Dehydration in Children

Dehydration is a state of negative fluid balance that develops when ongoing losses — most often from vomiting and diarrhea, but also fever, reduced oral intake during illness, osmotic diuresis, or excessive insensible losses — exceed intake. Infants and young children are disproportionately vulnerable: a much larger fraction of body weight is water (roughly 75-80% in a newborn versus 55-60% in an adult), fluid turnover per kilogram is higher, and a preverbal child cannot independently seek out or ask for fluids, so a given percentage fluid deficit develops faster and is tolerated less well than in an older child or adult. In most outpatient and emergency settings the degree of dehydration is estimated clinically from a pattern of history and examination findings rather than from a single laboratory value — clinically apparent signs often do not appear until a meaningful fraction of body weight has already been lost, so a well-appearing child can still have a real deficit.^[1] Getting the severity assessment right is the decision that drives everything downstream: oral rehydration therapy is the preferred first-line treatment for essentially all mild-to-moderate dehydration, nasogastric rehydration is an underused intermediate option when the oral route genuinely fails, and intravenous therapy is reserved for shock or true failure of both enteral routes.^[2]

Recognizing the degree of dehydration

No single sign reliably estimates the fluid deficit on its own — severity is judged from the whole clinical picture together: general appearance and behaviour, pulse, blood pressure, thirst, mucous membranes, tears, skin turgor and capillary refill, urine output, and (in infants under 3 months) the anterior fontanelle.^[3]

• Young age — infants and toddlers have a higher body-water percentage and higher insensible/metabolic fluid turnover per kilogram than older children, so a given illness produces a larger percentage fluid deficit, faster.
• Acute gastroenteritis (vomiting and/or diarrhea) — by far the most common cause of dehydration in children.
• Fever, which increases insensible fluid losses.
• Reduced oral intake during any acute illness, including from odynophagia (e.g., stomatitis, pharyngitis) or general malaise.
• Diabetic ketoacidosis or another cause of osmotic diuresis.
• Excessive renal losses (e.g., diabetes insipidus, a renal tubular disorder).
• Burns or another condition with high insensible or third-space losses.
• Inability to communicate thirst or access fluids independently — preverbal infants, and any child with reduced access to caregiving.
• A chronic condition with ongoing fluid losses, such as an ostomy, short bowel syndrome, or cystic fibrosis.
• Hot weather or high ambient temperature with limited fluid access.
• A prior episode of significant dehydration.

History

• Characterize the underlying cause: onset, frequency, and volume of vomiting and/or diarrhea, and any other source of fluid loss (fever, high-output ostomy, polyuria).
• Ask specifically about urine output — the frequency and approximate volume of the last several voids/wet diapers, and time since the last void.
• Ask about oral intake since the illness began, including whether the child is still breastfeeding or taking formula/fluids at all.
• Obtain a recent pre-illness weight if available (from a caregiver, growth record, or prior visit) — this is the most objective way to calculate the fluid deficit: deficit in litres equals pre-illness weight in kilograms minus current weight in kilograms.
• Ask about associated red-flag features: bilious or persistently forceful vomiting, blood in the stool or vomitus, abdominal pain out of proportion to a viral illness, or altered behaviour.

Examination

Heart rate, respiratory rate, blood pressure, temperature, oxygen saturation, capillary refill, and the full constellation of findings described in Recognize the Condition. Two simplified, complementary bedside tools are widely used alongside overall clinical judgment:

Two or more signs in the "some dehydration" column (with none more severe) classify the child as having some/moderate dehydration; two or more signs in the "severe dehydration" column classify the child as severely dehydrated.^[1]

The Clinical Dehydration Scale is a separate, formally validated 4-item bedside score: general appearance, eyes, mucous membranes, and tears, each scored 0 (normal), 1, or 2 (most abnormal), for a total of 0-8.

