitor SOB / ↑ RR first.
- Older adult w/ overload: expected findings include tachycardia, 2+ pitting edema, JVD, bilateral rhonchi.
- Serum K^+ 5.6\,\text{mEq/L} is most concerning vs. slightly low/high others.
- 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
- 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.
- 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.