Nephrology
Dehydration
Volume depletion versus water deficit — and why they need different fluids
"Dehydration" colloquially means fluid loss, but clinicians distinguish two different problems. Volume depletion is loss of isotonic fluid (extracellular volume) — from vomiting, diarrhea, hemorrhage, burns. Hyperosmolar dehydration is pure water loss with retained sodium — from diabetes insipidus, fever, inadequate intake. Both impair perfusion, but the treatments differ: volume needs isotonic saline; pure water deficit needs free water. Total body water is 60% of body weight; intracellular ~40%, extracellular ~20% (intravascular ~5%). Even 5% body weight loss causes significant symptoms.
- Total body water~60% of body weight (less in obese, elderly)
- ECF compartment~20% body weight (5% plasma, 15% interstitial)
- Volume depletion signTachycardia, low BP, postural drop
- Pure water loss signHypernatremia, thirst, neurologic symptoms
- Maintenance fluid~30 mL/kg/day adults
- Daily insensible losses~600-900 mL
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Why hydration assessment matters
- Sepsis resuscitation. 30 mL/kg crystalloid bolus within 3 hours, titrated by response.
- Pediatric gastroenteritis. ORS preferred to IV when child can drink; reduces hospitalization.
- Diabetic ketoacidosis. Profound volume depletion plus electrolyte deficits demand careful replacement.
- Surgical patients. NPO status, third-spacing, blood loss; goal-directed therapy improves outcomes.
- Heart failure. Distinguish congestion from intravascular depletion; cardiorenal axis.
- Heat illness. Heatstroke needs aggressive cooling plus targeted fluid; not just water.
- Geriatric falls. Postural hypotension from dehydration is reversible cause.
Common misconceptions
- "8 glasses of water a day" applies to all. Needs vary with activity, climate, comorbidity.
- Thirst always indicates dehydration. Elderly often don't sense thirst until severely depleted.
- IV is always better than oral. ORS works for most enteric losses and avoids line complications.
- Normal saline is "physiologic." 0.9% NaCl has supraphysiologic chloride causing hyperchloremic acidosis.
- Drink to prevent cramps. Cramps are mostly neuromuscular, not dehydration; sodium more relevant.
- Correct hypernatremia fast. Rapid correction causes cerebral edema; max 10 mEq/L per 24 hours.
Frequently asked questions
How do volume depletion and water deficit differ?
Volume depletion: isotonic loss (gastric content, diarrhea, blood). Plasma osmolality often normal. Treat with isotonic saline. Pure water deficit: hypotonic loss (insensible, DI). Plasma osmolality elevated, sodium rises. Treat with free water (D5W or hypotonic saline). Mixed losses are common — assess each axis separately.
How do you assess hydration?
History (intake, output, weight change). Vital signs: postural drop (>20 mmHg systolic) suggests >15% volume loss. Physical: dry mucosa, decreased skin turgor, sunken eyes, prolonged cap refill, low JVP. Labs: BUN/Cr ratio >20:1, hemoconcentration, urine sodium <20 mEq/L (if not on diuretic), urine specific gravity >1.020. Severe: oliguria, lethargy, shock.
What's the right IV fluid?
Resuscitation in shock: balanced crystalloid (lactated Ringer's, Plasma-Lyte) preferred over normal saline. SMART trial: balanced fluids reduce major adverse kidney events. Maintenance: D5 ½NS with K. Hyponatremia: 3% saline cautiously (correct ≤8 mEq/L per 24h to avoid osmotic demyelination). Hypernatremia: free water deficit corrected over 48 hours.
How do you calculate free water deficit?
Free water deficit = TBW × (current Na / 140 − 1). Example: 70 kg man with Na 160. TBW ≈ 0.6 × 70 = 42 L. Deficit = 42 × (160/140 − 1) = 6 L. Replace half over 24 h, rest over next 24-48 h. Lower Na by ≤0.5 mEq/L/h (≤10 mEq/L/day). Monitor q2-4 h. Use D5W IV or water orally/NG.
Why oral rehydration solution?
SGLT1 in small intestine cotransports sodium and glucose; water follows osmotically. Balanced ORS (sodium 75 mmol/L, glucose 75 mmol/L) absorbs efficiently even during cholera-grade diarrhea. WHO ORS has saved millions of children from rotavirus and cholera. Sports drinks have too much sugar and too little sodium for medical rehydration.
What about elderly dehydration?
Decreased thirst, kidney concentrating ability, total body water. Subtle presentation: confusion, falls, UTI, constipation. Hospitalization risk doubled. Use weight as gold standard for tracking. Encourage scheduled drinking (don't rely on thirst). Avoid over-aggressive diuretics; reassess after illness or hot weather.
How does exercise dehydration differ?
Sweat is hypotonic — losing more water than salt. Pure water replacement during prolonged exercise causes exercise-associated hyponatremia (EAH). Marathon and ultra-endurance runners have died from over-drinking dilute fluids. Recommendation: drink to thirst with electrolytes; track weight. Hypernatremia in athletes is rare; hyponatremia is the modern danger.