A total score of 0 corresponds to no clinically important dehydration, 1-4 to some dehydration, and 5-8 to moderate-to-severe dehydration — use it as a structured second check alongside overall clinical judgment, not a replacement for it. It is a genuinely validated point-sum score, but is taught here as a reference table rather than as a live calculator.^[5]

Classifying by serum sodium, once bloodwork is obtained

When electrolytes are checked, dehydration is further classified by serum sodium, which changes both the expected findings and the correction strategy.

Investigations

Most children with mild-to-moderate dehydration and a clear gastroenteritis history need no bloodwork at all — clinical reassessment during oral rehydration is the primary monitoring tool. Obtain serum sodium, potassium, chloride, urea, creatinine, glucose, and bicarbonate, plus a venous blood gas and urinalysis, when clinically indicated: moderate-to-severe dehydration, any child proceeding to IV therapy, suspected hyper- or hyponatremia, suspected diabetic ketoacidosis, or a presentation that is not straightforward gastroenteritis.^[3] A bicarbonate under 15 mmol/L at presentation is a reasonable trigger to lower the threshold for admission even when clinical severity looks borderline.

• Recognize and treat shock or impending shock immediately.
• Replace the estimated fluid deficit using the least invasive effective route — oral, then nasogastric, then intravenous.
• Correct electrolyte imbalances safely, without correcting serum sodium too quickly.
• Meet ongoing maintenance fluid needs plus any continuing losses while the deficit is being replaced.
• Prevent complications of both the dehydration itself and of its treatment — seizures from rapid sodium shifts, cerebral edema, and hospital-acquired hyponatremia from inappropriately hypotonic IV fluid.
• Resume a normal, age-appropriate diet as soon as rehydration is adequate, rather than prolonging fasting or dilute feeds.
• Educate the family on home fluid management and on the specific signs that should prompt them to seek care again.

Correct technique for giving ORS at home

• Give small volumes frequently rather than a large volume at once — roughly 5-10 mL (about 1-2 teaspoons) every 1-2 minutes for a reluctant or vomiting-prone child, using a spoon, oral syringe, or small cup, and gradually increasing the volume per attempt as it is tolerated.
• If the child vomits, wait about 10 minutes and then resume with smaller, more frequent volumes rather than stopping altogether — a single vomiting episode does not mean ORT has failed.
• Continue breastfeeding throughout rehydration in a breastfed infant — it does not need to be interrupted, though it should not be relied on as the sole rehydration fluid when a measured deficit needs to be replaced.
• Do not use plain water, sports drinks, undiluted fruit juice, or soft drinks as the rehydration fluid — none of these have an appropriate electrolyte and glucose composition, and plain water in particular risks hyponatremia and hypoglycemia in a young child.
• Once the calculated deficit has been replaced (typically over 4 hours for mild-to-moderate dehydration), resume the child's usual diet and usual fluids rather than continuing ORS alone.
• Weigh the child before and after rehydration when practical — the weight gained is an objective check that the estimated deficit was actually replaced.
• Teach caregivers the specific signs that should prompt a return to care — see Red Flags and Refer.

Pregnancy

Not applicable — dehydration in children is a pediatric-specific condition, assessed and treated using pediatric weight-based dosing throughout this chapter.

Neonates and young infants (under 1-3 months)

Have a lower threshold for admission and IV therapy at this age — classic exam signs are less reliable, deterioration can be rapid, and even moderate dehydration is often managed more cautiously than in an older child. Consider sepsis and a urinary tract infection as possible underlying causes of poor feeding or vomiting at this age, not only gastroenteritis.

