Fluid and Electrolyte Management

Body Fluids & Basics - Wet & Wild World

• TBW: 60% of body weight. 📌 60-40-20 Rule:
  • ICF: 40% (K+ dominant)
  • ECF: 20% (Na+ dominant); Interstitial 15%, Intravascular 5%.
• Electrolyte Distribution:
  • ICF: High K+, Mg2+, Phosphates.
  • ECF: High Na+, Cl-, HCO3-.
• Osmolality: Solute concentration per kg solvent. Plasma Osmolality $ \approx (2 \times Na^+) + (Glucose/18) + (BUN/2.8) $. Normal: 280-295 mOsm/kg.
• Tonicity: Describes ECF effect on cells. Solutions: Isotonic (e.g., NS, RL), Hypotonic (0.45% NaCl), Hypertonic (3% NaCl).
• Starling Forces: Capillary hydrostatic & oncotic pressures govern fluid exchange.

⭐ The primary determinant of ECF osmolality is Sodium concentration.

IV Fluids - Drip Drop Hydrate

• Classification:
  • Crystalloids: Small molecules (NS, RL, D5W). Types: Isotonic (0.9%NS, RL), Hypotonic (0.45%NS, D5W in-vivo), Hypertonic (3%NaCl).
  • Colloids: Large molecules (Albumin, HES).
• Common Fluids Overview:
  • 0.9% NS (Normal Saline): 154 Na+/Cl- mEq/L. Osm 308 mOsm/L. Use: Resuscitation. Risk: Hyperchloremic acidosis.
  • RL (Ringer's Lactate): 130 Na+, 4 K+, 28 Lactate (mEq/L). Osm 273 mOsm/L. Use: Burns, trauma. ⚠️ Liver failure, hyperK+.
  • D5W (5% Dextrose in Water): Osm 278 mOsm/L (in bag). Use: Free water. ⚠️ ↑ICP, hyperglycemia.
• Complications: Fluid overload, electrolyte imbalance (e.g., NS → hyperchloremic acidosis), colloid reactions.

⭐ Ringer's Lactate is most physiological but contains K+ (4 mEq/L); use cautiously in hyperkalemia or severe renal failure.

Fluid Therapy - Quenching the Thirst

• Maintenance Fluids:
  • Holliday-Segar (daily): $100 \text{ml/kg for first } 10\text{kg} + 50 \text{ml/kg for next } 10\text{kg} + 20 \text{ml/kg for rest} \text{ per day}$. (4/2/1 rule hourly).
  • Adults: 25-35 ml/kg/day.
• Daily Electrolytes (mEq/kg/day): Na⁺ 1-2, K⁺ 0.5-1, Cl⁻ 1-2.
• Assess Fluid Status:
  • Clinical: HR, BP, UO (target 0.5-1 ml/kg/hr), JVP, skin turgor, capillary refill.
  • Labs: Hct, BUN/Cr ratio.
• Deficit Replacement: Correct 50% in first 8h, remaining over next 16h.
• Insensible Losses: Approx. 400-600 ml/day; ↑ with fever, tachypnea.

⭐ In surgical patients, third space losses can significantly increase fluid requirements.

Electrolyte Imbalances - Ion Power Play

HyperK Mgmt

Acid-Base Balance - pH Balance Act

• Normal: pH 7.35-7.45, PaCO2 35-45 mmHg, HCO3 22-26 mEq/L.
• Steps: 1. pH? (Acidemia/Alkalemia) 2. Primary? (Resp/Metab) 3. Compensated?
• Anion Gap: $AG = Na^+ - (Cl^- + HCO_3^-)$ (Normal: 8-12). 📌 MUDPILES/GOLDMARK for ↑AG.

