Fluid Volume Deficit / Dehydration

The patient's blood pressure looks fine lying down — but the moment they stand, it drops 20 mmHg. That orthostatic shift is your earliest warning of fluid volume deficit before labs ever change.

Fluid volume deficit (FVD) occurs when fluid output exceeds intake, depleting the extracellular compartment. The body loses both water and electrolytes (isotonic FVD) or primarily water (hypertonic dehydration). Assessment findings follow a predictable progression: early signs include thirst, concentrated urine (specific gravity >1.030), dry mucous membranes, and orthostatic hypotension (systolic BP drop ≥20 mmHg or pulse increase ≥20 bpm upon standing). As deficit worsens, you see tachycardia, weak thready pulses, decreased skin turgor (test over the sternum or forehead in older adults, not the hand), flat neck veins when supine, and diminished urine output (<30 mL/hr or <0.5 mL/kg/hr). Lab markers include elevated BUN-to-creatinine ratio (>20:1), elevated hematocrit from hemoconcentration, elevated serum osmolality (>295 mOsm/kg), and increased urine specific gravity. Weight change is the most reliable short-term indicator — a 1 kg loss equals approximately 1 liter of fluid lost. Daily weights must be taken same time, same scale, same clothing. High-risk populations include older adults (blunted thirst response), clients with vomiting/diarrhea, burns, hemorrhage, and those on diuretics.

Key Distinctions

Don't confuse FVD (low volume — tachycardia, hypotension, concentrated urine) with fluid volume excess (high volume — bounding pulses, hypertension, dilute urine) — they mirror each other. Students confuse dehydration (water loss, elevated sodium/osmolality) with hypovolemia (proportional fluid and solute loss, sodium may be normal). Skin turgor testing on elderly hands gives false positives — tenting occurs even in euvolemic older adults due to age-related loss of skin elasticity — always use the sternum or forehead.

Clinical Pearl

Weight is the gold standard: 1 kg lost = 1 liter lost. If the chart says the client lost 2 kg overnight, that's 2 liters — act on the number, not the appearance.