Total Body Water (TBW) is ~60% of body weight in men, ~50% in women.
Intracellular Fluid (ICF): ⅔ of TBW.
Extracellular Fluid (ECF): ⅓ of TBW.
Interstitial fluid: ¾ of ECF.
Intravascular volume: ¼ of ECF.
Distribution of common IV fluids:
0.9% Normal Saline (NS): Stays entirely in ECF (¼ intravascular, ¾ interstitial). Best suited for intravascular volume expansion.
D5W (5% Dextrose in Water): Dextrose is rapidly metabolized, leaving free water. Distributes across TBW (⅔ ICF, ⅓ ECF). Only ~8% remains intravascularly. Never use for acute volume resuscitation.
0.45% Half-Normal Saline (½ NS): Distributes as 50% free water and 50% isotonic saline.
3% NaCl: 513 mEq/L Na+, 513 mEq/L Cl-. Used strictly for severe symptomatic hyponatremia or elevated intracranial pressure (ICP).
Colloids:
Albumin (5% or 25%): Large proteins that remain entirely intravascular. No survival benefit over crystalloids in general sepsis/shock, but indicated in specific settings (e.g., cirrhosis with spontaneous bacterial peritonitis, large-volume paracentesis).
Resuscitation vs. Maintenance Principles
Volume Resuscitation (Emergency):
Goal: Restore tissue perfusion and intravascular volume.
Indication: Shock (septic, hypovolemic), severe dehydration.
Fluid: Isotonic crystalloid (NS or LR). Never use hypotonic fluids (e.g., D5W, ½ NS) for resuscitation.
Standard Dose: 30 mL/kg bolus in adults with sepsis; 20 mL/kg bolus in pediatric patients. c
Maintenance Therapy:
Goal: Maintain fluid and electrolyte balance in stable, NPO patients.
Rule of thumb: Holliday-Segar method (4-2-1 Rule):
4 mL/kg/hr for first 10 kg.
2 mL/kg/hr for next 10 kg.
1 mL/kg/hr for each kg above 20 kg.
Adult shortcut (>20 kg): Weight in kg + 40 mL/hr.
Fluid of Choice: D5 ½ NS + 20 mEq/L KCl.
High-Yield Safety Pearl: Do NOT add potassium (K+) to IVF if patient has oliguria, anuria, or acute kidney injury (AKI) to avoid severe hyperkalemia. Verify urine output (UOP) first.
High-Yield Clinical Scenarios
Diabetic Ketoacidosis (DKA) / HHS:
Phase 1: Aggressive resuscitation with NS or LR to restore volume.
Phase 2: Transition to D5 ½ NS (or D5 NS) when serum glucose falls <250 mg/dL (DKA) or <300 mg/dL (HHS) to prevent cerebral edema and hypoglycemia while insulin infusion continues.
Severe Burns:
Fluid of Choice: Lactated Ringer’s (LR) (reduces hyperchloremic acidosis risk).
Parkland Formula: 4 mL x weight (kg) x % TBSA burned.
Give 1st half of total calculated volume over first 8 hours from the time of injury.
Give 2nd half over the next 16 hours.
Trauma & Hemorrhagic Shock:
First step: Limit crystalloids (max 1-2L NS/LR) to prevent dilutional coagulopathy, hypothermia, and acidosis (“lethal triad”).
Next best step: 1:1:1 Balanced Transfusion (PRBCs, FFP, PLT).
Traumatic Brain Injury (TBI) / Elevated ICP:
Fluid of Choice: NS 0.9% (maintains mild hypertonicity).
Absolute Contraindication: Hypotonic fluids (e.g., ½ NS, D5W) as they cause cerebral edema and herniation.
Large-Volume Paracentesis (>5L removal):
Administer Albumin (6-8 g per liter of fluid removed) to prevent post-paracentesis circulatory dysfunction and AKI.
Pancreatitis:
Preferred fluid: Lactated Ringer’s (LR) (associated with lower rates of systemic inflammatory response syndrome (SIRS) and hypocalcemia compared to NS).
Fluid-Induced Complications & Monitoring
Hyperchloremic NAGMA:
Cause: Infusing large volumes of 0.9% NS (high Cl- concentration pushes bicarbonate into cells or promotes renal bicarb excretion).
Volume Overload & Pulmonary Edema:
Patients with CHF, CKD, and Cirrhosis have highly reduced tolerance.
Monitor for crackles, S3 gallop, jugular venous distention (JVD), and dyspnea.
Iatrogenic Hyponatremia:
Cause: Excessive hypotonic fluid administration (e.g., D5W, 0.45% NS) in postoperative or sick patients with elevated ADH secretion.
Clinical Monitoring Goals:
UOP: >0.5 mL/kg/hr in adults, >1 mL/kg/hr in pediatrics.