Breastfeeding infants

Continue breastfeeding throughout rehydration and afterward — it does not need to be interrupted for oral rehydration therapy, ondansetron, or IV therapy, and should resume as the primary feed as soon as the child can tolerate oral intake.^[7]

Hypernatremic dehydration

Correct slowly to avoid cerebral edema and seizures — the goal is to lower serum sodium by no more than 10-15 mmol/L per day and to restore normal hydration over no less than 48 hours. If serum sodium is falling faster than this (over 10-15 mmol/L per day, or over 1 mmol/L every 2 hours), the correction fluid needs adjustment. Shock, when present, is still treated urgently with isotonic crystalloid regardless of the underlying sodium type.^[8]

Hyponatremic dehydration

A symptomatic child (e.g., seizures) needs urgent, monitored correction with hypertonic saline to a serum sodium of about 125 mmol/L, with the remaining deficit corrected over the following 24-48 hours once the child is asymptomatic — see Drug Tables for dosing. Avoid hypotonic IV fluid in this situation, since it worsens rather than corrects the problem.^[9]

Diabetic ketoacidosis

Dehydration from DKA needs its own specific fluid and insulin protocol rather than standard gastroenteritis-pattern rehydration — recognize the possibility (a polyuria/polydipsia history, hyperglycemia, ketosis/acidosis on the initial gas and glucose) and manage per a dedicated DKA pathway.

Chronic conditions with ongoing losses

A child with an ostomy, short bowel syndrome, cystic fibrosis, or a renal concentrating defect may need individualized maintenance fluid and electrolyte targets that differ from the standard formulas in this chapter — involve the child's specialist team when adjusting their usual regimen.

Malnutrition

A significantly malnourished child tolerates rapid fluid shifts poorly and is at higher risk of fluid overload and cardiac complications with standard bolus volumes — use smaller, more cautious boluses, reassess more frequently, and involve a specialist team experienced in rehabilitating a malnourished child wherever possible.

• Reassess hydration status clinically at set intervals during ORT — improving alertness, moistening mucous membranes, and resumption of urine output are the most useful bedside signs that rehydration is working.
• Track cumulative intake (oral, NG, or IV) against the calculated deficit plus maintenance plus ongoing losses.
• Weigh the child again once rehydration is thought to be complete, to confirm the estimated deficit was actually replaced.
• For a child discharged home on oral rehydration, arrange a specific follow-up plan (a return visit or a phone check) within 24 hours, and give caregivers clear, specific written red-flag instructions for when to return sooner.
• For a child on IV fluid, recheck electrolytes at an interval appropriate to the degree of derangement found initially and to the correction strategy in use, and confirm urine output is established before adding potassium to the IV fluid.
• For hypernatremic or hyponatremic dehydration, monitor serum sodium closely enough to confirm the correction rate is on target — not too fast, not too slow.

1. Before assuming ORT has failed, confirm the technique was actually correct — small frequent volumes, resuming after a vomiting episode rather than stopping, and an adequate trial (at least the first hour) before switching strategies.
2. If vomiting is the specific barrier, confirm a weight-based ondansetron dose was actually given and that ORT was restarted 15-30 minutes afterward; if not yet tried, try it now before escalating further.
3. If oral intake is still genuinely failing despite correct technique and an ondansetron trial, and there is no contraindication (shock, impaired consciousness, paralytic ileus, protracted vomiting despite optimized technique, or suspected monosaccharide malabsorption), move to nasogastric rehydration rather than skipping straight to IV.
4. If NG rehydration is not tolerated, is contraindicated, or the child deteriorates at any point, proceed to IV access and isotonic fluid therapy.
5. Reassess the diagnosis if the response to appropriate therapy is not as expected — consider a surgical abdomen, diabetic ketoacidosis, sepsis, or a non-gastroenteritis cause of the fluid losses.
6. Admit for IV therapy and inpatient monitoring for shock, significant hypo-/hypernatremia, intractable vomiting or diarrhea despite the above steps, altered sensorium, or a bicarbonate under 15 mmol/L at presentation.

