Ch02: Fluid, Electrolyte & Acid-Base Balance（水电解质代谢与酸碱平衡）

Preparatory Mindset

Every surgical patient has fluid/electrolyte issues — the body water distribution, the sodium/potassium disorders (isotonic, hypotonic, hypertonic dehydration (CM exam tested); water intoxication), and the acid-base disorders (metabolic/respiratory acidosis & alkalosis) are exam staples. The Chinese prep PDFs give the exact rules: potassium replacement (分次补钾, 浓度<40mmol/L即0.3%, 速度<20mmol/h, 尿量>40ml/h), isotonic dehydration treatment (平衡盐液或等渗盐), and acid-base buffering (缓冲系统, 肺, 肾).

Exam mindset: know the ECF/ICF electrolyte table (ECF main cation = Na+; ICF main cations = K+ & Mg2+), the four water-sodium disorders, and the classic ABG patterns. The 协和 Q&A PDF asks "人体通过哪些机制维持体液酸碱平衡? (体内缓冲系统, 肺的呼吸, 肾的调节)".

Core Concepts

1. Body fluid distribution (MUST KNOW)

2. Electrolyte concentrations (MUST KNOW)

• ECF: main cation = Na+; main anions = Cl-, HCO3-, protein
• ICF: main cations = K+ & Mg2+; main anion = HPO4²⁻ (and protein)
• Functional vs non-functional ECF: non-functional (CSF, joint, digestive, connective tissue fluid, 1-2% BW) — significant loss changes body composition

3. Classification of body fluid imbalance

1. Isotonic dehydration (等渗性缺水) — most common in surgery; water & Na lost in proportion 2. Hypotonic dehydration (低渗性缺水) — Na loss > water; ECF hypotonic 3. Hypertonic dehydration (高渗性缺水) — water loss > Na; ECF hypertonic 4. Water intoxication (水中毒) — excess water, dilutional hyponatremia

• Three types: volume imbalance (容量失调), concentration imbalance (浓度失调), composition imbalance (成分失调)
• Water & sodium disorders (水钠代谢紊乱):

4. Isotonic dehydration (等渗性缺水) — MUST KNOW

• Causes: ① acute loss of digestive fluids (vomiting, diarrhea, fistulae, NG suction); ② fluid loss into infected/soft tissue (third-space: burns, peritonitis, trauma)
• Clinical features: low-Na symptoms (nausea, anorexia, fatigue — may NOT have thirst); water-loss symptoms (oliguria, dry tongue, sunken eyes, dry loose skin). Loss >5% BW → hypovolemia (thready rapid pulse, cold extremities, unstable/decreased BP); 6–7% → severe shock + metabolic acidosis
• Treatment: treat the cause + replace volume with balanced salt solution (平衡盐液) or isotonic saline; if pulse fast & BP unstable (≥5% loss, ~3000 ml in a 60 kg adult), give rapid IV fluids containing sodium (prevent hyponatremia)

5. Potassium disorders (MUST KNOW)

- Causes: inadequate intake, GI loss (vomiting, diarrhea, fistulae), renal loss (diuretics, aldosterone), shifts (alkalosis, insulin) - Clinical: muscle weakness, hyporeflexia, ileus, cardiac arrhythmias (U waves, ST depression), alkalosis (paradoxical aciduria) - IV replacement rules (MUST KNOW): 分次补钾 (divide doses), 边补边观察 (monitor), concentration < 40 mmol/L (<0.3%), rate < 20 mmol/h, only after urine output > 40 ml/h (见尿补钾); total daily replacement usually ≤ 100–200 mmol; NEVER bolus/IV push K+

- Causes: renal failure, tissue breakdown (crush, burns), hemolysis, excessive replacement, acidosis - Clinical: muscle weakness, peaked T waves → widened QRS → VF/asystole (ECG emergencies) - Treatment: calcium gluconate (cardiac membrane stabilization), insulin + glucose (shift K+ in), sodium bicarbonate (acidosis), β-agonist (salbutamol), cation-exchange resin (kayexalate), dialysis; stop K+ intake

• Hypokalemia (低钾血症): K+ < 3.5 mmol/L
• Hyperkalemia (高钾血症): K+ > 5.5 mmol/L
• Paradoxical aciduria (反常性酸性尿): in hypokalemic alkalosis, the kidney excretes H+ instead of K+ (K+ depletion → H+ reabsorption) → urine becomes acidic despite systemic alkalosis — a classic exam question

6. Acid-base balance (MUST KNOW)

| Disorder | pH | Primary change | Compensation | |---|---|---|---| | Metabolic acidosis | ↓ | ↓ HCO3⁻ | Hyperventilation (↓PaCO2) | | Metabolic alkalosis | ↑ | ↑ HCO3⁻ | Hypoventilation (↑PaCO2) | | Respiratory acidosis | ↓ | ↑ PaCO2 | ↑ HCO3⁻ (renal) | | Respiratory alkalosis | ↑ | ↓ PaCO2 | ↓ HCO3⁻ (renal) |

• Mechanisms maintaining acid-base balance: ① body buffer systems (缓冲系统), ② lung ventilation (肺的呼吸), ③ kidney regulation (肾的调节)
• Normal values: pH 7.35–7.45; PaCO2 35–45 mmHg; HCO3⁻ 22–27 mmol/L; BE ±3
• Four disorders:
• Metabolic acidosis (代谢性酸中毒): most common surgical acid-base disorder — causes: diarrhea/fistula (HCO3⁻ loss), lactic acidosis (shock, sepsis), DKA, renal failure, starvation; treatment: treat cause, correct with bicarbonate only if severe (pH < 7.2)
• Metabolic alkalosis (代谢性碱中毒): vomiting/NG suction (H+ loss), diuretics, hypokalemia; treatment: saline + KCl, treat the cause
• Respiratory acidosis: hypoventilation (airway obstruction, anesthesia, COPD, sedatives); treatment: improve ventilation
• Respiratory alkalosis: hyperventilation (pain, anxiety, fever, salicylate, hypoxia); treatment: treat cause

High-Yield Points

• ECF cation = Na+; ICF cations = K+ & Mg2+ — the table is an exam staple
• Isotonic dehydration: most common surgical dehydration; balanced salt solution; 5% loss → hypovolemia
• K+ replacement rules (memorize): 浓度<40mmol/L (0.3%), 速度<20mmol/h, 尿量>40ml/h (见尿补钾), 分次补钾
• Hypokalemia ECG: U waves; Hyperkalemia ECG: peaked T → widened QRS → arrest
• Paradoxical aciduria: hypokalemia + alkalosis → acidic urine
• Acid-base balance: 缓冲系统 + 肺 + 肾
• Metabolic acidosis = most common surgical acid-base disorder (shock/lactic)
• Total body water: male 60% / female 50%

Topic Summary

Body fluids: male 60% / female 50% water; ECF (Na+ dominant, 20%) vs ICF (K+/Mg2+ dominant, 40%); functional vs non-functional ECF. Water-sodium disorders: isotonic (most common surgical), hypotonic, hypertonic, water intoxication. Isotonic dehydration from GI loss/third-space → replace with balanced salt solution. Potassium: hypokalemia (U waves, ileus; replace at <0.3%, <20 mmol/h, after urine >40 ml/h, divided doses) vs hyperkalemia (peaked T; calcium/insulin-glucose/bicarbonate/resin/dialysis). Acid-base: buffer + lungs + kidneys; metabolic acidosis (most common surgical) vs alkalosis; respiratory disorders.