Fluid, Electrolyte, and Acid-Base Imbalances – Core Vocabulary

Body Water, Homeostasis, and Functions

• Water ≈ 60 % of adult male body weight, 50 % female, 70 % infant (↓ to ≈45 % in elderly women)
• Central to homeostasis; acts as:
  • Medium for metabolic reactions
  • Transport system (blood, lymph, secretions)
  • Lubricant & cushion for joints, organs, CSF
  • Facilitator of movement (muscle, GI, lungs)
• Input must equal output (≈2500\,\text{mL·day}^{-1}):
  • Sources: liquids (1200 mL), solid foods (1000 mL), cell metabolism (300 mL)
  • Losses: urine (1400 mL), feces (200 mL), lungs (400 mL), skin (500 mL)

Fluid Compartments and Typical Volumes

• Intracellular Fluid (ICF):
  • ≈28\text{ L} (40 % body weight adult male)
• Extracellular Fluid (ECF): subdivided into
  • Intravascular fluid (IVF/plasma) ≈4–5 %
  • Interstitial fluid (ISF) ≈10–15 %
  • Cerebrospinal fluid (CSF)
  • Transcellular fluids (synovial, pericardial, pleural, ocular, GI secretions)
• Total water ≈43\text{ L} (60 % adult male)

Regulation of Water & Electrolytes

• Thirst → osmoreceptors (hypothalamus) trigger intake
• Antidiuretic Hormone (ADH) → ↑ water re-absorption in renal distal tubules & collecting ducts
• Aldosterone → ↑ Na^+ & water re-absorption
• Atrial & B-type Natriuretic Peptides (ANP, BNP/T-type) → ↑ Na^+/water excretion, inhibit RAAS, vasodilate

Movement of Water Between Compartments

• Governed by filtration & osmosis across semipermeable capillary membranes
  • Hydrostatic pressure (pushing) vs. Osmotic/oncotic pressure (pulling)
  • Water flows from \text{low\ solute}\;\to\;\text{high\ solute} concentration
• Normal starling forces example (arteriolar end):
  • P_{hydro}^{IVF}=30\,\text{mmHg} outward
  • \pi_{oncotic}^{IVF}=25\,\text{mmHg} inward

Fluid Excess — Edema

• Definition: excess ISF → visible swelling, possible weight gain
• Clinical forms: localized (injury, allergic), generalized (anasarca), pulmonary, cerebral, ascites

Etiologies

• ↑ Capillary hydrostatic pressure (HTN, hypervolemia, CHF, pregnancy) → forces fluid out
• ↓ Plasma oncotic pressure (loss of albumin via malnutrition, nephrosis, burns)
• ↑ Capillary permeability (inflammation, infection, toxins, large burns)
• Lymphatic obstruction (tumor, surgical removal, radiation) → protein-rich localized edema

Consequences

• Pitting edema (indent persists)
• Functional impairment: ↓ joint ROM, ↓ vital capacity, impaired diastolic filling
• Pain (pressure on nerves; organ capsule stretch)
• ↓ arterial circulation → ischemia, skin breakdown, ulcers, infection; dental fitting issues
• Drug trapping in ISF (↓ therapeutic effect)

Fluid Deficit — Dehydration

• Caused by ↓ intake, ↑ loss, or both; severe in infants & elderly

Common Causes

• Vomiting/diarrhea, excessive sweating, diabetic ketoacidosis (osmotic diuresis), limited access to water, concentrated infant formula

Manifestations

• Dry mucosa, ↓ skin turgor, sunken eyes & fontanelles (infant)
• Rapid weak pulse, ↓ BP, orthostatic hypotension
• ↑ hematocrit, ↑/variable electrolytes, high urine specific gravity with low volume
• Fatigue, dizziness, confusion → LOC

Compensation

• ↑ thirst & HR, cutaneous vasoconstriction, oliguria with concentrated urine

Third-Spacing

• Fluid trapped in cavity/ISF (burns: ↑ ISF osmotic pressure; sepsis: ↑ capillary permeability). Non-functional until reabsorbed

Major Electrolyte Distribution (typical, mEq·L⁻¹)

• [Na^+]{ICF}\approx10 vs [Na^+]{plasma}\approx142
• [K^+]{ICF}\approx160 vs [K^+]{plasma}\approx4
• [Ca^{2+}]_{plasma}\approx5 (ionized) — largely extracellular
• [Mg^{2+}]{ICF}\approx35,\ [Mg^{2+}]{plasma}\approx3
• Bicarbonate [HCO3^-]{plasma}\approx27; Chloride [Cl^-]{plasma}\approx103; Phosphate [HPO4^{2-}]_{ICF}\approx140

Sodium Imbalances

Hyponatremia (Na^+ < 135 mEq·L⁻¹)

• Loss via sweating, vomiting, diarrhea; diuretics + low-salt diet; endocrine (↓ aldosterone, adrenal insuff., ↑ ADH), excessive water intake/IV D5W, renal losses (osmotic diuresis)
• Effects:
  • Cellular swelling → fatigue, cramps, nausea
  • ↓ ECF osmotic pressure → hypovolemia, ↓ BP
  • Cerebral edema → headache, confusion, seizures