• Going straight to IV fluid for a child who is not in shock, without first attempting oral rehydration (with ondansetron if vomiting is the barrier) and, where appropriate, nasogastric rehydration.
• Giving ORS in large volumes at once instead of small, frequent amounts — this itself provokes more vomiting and looks like treatment failure.
• Stopping oral rehydration entirely after a single vomiting episode instead of pausing briefly and resuming with smaller volumes.
• Using plain water, sports drinks, undiluted juice, or soft drinks as the rehydration fluid instead of an appropriate oral rehydration solution.
• Withholding breastfeeding during rehydration — it does not need to be interrupted.
• Prolonging a clear-fluid or dilute-formula phase after rehydration instead of resuming the normal diet promptly — this is outdated practice that delays recovery rather than helping it.
• Correcting hypernatremia too quickly, risking cerebral edema and seizures.
• Missing a surgical abdomen or another non-gastroenteritis cause of vomiting because the presentation was assumed to be simple dehydration from the outset.
• Underusing nasogastric rehydration — it is frequently skipped in practice even though the evidence supports trying it before IV access.
• Defaulting to a hypotonic IV maintenance solution instead of an isotonic one, which raises the risk of hospital-acquired hyponatremia.
• Reaching for metoclopramide or domperidone as the antiemetic of choice instead of ondansetron, which has both better pediatric safety data and stronger evidence in this setting.
• Not weight-basing the ondansetron dose correctly, or giving it and then not actually restarting oral rehydration afterward.

How to use this table: Doses are typical pediatric doses and require patient-specific confirmation, including for renal impairment or a chronic condition affecting fluid balance. Select any drug name to open its complete Pharmasist Drug Database page.

Oral rehydration solutions (ORS)

The reduced-osmolarity formulation (sodium 75 mmol/L, potassium 20 mmol/L, chloride 65 mmol/L, citrate 10 mmol/L, glucose 75 mmol/L, total osmolarity 245 mOsm/L) is the current WHO/UNICEF standard and is preferred over the older higher-osmolarity formula, homemade sugar-salt solutions, or high-osmolarity sports drinks.^[10]

Antiemetics

Reserved for vomiting that is genuinely limiting oral intake - not routine for every child with gastroenteritis.^[6]

Take-home dosing evidence: a large pragmatic trial supports providing take-home ondansetron doses for ongoing vomiting after an emergency or office visit, rather than a single dose only.^[11]

Not preferred: Metoclopramide and domperidone are not preferred antiemetics for pediatric gastroenteritis-related vomiting - metoclopramide carries a meaningful extrapyramidal reaction risk in children, and domperidone carries a QT-prolongation caution; ondansetron has both better pediatric safety data and stronger evidence for this specific indication.

Intravenous fluids

Isotonic maintenance fluid (with appropriate potassium chloride and dextrose) is now preferred over hypotonic solutions for essentially all children requiring maintenance IV fluid - hypotonic maintenance fluid is a leading preventable cause of hospital-acquired hyponatremia.^[8]

Correction-rate evidence: the staged hypertonic-saline correction target above follows current guidance on safely managing symptomatic hyponatremia in hospitalized children.^[9]

Calculating the fluid deficit, maintenance rate, and sodium deficit

When a pre-illness weight is known, fluid deficit in litres equals pre-illness weight in kilograms minus current weight in kilograms. When it is not known, estimate the deficit as the percentage suggested by the clinical severity tier (see Recognize the Condition) multiplied by the current weight in kilograms.

Maintenance fluid rate (the Holliday-Segar method): 4 mL/kg/hour for the first 10 kg of body weight, plus 2 mL/kg/hour for each kilogram from 11-20 kg, plus 1 mL/kg/hour for each kilogram over 20 kg.^[8]

Sodium deficit in mmol equals (desired serum sodium minus actual serum sodium, in mmol/L) multiplied by body weight in kilograms multiplied by total body water in litres per kilogram (roughly 0.6 L/kg in a child, higher in a young infant) - use this only alongside specialist input for a significant sodium derangement.^[9]

Evidence note: Drug inventory, dosing, and comments reflect standard Canadian pediatric practice, cross-checked against current WHO, AAP, and Canadian Paediatric Society guidance.^[3]