Isotonic, hypotonic, hypertonic — where the fluid goes and why it matters.
Read the tonicity cards first, then work the questions. Tap "Hide answers" to quiz yourself; tap "Show the answer" on any question to check your reasoning.
"Where is the fluid right now… and where do I need it to go?"
Everything about IV fluids comes back to fluid shifts. There are only three compartments and water always moves toward the saltier one.
Normal serum osmolality is 275–295 mOsm/kg. A solution is isotonic if it sits in that range, hypotonic below it, hypertonic above it.
If you learn one table on this page, learn this one. Every question below is an application of it.
| ISOTONIC | HYPOTONIC | HYPERTONIC | |
|---|---|---|---|
| Osmolality | ~275–295 mOsm/L — same as plasma | <275 mOsm/L — more dilute than plasma | >295 mOsm/L — more concentrated than plasma |
| Where the water goes | Stays in the vessel (extracellular space) | Moves INTO the cell | Pulled OUT of the cell into the vessel |
| What the cell does | Stays the same size | Swells (and can burst) | Shrinks |
| Main examples | 0.9% NS, Lactated Ringer's, D5W in the bag | 0.45% NS, 0.33% NS, D5W once metabolized | 3% NS, 5% NS, D10W, D5½NS, D5NS, D5LR |
| Use it when… | The vascular space is empty — hemorrhage, shock, dehydration, sepsis, DKA, vomiting/diarrhea | The cells are dry — hypernatremia, cellular dehydration, DKA after the pressure is restored, maintenance fluid | The cells are too full — severe symptomatic hyponatremia, cerebral edema, raised ICP |
| Biggest danger | Fluid overload → pulmonary edema | Cerebral edema and cardiovascular collapse (fluid leaves the vessel) | Fluid overload and pulmonary edema — the vascular space fills fast |
| One-line memory | "Fills the tank." | "Feeds the cell." | "Drains the cell." |
Think: "Patient is dry and needs volume."
| Situation | Why it matters |
|---|---|
| Heart failure, renal failure, older adults | These patients cannot mobilize an extra liter — run slowly, weigh daily, listen to lungs every shift |
| Large volumes of 0.9% NS | The chloride load causes hyperchloremic metabolic acidosis; this is why LR is often preferred for big resuscitations |
| LR in liver failure | Avoid — the liver is what converts lactate to bicarbonate. A failing liver cannot, so the lactate accumulates |
| LR with blood products | Never — the calcium in LR binds the citrate anticoagulant and can clot the line. 0.9% NS only with blood |
| LR in hyperkalemia / severe renal failure | LR contains 4 mEq/L of potassium — small, but avoided when potassium is already high |
| LR in lactic acidosis | Adds more lactate to a patient who already cannot clear it |
| NS in hypernatremia | 0.9% NS has 154 mEq/L of sodium — it will not bring a high sodium down |
Think: "Cells are thirsty — they are leaking fluid."
Think: "Pull fluid OUT of swollen cells."
One row per bag you will actually hang. Cover the right-hand columns and quiz yourself off the name.
| Solution | Tonicity (mOsm/L) | Give it for | Watch for / avoid |
|---|---|---|---|
| 0.9% NS | Isotonic — 308 | Shock, hemorrhage, dehydration, sepsis, DKA, hypercalcemia; the only fluid compatible with blood; medication carrier | Fluid overload; hyperchloremic metabolic acidosis with large volumes; will not lower a high sodium |
| Lactated Ringer's | Isotonic — 273 | Burns, trauma, surgery, large-volume resuscitation, metabolic acidosis (lactate → bicarbonate) | Not in liver failure. Never with blood products. Avoid in hyperkalemia (K⁺ 4 mEq/L) and lactic acidosis |
| D5W | Isotonic in the bag (252) → hypotonic in the body | Free water, keeping a vein open, diluting IV medications, hypernatremia; supplies ~170 kcal/L | Never in head injury or raised ICP. Not for resuscitation. Raises glucose — caution in diabetes |
| 0.45% NS | Hypotonic — 154 | Hypernatremia, cellular dehydration, DKA after pressure is restored, routine maintenance | Never in head injury, stroke, or burns. Cerebral edema; worsens hypotension |
| 0.33% NS | Hypotonic — ~103 | Free-water replacement with a little sodium; cellular rehydration | Same as 0.45% NS, and more so — cell swelling and hemolysis |
| 3% NS | Hypertonic — ~1026 | Symptomatic hyponatremia, cerebral edema, raised ICP | Infusion pump + frequent sodium checks, always. Pulmonary edema, CHF, ODS from over-fast correction |
| D10W | Hypertonic — 505 | Hypoglycemia, calorie support, weaning off TPN | Hyperglycemia, vein irritation, rebound hypoglycemia if stopped abruptly |
| D5½NS | Hypertonic in the bag — 406 | Very common maintenance fluid; often ordered with 20 mEq KCl | Fluid overload; glucose; behaves closer to isotonic once dextrose is used up |
| Albumin (colloid) | Acts hypertonic — oncotic pull | Third-spacing, burns, liver failure with ascites, low-albumin shock | Fluid overload and pulmonary edema; it is a blood product — monitor for reaction |
D5W is the one bag that changes its answer depending on where it is.
