Restlessness, irritability, confusion; seizures when severe
Fluid, Electrolyte & Acid-Base · Topic 9 of 23
Fluid Volume Deficit / Dehydration
The patient's blood pressure looks fine lying down — but the moment they stand, it drops 20 mmHg.
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Fluid volume deficit
Na⁺SodiumPriority
K⁺PotassiumWNL
Cl⁻ChlorideH
Ca²⁺CalciumWNL
Mg²⁺MagnesiumWNL
PO₄³⁻PhosphateWNL
ECFVolume (ECF)L
pHpHWNL
Bedside monitor · pattern
Hypernatremia
Too little water relative to sodium raises serum tonicity, pulling water out of brain cells.
Direct physiology↑ serum tonicity→Water leaves neurons→Cellular dehydration
ECG responseNormal sinus rhythm
Neuronal excitabilityNormal
SuppressedBalancedHyper
PerfusionHypovolemic
84%−2.2 kg
Orthostatic drop, flat neck veins, dry mucous membranes, BUN:creatinine climbing past 20:1. Fall precautions — they will be dizzy standing up.
What the nurse would notice
Hypernatremia across body systems
Also on this panel
Thirst, dry sticky mucous membranes, flushed skin
Weakness, twitching, hyperreflexia
Dilute urine in DI, concentrated urine with other water loss
Teaching model, not a clinical calculator. Patterns show high-yield directional relationships under the stated assumption. Real symptoms, ranges and treatment depend on cause, acuity, comorbidities and local protocols.
Signs & symptoms
Present on this panel. Tap any sign to see which values produce it.
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Tap to start15 cards · about 6 min · +45 XP
Fluids & Electrolytes · Volume
Fluid volume deficit
More fluid going out than coming in. The tank runs low — the pulse speeds up, the pressure drops and the kidneys make less urine.
Urine under 30 mL/hr → report it
22 mL in 1 hour — under 30. Report it.
Hourly urine meter · example patient
01 · The scaleDaily weight: 1 kg ≈ 1 L
The best measure of fluid change · same time, same scale, same clothing
02 · The standing testOrthostatic hypotension
BP drops on standing: systolic ↓ 20+ or diastolic ↓ 10+ within 3 minutes
03 · The fixGive isotonic fluid (NS or LR)
0.9% normal saline or lactated Ringer's · low BP + fast pulse → IV bolus now
01
Take orthostatic vitals: lying, sitting, standing
Try itExample patient · after two days of vomitingBaseline
118/76mmHg88pulse / minlying · baseline
Systolic dropcriteria ≥ 200
Diastolic dropcriteria ≥ 100
Pulse riseover 15 bpm (supporting sign)0
Timer runs fast: the 3 standing minutes play in a few seconds. Orthostatic hypotension = systolic ↓ 20 or more, or diastolic ↓ 10 or more, within 3 minutes of standing; a pulse rise over 15 bpm on standing also points to low volume (supporting sign, not one of the criteria).
02
What causes it
Watch forMore fluid out than in
Fluid lost
VomitingDiarrheaHemorrhageBurnsFever & heavy sweating
Kidneys lose it
DiureticsDiabetes insipidus
Too little in
Poor intakeOlder adultsblunted thirst
Older adults feel less thirst, so losses go unreplaced — thirst is a poor guide in them.
03
What you'll see
SignsThirst · dry mucous membranes early
Scant, dark (concentrated) urine early
Orthostatic hypotension (BP drops on standing) — dizzy when standing earlysystolic ↓ 20 or more, or diastolic ↓ 10 or more, within 3 min · a pulse rise over 15 bpm also points to low volume
Tachycardia · weak, thready pulse worse
Hypotension · flat neck veins · slow capillary refill worse
Confusion · lethargy (very sleepy) late
Older adult: test skin turgor below the clavicle (upper chest) or on the foreheadthe back of the hand tents falsely (lost elasticity)
Infant: sunken anterior fontanel, no tearsa bulging fontanel points the other way
Orthostatic changes are an early sign — but thirst, a pulse rise on standing and falling urine output can come first.
