19.1 Maintenance Fluids & Dehydration

Maintenance Fluids & Dehydration

Why This Matters for the Exam

Fluid and electrolyte questions are a core, heavily tested domain on the Arab Board of Health Specializations (ABHS) Pediatrics Part 1 written paper. Examiners favor scenario-based, single-best-answer questions that require you to calculate a maintenance fluid rate, classify dehydration severity from clinical signs, or choose a safe replacement strategy. Precision with numbers is essential — the same clinical vignette can have different correct answers depending on whether the child is mildly, moderately, or severely dehydrated.

The Holliday-Segar 4-2-1 Rule

The Holliday-Segar formula is the standard method for calculating maintenance fluid requirements — the volume needed to replace normal, ongoing insensible losses (skin, respiratory tract), urine output, and stool water in a child who is not eating or drinking normally. It is based on estimated caloric expenditure, since roughly 1 mL of water is required per kilocalorie metabolized.

Daily Maintenance Volume

Hourly Rate — the 4-2-1 Shortcut

For bedside infusion-pump calculations, the same weight bands are converted (and rounded) to an hourly rate:

Exam Alert: The 4-2-1 hourly figures are a rounded approximation of the daily formula divided by 24 hours (100/24 ≈ 4.17, 50/24 ≈ 2.08, 20/24 ≈ 0.83). Both methods are taught and accepted; expect the hourly shortcut in most exam vignettes because it is faster to apply at the bedside.

Worked Example

A 24 kg child is kept nil by mouth after surgery and needs maintenance intravenous (IV) fluids. Using the 4-2-1 rule:

• First 10 kg → 10 × 4 mL/kg/hour = 40 mL/hour
• Next 10 kg (10–20 kg) → 10 × 2 mL/kg/hour = 20 mL/hour
• Remaining 4 kg (20–24 kg) → 4 × 1 mL/kg/hour = 4 mL/hour
• Total maintenance rate = 40 + 20 + 4 = 64 mL/hour

Using the daily formula for the same child: (10 × 100) + (10 × 50) + (4 × 20) = 1000 + 500 + 80 = 1580 mL/day, which is consistent with the hourly figure once rounding is accounted for.

Modern Practice Note: Older teaching favored hypotonic maintenance fluids (e.g., 0.18–0.45% saline). Because hypotonic fluids are linked to hospital-acquired hyponatremia, current guidelines (AAP 2018 and most regional protocols) recommend isotonic fluids — 0.9% saline, with appropriate dextrose and potassium once urine output is confirmed — as the default maintenance fluid for most hospitalized children.

Dehydration Severity Assessment

Dehydration is a net deficit of total body water, most commonly caused by gastroenteritis (vomiting and diarrhea), reduced oral intake, or increased insensible losses from fever or tachypnea. Severity is graded by the estimated percentage of body weight lost as fluid. When a reliable pre-illness weight is available, percent dehydration can be calculated directly as: (pre-illness weight − current weight) ÷ pre-illness weight × 100 — a favorite calculation vignette on the written paper.

Exam Alert: Delayed capillary refill, abnormal skin turgor, and an abnormal respiratory pattern are the three individual signs with the strongest evidence for predicting at least 5% dehydration — a favorite "which finding is most useful" style distractor.

Fluid Deficit Replacement Principles

Once severity is estimated, the fluid deficit in milliliters can be calculated as:

Deficit (mL) = % dehydration × weight (kg) × 10

This works because each 1% of body weight lost as fluid is approximately equal to 10 mL/kg. Total fluid prescribed over the replacement period equals maintenance + deficit + ongoing losses (e.g., continuing diarrhea or nasogastric losses, replaced mL-for-mL as they occur).

• Mild-to-moderate dehydration: oral rehydration solution (ORS) is first-line per World Health Organization (WHO) guidance — roughly 50–100 mL/kg given over about 4 hours, with clinical reassessment before continuing. ORS replaces both water and electrolytes (glucose plus sodium are the key components); reassess every 1–2 hours for improved mucous membranes, urine output, and mental status before giving more.
• Severe dehydration or shock: give an immediate IV bolus of 20 mL/kg of an isotonic crystalloid (0.9% saline or lactated Ringer's solution), reassess, and repeat as needed (up to roughly 60 mL/kg) before moving to slower deficit replacement.
• Isotonic (isonatremic) dehydration: replace the remaining deficit over 24 hours — classically half over the first 8 hours and the remaining half over the next 16 hours, in addition to ongoing maintenance fluids.
• Hypernatremic dehydration is the key exception: the deficit is replaced much more slowly, typically over 48 hours, to avoid a dangerous osmotic shift of water into brain cells — the mechanism explored in the next section.