Exam consequences: D5W is never appropriate for a head-injured patient, a stroke patient, or anyone with raised ICP, and it is not a resuscitation fluid — it will not hold a blood pressure up. If an option says D5W "stays in the bloodstream" or "pulls water out of cells," it is wrong.
Sodium and water are the same question asked two ways. Fix the water and the sodium fixes itself.
| HIGH sodium (hypernatremia >145) | LOW sodium (hyponatremia <135) | |
|---|---|---|
| What it really means | Too little WATER | Too much WATER |
| The cells are | Dehydrated / shrunken | Swollen — including brain cells |
| Signs | Thirst, dry sticky mucous membranes, flushed skin, restlessness → agitation, weakness | Headache, confusion, lethargy, muscle cramps, nausea, seizures |
| Think… | Hypotonic fluids (0.45% NS, D5W) — give the water back | Hypertonic fluid (3% NS) if symptomatic — otherwise fluid restriction |
| Correction rule | Lower it slowly — dropping sodium too fast swells the brain | Raise it slowly — roughly no more than 8–12 mEq/L in 24 hours; too fast causes osmotic demyelination |
Both directions are corrected slowly, and both are corrected with the fluid that is the opposite of the problem.
| The stem says… | You choose… |
|---|---|
| Dry patient — low BP, tachycardia, dry mucosa, poor turgor, low urine output | → ISOTONIC (0.9% NS or LR) — fill the vessel first, always |
| Swollen brain — severe hyponatremia with confusion or seizures, cerebral edema | → HYPERTONIC (3% NS) — pull water out of the cells |
| High sodium — thirst, dry sticky membranes, Na >145 | → HYPOTONIC carefully (0.45% NS) — slow, with neuro checks |
| Fluid overload — crackles, edema, SOB, weight gain, JVD | → Slow the fluid, reassess, sit the patient upright, notify the provider — anticipate a diuretic |
| Head injury / raised ICP | → Never hypotonic. 3% saline is the anticipated fluid |
| Blood transfusion running | → 0.9% NS only — no LR, no dextrose solutions |
| Burns in the first 24 hours | → Lactated Ringer's, titrated to urine output |
| DKA | → 0.9% NS first for the pressure, then 0.45% NS, then add dextrose once glucose reaches about 200 mg/dL |
🤖 Ask Claire: "Give me ten one-line IV fluid scenarios. Make me pick isotonic, hypotonic, or hypertonic and say what I would assess before hanging it."
Never hang 0.45% NS, 0.33% NS, or D5W on a patient with a head injury, stroke, brain tumor, or any raised ICP. Free water moves straight into brain cells that are already swollen inside a skull that cannot expand. If you see one of these ordered for a neuro patient, that is the order you question. The right fluid for cerebral edema is 3% saline.
Answer: 3 — Lactated Ringer's. This client is hypovolemic and needs the vascular space refilled right now. Isotonic fluid stays where you put it — in the bloodstream — so BP rises, heart rate falls, and the kidneys get perfused again. LR also replaces the electrolytes lost in vomit and stool.
Why the others are wrong:
1. 0.45% NS is hypotonic — it shifts water into the cells and leaves the vessel, dropping an already low BP further.
2. 3% saline is for symptomatic hyponatremia and cerebral edema, not volume loss; it would also pull water out of cells that are already dry.
4. D5W acts hypotonic once the dextrose is metabolized — no lasting volume expansion.
Answer: 4 — 0.45% Normal Saline. This client's sodium is already low and the brain is already swelling. A hypotonic fluid adds more free water, drives it into the brain cells, and makes the cerebral edema worse. Read the word "question" carefully — it is asking for the wrong order, not the right one.