04
Labs & diagnostics
CheckWeight down, labs concentrated
Daily weight: 1 kg lost ≈ 1 L lostsame time, same scale, same clothing · more than 3% down over 7 days suggests dehydration
Hematocrit uphemoconcentration — less water, same cells
Serum osmolality above 295normal 275 – 295 mOsm/kg · rises most when mostly water is lost
Urine specific gravity highconcentrated urine — the kidneys are saving water
BUN : creatinine above 20 : 1low volume (prerenal), not kidney damage
Serum Na⁺: normal or high?normal when water and sodium are lost together (isotonic) · high when mostly water is lost (dehydration)
Urine sodium below 20 mEq/Lthe kidneys are holding on to sodium
05
Red flags
Act nowUrine output below 30 mL/h report nowoliguria — the kidneys aren't perfused · weight-based: under 0.5 mL/kg/h for 2 h in a row
Low BP with a fast, thready pulse report nowIV bolus now
New confusion or lethargy report now
Crackles or new shortness of breath during a bolus report nowfluid overload — slow the fluid and call
Infant: sunken fontanel and no tears
06
Treatment: fill the tank
IsotonicReplace volume with isotonic fluid — NS or LR
Do right now
1
→Check BP, pulse, mental statuslow BP + fast pulse → bolus now
2
→Start isotonic IV fluid0.9% saline or LR, as ordered
3
→Measure urine every hourreport below 30 mL/h
4
Reassess after each boluspulse, urine, lung sounds
Oral rehydrationMild · gut working
Oral rehydration solution
Small, frequent sips — children about 5 mL every 1 – 2 minutes.
First choice when it's mildIV isotonicModerate to severe
0.9% normal saline Lactated Ringer's
Bolus for low BP with a fast pulse, then reassess.
Listen for cracklesFall safetyOrthostatic BP
Rise slowly — sit, dangle the legs, then stand with help.
Dizzy on standing = fall riskThe fluids are working when…
Urine ≥ 30 mL/h
The kidneys are perfused again
Pulse falling
The heart no longer racing to compensate
Lungs still clear
Crackles during a bolus = overload
slow the fluid and call
Doses & special cases
Child in shock: 0.9% saline 20 mL/kg IV over about 20 minthen reassess
Water-loss (high-sodium) dehydrationisotonic fluid first to restore volume, then free water slowly — lower Na⁺ no more than 10 – 12 mEq/L in 24 h
07
Nursing priorities
In orderCheck BP, pulse and mental statuslow BP + fast pulse → IV bolus now
Start isotonic fluid as orderedNS or LR · lung sounds during boluses
Hourly urine outputreport below 30 mL/h
Daily weightsame time, same scale, same clothing · 1 kg ≈ 1 L
Orthostatic vitals · fall precautionslying, then standing up to 3 min · rise slowly
Strict I&O · oral fluidssmall, frequent sips
08
Deficit or excess?
| Daily weight | Down — 1 kg lost ≈ 1 L lost |
|---|---|
| Pulse | Fast, weak, thready |
| Blood pressure | Low · drops on standing |
| Neck veins | Flat |
| Lungs | Clear |
| Labs | Concentrated — hematocrit, osmolality, urine SG up |
| Fix | 0.9% normal saline or lactated Ringer's |
09
Teach your patient
DischargeDrink more — not less — with vomiting, diarrhea or fever
Dark urine or dizziness on standing means you're falling behind
Rise slowly from lying to standingsit, dangle your legs, then stand — prevents a fall
Older adults: drink on a schedulethirst is a poor guide
In fluid volume deficit, weight, BP and urine output fall while the pulse climbs. Replace with isotonic fluid and watch the urine climb back to 30 mL/h.
Sources · Signs, fluids and nursing care from OpenStax (Medical-Surgical Nursing 10.2, Clinical Nursing Skills 19.2 and 19.3); criteria checked against StatPearls (orthostatic hypotension, oliguria, hypovolemia, hypovolemic shock, azotemia, pediatric dehydration, hypernatremia), NICE CG84 (infant signs) and the 0.9% sodium chloride label (DailyMed). Typical adult values — follow your protocol.
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), patients 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 patient lost 2 kg overnight, that's 2 liters — act on the number, not the appearance.
Knowledge Check
3 quick questions on the must-knows for this topic.
Question 1 of 3
A 1 kg weight loss equals about how much fluid lost?
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Go further
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- GuideHyperkalemia ECG Changes in Order: Peaked T Waves to Sine Wave, and What the Nurse Does FirstThe fixed sequence — peaked T, long PR, wide QRS, sine wave — with approximate potassium levels, what IV calcium actually does (protects the heart, does not lower the potassium), the stabilize–shift–eliminate treatment order, and what the nurse monitors. Includes a live ECG you can drag.
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