Hypernatremia (Na^+ > 145 mEq·L⁻¹)

• Etiology: ↓ ADH (diabetes insipidus), lack of thirst, watery diarrhea, prolonged tachypnea, excessive Na^+ intake (tube feeding, hypertonic IV) w/ inadequate water
• Effects: thirst, dry tongue, rough mucosa, edema, agitation, ↑ BP

Potassium Imbalances (life-threatening cardiac effects)

Hypokalemia (K^+ < 3.5 mEq·L⁻¹)

• Causes: diarrhea, loop/thiazide diuretics, hyperaldosteronism/Cushing, insufficient intake, insulin treatment of DKA
• Manifestations:
  • Cardiac dysrhythmias → cardiac arrest (flattened T-wave, U-wave)
  • Neuromuscular: muscle weakness, paresthesias, ↓ GI motility, shallow respirations, polyuria (↓ ADH response)

Hyperkalemia (K^+ > 5 mEq·L⁻¹)

• Causes: renal failure, K^+-sparing diuretics, hypoaldosteronism, extensive tissue damage/burns, acidosis (H^+/K^+ shift)
• Manifestations:
  • ECG changes (tall peaked T, widened QRS) → VT/VF, arrest
  • Muscle weakness → paralysis, respiratory failure
  • Paresthesias, nausea, oliguria
• Relationship: acidosis drives K^+ extracellularly (and vice-versa)

Calcium Imbalances

Hypocalcemia (Ca^{2+} < 4.5 mEq·L⁻¹ or < 2.2 mmol·L⁻¹ ionized)

• Etiologies: hypoparathyroidism, malabsorption/vit D deficit, ↓ albumin, alkalosis, renal failure
• Effects: ↑ neuromuscular excitability → tetany (Chvostek & Trousseau signs), muscle twitch, paresthesias; weak heart contractions → dysrhythmia, ↓ BP

Hypercalcemia (Ca^{2+} > 5.5 mEq·L⁻¹)

• Causes: hyperparathyroidism, malignancy (bone tumors), immobility, excess vit D/Ca intake, milk-alkali syndrome
• Effects: ↓ neuromuscular activity (weakness, lethargy), GI upset, polyuria (ADH resistance), renal stones, ↑ cardiac contractility & dysrhythmias

Magnesium

• Hypomagnesemia: malnutrition, alcoholism, diuretics, DKA, hyperthyroid → ↑ neuromuscular irritability, cardiac dysrhythmia
• Hypermagnesemia: renal failure → CNS depression, hyporeflexia

Phosphate

• Functions: bone/teeth, ATP, buffer, cell membrane; inverse with Ca^{2+}
• Hypophosphatemia: malabsorption, diarrhea, antacids
• Hyperphosphatemia: renal failure

Chloride

• Major ECF anion; parallels Na^+
• Hypochloremia usually with alkalosis (early vomiting → loss of HCl, chloride shift: HCO_3^- exits RBC to plasma)
• Hyperchloremia: excess NaCl intake → metabolic acidosis possibility

Acid–Base Balance Basics

• Normal serum pH 7.35\text{–}7.45; death < 6.8 or > 7.8
• Primary buffer: \text{NaHCO}3/\text{H}2\text{CO}_3 with ideal ratio 20:1
• 3 lines of defense:
  • Buffers (instantaneous)
  • Respiratory (minutes) alters CO2 → H2CO_3
  • Renal (hours–days) excrete H^+, regenerate HCO_3^- (most powerful)

Classification of Imbalances

• Respiratory Acidosis (↑ PCO₂) : hypoventilation, COPD, drugs, airway obstruction
• Respiratory Alkalosis (↓ PCO₂) : hyperventilation (anxiety, pain, fever, ASA OD), brain stem lesion
• Metabolic Acidosis (↓ HCO₃⁻) : diarrhea, DKA, shock/lactic, renal failure
• Metabolic Alkalosis (↑ HCO₃⁻) : vomiting/NG suction, hypokalemia, antacid excess

Compensation & Decompensation

• Compensation seeks to restore 20:1 ratio, may normalize pH but underlying values abnormal
• Decompensation when buffering/respiratory/renal limits exceeded → life-threatening

Representative Lab Patterns (Table 2-8)

• Example compensated respiratory acidosis: ↑ PCO₂, ↑ HCO₃⁻, pH ≈7.38

Clinical Effects

• Acidosis: CNS depression → headache, lethargy → coma; Kussmaul respirations; acidic urine
• Alkalosis: CNS irritability → restlessness, tetany, seizures; hypokalemia often accompanies

Treatment Principles

• Correct underlying etiology (e.g., insulin for DKA, antidote for salicylates, adjust ventilation)
• Replace or remove fluids/electrolytes cautiously to avoid rapid shifts
• Bicarbonate infusion for severe metabolic acidosis
• Modify diet (electrolyte content, protein, fluid)
• Dialysis or mechanical ventilation where indicated