Fluid Management

History - Hypovolemia: vomiting, diarrhea, fever, sepsis, trauma, prolonged NPO - Hypervolemia: weight gain, edema, AKI, ascites

Physical exam - Hypovolemia: skin turgor, thready pulse, dry mucous membranes, tachycardia, orthostasis, ↓ UOP - Hypervolemia: pitting edema, rales, wheezing, elevated JVP

Labs / studies - Hypovolemia: rising Hct, contraction alkalosis then metabolic acidosis, urine SG > 1.010, urine Na < 10, urine osm > 450, hypernatremia, BUN:Cr > 10:1 - IVC ultrasound: < 1.7 cm OR > 50% collapse on inspiration → volume responsive - Hypervolemia: ↑ pulmonary vascular markings on CXR

Always trend and combine modalities.

• Vitals: HR, BP trends; account for PPV and anesthetic effects
• Pulse oximetry waveform variability — change with respiration suggests volume responsiveness
• PVI (Pleth Variability Index): > 12–16% → volume responsive
• Urine output: ADH elevated intraop → less reliable
• Arterial line:
• Serial ABGs (pH, Hct, lytes)
• Pulse Pressure Variation (PPV): = (PP_max − PP_min) / PP_mean
• PPV > 10% → likely fluid responsive
• NOT reliable if non-sinus rhythm, open chest, no PPV, or TV < 8 mL/kg
• CVP: absolute value unreliable; trend may be meaningful
• PA catheter: RV failure, pulm HTN, valvular disease, LV dysfunction
• TEE: transgastric view best for volume assessment; gold standard in cardiac/liver

Total body water: 60% of body weight (♂), 50% (♀).

5–15–40 rule (% body weight): - 5% intravascular (~3.5 L in 70 kg) - 15% interstitial (~10 L) - 40% intracellular (~28 L) - 20% extracellular total

Regulation: - Aldosterone: ↑ Na reabsorption → ↑ intravascular volume - ADH (vasopressin): ↑ water reabsorption - ANP / BNP: ↑ Na and water excretion

NS: preferred in brain injury/swelling (hyperosmolar) and to dilute pRBCs. Large volumes → hyperchloremic metabolic acidosis → ↓ GFR / AKI risk.

LR: more physiologic balanced crystalloid. Lactate → HCO₃⁻ in liver. Caution with K in renal failure. Ca²⁺ theoretically interferes with citrate in pRBCs (debated).

Use cases: - Initial volume resuscitation when crystalloid is inadequate or when > 3–4 L would be needed - ½-life ~3–6 h vs 20–30 min for crystalloid - Large protein losses / ↓ oncotic pressure (cirrhosis, burns) - Hemorrhagic shock when blood not available (1 mL colloid per mL blood lost)

Albumin (5% and 25%) — pooled donor, heat-treated. Minimal viral risk. Expensive. 5% for hypovolemia; 25% for hypovolemia with fluid/Na restriction.

Hetastarch (HES) — rarely used. ↑ PTT, anaphylactoid reactions, platelet dysfunction. Max 15–20 mL/kg/day. Contraindicated in coagulopathy, heart failure, renal failure.

Volume overload causes: - ↑ mortality and length of stay - Pulmonary and cerebral edema - Bowel edema → ileus, anastomotic dehiscence - Coagulopathy from dilution - Cardiac dysfunction

Liberal (older paradigm): generous replacement of insensible losses + "third space" losses.

Restrictive / goal-directed (modern): replace ongoing losses; minimize maintenance; use dynamic indices to guide boluses. Particularly in colorectal, hepatobiliary, thoracic surgery.

4-2-1 rule (Holliday-Segar): - 4 mL/kg/h for first 10 kg - 2 mL/kg/h for next 10 kg - 1 mL/kg/h thereafter

NPO deficit: rate × hours NPO. Classical teaching: replace ½ in hour 1, ¼ in hour 2, ¼ in hour 3.

Surgical losses: match estimated blood loss 3:1 with crystalloid or 1:1 with colloid/blood (depending on hemoglobin target).