🔥 Fluids & Electrolytes Rapid Review
🧠 Fluid Compartments
- ICF = 2/3 of total body water
- ECF = 1/3 of total body water
- Na⁺ = major extracellular cation
- K⁺ = major intracellular cation
- Na/K pump = 3 Na⁺ out, 2 K⁺ in
- Do NOT flip Na⁺ and K⁺ locations.
- Na⁺ drives ECF osmolality and ECF volume changes.
- Hypoxia/pump failure → cell swelling.
💧 Tonicity
- Isotonic → no major cell size change
- Hypotonic → cells swell
- Hypertonic → cells shrink
- D5W is isotonic in the bag but acts hypotonic after metabolism.
- 3% saline is hypertonic and pulls water out of cells.
🧂 Sodium
- Normal Na⁺: 135–145 mEq/L
- Hypernatremia = water loss greater than Na⁺ loss
- Hyponatremia = Na⁺ loss or water excess
- Correct hypernatremia slowly → rapid correction can cause brain edema/seizures.
- Correct hyponatremia slowly → rapid correction can cause central pontine myelinolysis/osmotic demyelination.
- Na⁺ disorders are neurologic symptom traps.
⚡ Potassium
- K⁺ is primarily intracellular.
- Hyperkalemia → peaked T waves, dysrhythmia risk.
- Hypokalemia → weakness, U waves, dysrhythmia risk.
- Insulin shifts K⁺ into cells.
- Dextrose supports insulin-mediated shift.
- Beta agonists shift K⁺ into cells.
- Calcium stabilizes the cardiac membrane but does NOT lower K⁺.
🧪 Calcium
- Hypocalcemia → increased neuromuscular excitability.
- Chvostek sign and Trousseau sign are classic.
- Hypocalcemia = twitchy/tetany.
- Hypercalcemia = weakness/decreased excitability.
- Do not confuse calcium treatment in hyperkalemia with potassium-lowering therapy.
🧬 Magnesium
- Magnesium affects acetylcholine release.
- High Mg²⁺ can potentiate neuromuscular blockade.
- Low Mg²⁺ increases excitability/dysrhythmia risk.
- Mg²⁺ decreases ACh release at the NMJ.
- Hypermagnesemia may reduce opioid requirement and blunt sympathetic response.
- Mg²⁺ + nondepolarizer = stronger/longer blockade risk.
💉 Crystalloids
- Crystalloids distribute in the ECF.
- LR is more physiologic for large-volume replacement.
- NS can cause hyperchloremic metabolic acidosis.
- Crystalloid blood-loss replacement = 3:1.
- NS is useful with PRBCs and metabolic alkalosis, but large volumes can cause acidosis.
- LR contains lactate that is converted to bicarbonate.
- D5W is not a resuscitation fluid for intravascular volume.
🧫 Colloids
- Colloids stay intravascular longer than crystalloids.
- Colloid replacement = about 1:1.
- Albumin is a natural colloid.
- Hetastarch: avoid/caution in renal dysfunction and bleeding risk.
- Dextran: bleeding risk, anaphylaxis, and can interfere with blood typing.
🧮 Maintenance + Deficit
- 4-2-1 rule: first 10 kg = 4 mL/kg/hr, second 10 kg = 2 mL/kg/hr, remaining kg = 1 mL/kg/hr.
- Shortcut for adults: weight + 40.
- NPO deficit = maintenance × hours NPO.
- Replace 50% in first hour.
- Replace 25% in second hour.
- Replace 25% in third hour.
🩸 Blood Loss + Transfusion
- 4x4 sponge ≈ 10 mL
- Lap sponge ≈ 100 mL
- 1 gram weight ≈ 1 mL blood
- Subtract irrigation from suction canister volume.
- 1 unit PRBC → Hgb ↑ about 1 g/dL; Hct ↑ about 3%.
- Adult male EBV ≈ 70 mL/kg
- Adult female EBV ≈ 65 mL/kg
- Infant EBV ≈ 80 mL/kg
- Common transfusion trigger: Hgb 7–8 or symptomatic/significant loss.
💀 Last-Minute Teacher Traps
- Calcium stabilizes the heart; it does not lower potassium.
- D5W looks isotonic in the bag but acts hypotonic in the body.
- Hypernatremia correction too fast → brain edema.
- Hyponatremia correction too fast → osmotic demyelination.
- NS large volume → hyperchloremic metabolic acidosis.
- Crystalloid = 3:1; colloid/PRBC = 1:1.
- Do not forget to subtract irrigation from EBL.
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