Start["🔬 ABG Analysis
• Arterial blood gas• Check acid-base"]

PHVal{"📋 pH Value?
• Measure serum pH• Identify acidity"}

Normal["✅ Normal/Comp
• pH 7.35 to 7.45• Homeostasis met"]

AcidCause{"📋 Acid Cause?
• pH < 7.35• Acidemia state"}

AlkCause{"📋 Alk Cause?
• pH > 7.45• Alkalemia state"}

RespAcid["🩺 Resp Acidosis
• ⬆️ PaCO2 levels• Hypoventilation"]

MetAcid["🩺 Metab Acidosis
• ⬇️ HCO3 levels• Renal/metabolic"]

RespAlk["🩺 Resp Alkalosis
• ⬇️ PaCO2 levels• Hyperventilation"]

MetAlk["🩺 Metab Alkalosis
• ⬆️ HCO3 levels• GI/renal loss"]

AnionGap{"📋 Anion Gap?
• Na - Cl + HCO3• Gap calculation"}

HAGMA["⚠️ HAGMA
• High Gap Acidosis• MUDPILES causes"]

NAGMA["🩺 NAGMA
• Normal Gap Acid• Diarrhea/RTA"]

Comp["👁️ Compensation
• Assess response• Winters formula"]

Start --> PHVal PHVal -->|Normal| Normal PHVal -->|Acidemia| AcidCause PHVal -->|Alkalemia| AlkCause

AcidCause -->|⬆️ PaCO2| RespAcid AcidCause -->|⬇️ HCO3| MetAcid

AlkCause -->|⬇️ PaCO2| RespAlk AlkCause -->|⬆️ HCO3| MetAlk

MetAcid --> AnionGap AnionGap -->|High| HAGMA AnionGap -->|Normal| NAGMA

RespAcid --> Comp MetAcid --> Comp RespAlk --> Comp MetAlk --> Comp

style Start fill:#FFF7ED, stroke:#FFEED5, stroke-width:1.5px, rx:12, ry:12, color:#C2410C style PHVal fill:#FEF8EC, stroke:#FBECCA, stroke-width:1.5px, rx:12, ry:12, color:#854D0E style Normal fill:#F6F5F5, stroke:#E7E6E6, stroke-width:1.5px, rx:12, ry:12, color:#525252 style AcidCause fill:#FEF8EC, stroke:#FBECCA, stroke-width:1.5px, rx:12, ry:12, color:#854D0E style AlkCause fill:#FEF8EC, stroke:#FBECCA, stroke-width:1.5px, rx:12, ry:12, color:#854D0E style RespAcid fill:#F7F5FD, stroke:#F0EDFA, stroke-width:1.5px, rx:12, ry:12, color:#6B21A8 style MetAcid fill:#F7F5FD, stroke:#F0EDFA, stroke-width:1.5px, rx:12, ry:12, color:#6B21A8 style RespAlk fill:#F7F5FD, stroke:#F0EDFA, stroke-width:1.5px, rx:12, ry:12, color:#6B21A8 style MetAlk fill:#F7F5FD, stroke:#F0EDFA, stroke-width:1.5px, rx:12, ry:12, color:#6B21A8 style AnionGap fill:#FEF8EC, stroke:#FBECCA, stroke-width:1.5px, rx:12, ry:12, color:#854D0E style HAGMA fill:#FDF4F3, stroke:#FCE6E4, stroke-width:1.5px, rx:12, ry:12, color:#B91C1C style NAGMA fill:#F7F5FD, stroke:#F0EDFA, stroke-width:1.5px, rx:12, ry:12, color:#6B21A8 style Comp fill:#EEFAFF, stroke:#DAF3FF, stroke-width:1.5px, rx:12, ry:12, color:#0369A1

> ⭐ Winters' formula ($Expected PaCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2$) for resp. compensation in metabolic acidosis.

![Davenport Diagram: Acid-Base Balance Nomogram](https://ylbwdadhbcjolwylidja.supabase.co/storage/v1/object/public/notes/L1/Surgery_General_Surgery_Principles_Fluid_and_Electrolyte_Management/d1445c15-7bf5-46ed-91da-155bacd14ad9.png)


## High‑Yield Points - ⚡ Biggest Takeaways
> * **Daily fluid needs**: **25-35 mL/kg**; Na+ **1-2 mmol/kg**, K+ **0.5-1 mmol/kg**.
> * **Isotonic crystalloids** (NS, RL) are primary for initial **volume resuscitation**.
> * **Hypokalemia** is the most common surgical electrolyte imbalance, often from **GI losses**.
> * Monitor **urine output (>0.5 mL/kg/hr)** as a key indicator of adequate **hydration & perfusion**.
> * **Hypertonic saline (3% NaCl)** for severe symptomatic **hyponatremia** or ↑ICP; administer cautiously.
> * **Third space fluid loss** is a major contributor to **postoperative hypovolemia**.