Why the others are appropriate:
1. Seizure precautions are exactly right — severe hyponatremia seizes.
2. Frequent neuro checks are how you detect deterioration and how you monitor the response to treatment.
3. 3% saline is the treatment for symptomatic hyponatremia — on a pump, with frequent sodium levels.
Answer: 3 — 3% Normal Saline. Hypertonic saline raises serum osmolality, so water is pulled out of the swollen brain tissue and back into the vasculature — brain volume drops and ICP falls. Give it on a pump with frequent sodium levels and continuous neuro assessment.
Why the others are wrong:
1. 0.45% NS is hypotonic — the single most dangerous fluid choice in a head injury.
2. D5W behaves hypotonic in the body for the same reason. Same danger.
4. Sterile water is never infused IV — it causes hemolysis.
Answer: 1 — sodium 154 mEq/L. Hypernatremia is a water deficit, and the cells are shrunken. Hypotonic fluid supplies free water that moves into the cells and dilutes the sodium back toward normal — done slowly, with neuro checks.
Why the others are wrong:
2. Pulmonary edema means there is already too much fluid; this patient needs a diuretic, not more volume.
3. Burns with hypotension need isotonic LR for aggressive volume resuscitation — hypotonic fluid would leave the vessel and worsen the shock.
4. A TBI patient must never receive hypotonic fluid — it worsens cerebral edema.
Answer: 3 — bilateral basilar crackles. Crackles mean fluid has moved into the alveoli. Because isotonic fluid stays entirely in the extracellular space, overload shows up in the lungs first. Slow the infusion, sit the patient upright, check O₂ saturation and vitals, and notify the provider.
Why the others are wrong:
1. Dry mucous membranes are a sign of fluid deficit — the opposite problem.
2. Tachycardia accompanies overload sometimes, but it is nonspecific — pain, fever, anxiety, and hypovolemia all cause it.
4. Poor skin turgor is also a deficit sign; overload gives you edema instead.
Answer: 3 — it eventually acts hypotonic. D5W is isotonic in the bag at 252 mOsm/L, but the cells burn the dextrose within minutes and only free water is left behind — which then shifts into the cells. That is why D5W is a free-water source, not a volume expander.
Why the others are wrong:
1. It leaves the vascular space almost immediately; nothing stays there "permanently."
2. Hypertonic is backwards — the solution gets more dilute as the sugar disappears.
4. Pulling water out of cells is what 3% saline and D10W do; D5W pushes water in.
Answer: 2 — 3% saline with new shortness of breath. Hypertonic saline expands the vascular space fast, and dyspnea is the first sign of pulmonary edema. This is an airway/breathing problem developing in real time. Stop or slow the infusion, sit the patient upright, get O₂ saturation and lung sounds, and notify the provider.
Why the others are wrong:
1. BP 110/72 on LR is a normal, reassuring finding — the fluid is doing its job.
3. Glucose 128 mg/dL with a dextrose infusion running is expected and not urgent.
4. 0.45% saline for a sodium of 148 is appropriate therapy — hypotonic fluid is exactly what you give for hypernatremia. Monitor, but nothing is wrong.
Answer: 3 — urine output increases. Urine output is the bedside measure of kidney perfusion (goal at least about 0.5 mL/kg/hr, or roughly 30 mL/hr in an adult). The kidneys only make urine when there is enough volume and pressure reaching them, so rising output means the tank is filling. Expect the heart rate to fall and the BP to rise alongside it.
Why the others are wrong:
1. A rapid sodium drop is a complication, not a goal — fast shifts swell the brain.
2. Crackles mean you have overshot into fluid overload.
4. Increasing confusion signals a worsening neurologic or perfusion state, never improvement.
Answer: 4 — 0.45% saline. At 154 mOsm/L it is roughly half the concentration of plasma. Water moves from the dilute solution toward the saltier inside of the cell, so the cell takes on water and swells. That is useful in hypernatremia and dangerous in a brain injury.
Why the others are wrong:
1. 3% saline is hypertonic — it shrinks cells.
2. LR is isotonic — cells stay exactly the same size.
3. D10W is hypertonic in the bag (505 mOsm/L) and is given for hypoglycemia and calories, not free water.
Answer: 3 — 150 mL/hr of 3% saline. Two problems stacked on each other: a hypertonic fluid that rapidly pulls water into the vascular space, running at a high rate, in a heart that already cannot handle its preload. Expect flash pulmonary edema. Verify the order, use a pump, and monitor lung sounds, respiratory status, and sodium closely.
Why the others are wrong:
1. 25 mL/hr of NS is a deliberately low, CHF-appropriate rate.
2. 50 mL/hr of LR is modest and isotonic — monitor, but reasonable.
4. A saline lock delivers no continuous volume at all, which is the safest option on the list.
Answer: 3 — isotonic. The patient is hypovolemic and needs circulating volume replaced fast; isotonic fluid stays in the bloodstream where the deficit is.
Why not the others: hypotonic would shift fluid out of the vessel and drop the BP further; hypertonic would dehydrate cells that are already losing water, and is not a resuscitation fluid.
Answer: 1 — hypotonic. Hypernatremia means the cells are dehydrated; hypotonic fluid moves water back INTO the cells and dilutes the sodium. Lower it slowly with neuro checks.
Why not the others: hypertonic would raise a sodium that is already dangerously high; isotonic 0.9% NS carries 154 mEq/L of sodium, so it will not bring 158 down. (If the patient were profoundly hypotensive, isotonic goes first to save the pressure, then you switch.)
Answer: 2 — hypertonic. Seizures make this symptomatic hyponatremia. 3% saline raises serum osmolality and pulls water out of the swollen brain cells. Pump, frequent sodium levels, seizure precautions, airway ready.
Why not the others: hypotonic would add more free water to a swelling brain — the worst possible choice; isotonic will not raise the sodium quickly enough to stop the seizing.
Answer: 2 — hypertonic. It pulls water OUT of swollen brain cells, shrinking brain volume and lowering ICP.
Why not the others: hypotonic pushes water into the brain — contraindicated in any raised-ICP patient; D5W is functionally hypotonic once the dextrose is metabolized, so it is the same mistake wearing a different label.
Answer: 1 — isotonic. Sepsis causes vasodilation and capillary leak, so the vascular space is functionally empty. Isotonic crystalloid is the first-line resuscitation (commonly 30 mL/kg), before vasopressors.
Why not the others: hypotonic leaves the vessel and worsens the hypotension; hypertonic is not a sepsis resuscitation fluid and risks pulmonary edema in a leaky vasculature.
Answer: 3 — hypotonic. A solution more dilute than the cell creates a gradient that pulls water inward, which is exactly what a dehydrated cell needs.
Why not the others: hypertonic pulls water out — the opposite; isotonic creates no gradient at all, so the cell size does not change.
Answer: 1 — isotonic. Every drop of isotonic fluid stays in the extracellular space, so in a failing heart the volume backs up into the pulmonary circulation → crackles and pulmonary edema.
Why not the others: hypotonic shifts fluid into cells and is the less likely culprit for vascular overload; "no fluid type causes this" is simply false — fluid overload is the most common IV therapy complication, and hypertonic solutions do it too.
Answer: 1 — hypertonic. Sodium 118 mEq/L with confusion is severe symptomatic hyponatremia. Hypertonic saline pulls water out of the swollen cells and raises the sodium — slowly, no more than about 8–12 mEq/L in 24 hours.
Why not the others: hypotonic adds free water and deepens the hyponatremia; D5W acts hypotonic and would do the same damage.
Answer: 2 — isotonic. This is classic dehydration with hypovolemia. Isotonic fluid replaces vascular volume so the kidneys are perfused and urine output comes back up.
Why not the others: hypertonic would pull water out of already dry cells; hypotonic would not hold the blood pressure. Vascular volume is always restored before you worry about cell water.
Answer: 3 — hypotonic. Hypotonic fluid moves water INTO cells, including brain cells, and worsens cerebral edema and ICP.
Why not the others: hypertonic is the treatment for raised ICP; isotonic (0.9% NS) is the standard maintenance fluid for neuro patients because it does not shift water across the membrane at all.
Seven things. If you can say these out loud, you can answer any IV fluid question on the exam.
🤖 Ask Claire: "Act as my instructor. Give me eight IV fluid scenarios — include a head injury, a CHF patient, a septic patient, a blood transfusion, and a sodium of 118 — and make me name the fluid, the rate concern, and the one thing I would assess before hanging it."