teach me fluids and electrolytes as a paediatrician for 1st year fron nelson
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Fluids & Electrolytes in Paediatrics
Based on Nelson Textbook of Paediatrics (Harriet Lane Handbook, 9780323876988)
I. INTRODUCTION - Think of IVF as a Drug
Intravenous fluids (IVFs) are a medication - always prescribe with intent. Since Holliday and Segar (late 1950s), maintenance fluids were calculated based on sodium in human milk (~3 mEq Na + 2 mEq K per 100 mL water), favouring 1/4 NS (hypotonic). However, AAP guidelines now recommend isotonic fluids as the maintenance choice for most hospitalised children due to risk of hyponatraemia with hypotonic fluids.
II. FLUID RESUSCITATION
A. Maintenance Fluid Volume - The Holliday-Segar Method
The most widely used method - estimates caloric expenditure by weight categories, assuming ~100 mL water per 100 kcal metabolised.
| Body Weight | mL/kg/day | mL/kg/hr ("4-2-1 rule") |
|---|---|---|
| First 10 kg | 100 | ~4 |
| Second 10 kg | 50 | ~2 |
| Each additional kg | 20 | ~1 |
Maximum recommended rate: 120 mL/hr (unless cardiac, hepatic, or renal pathology exists)
Note: Holliday-Segar NOT suitable for neonates <14 days old - it overestimates needs. Use Chapter 18 neonatal protocols.
Worked Example (25 kg, 8-year-old):
- First 10 kg: 4 × 10 = 40 mL/hr
- Second 10 kg: 2 × 10 = 20 mL/hr
- Additional 5 kg: 1 × 5 = 5 mL/hr
- Total = 65 mL/hr
B. Calculating Fluid Loss
Total Body Water (TBW):
- TBW = weight (kg) × 0.6 (children)
- TBW = weight (kg) × 0.75 (infants)
- 1 L water = 1 kg (use pre-illness weight)
Fluid Deficit Calculation:
There are two methods:
Equation 11.2 (% dehydration method):
Fluid deficit (mL) = % dehydration × weight (kg) × 10
Equation 11.3 (osmolality-based):
Free Water Deficit (mL) = TBW × [(current serum Na / desired serum Na) - 1]
Assessing Dehydration by Clinical Signs:
| Feature | Mild (3-5%) | Moderate (6-9%) | Severe (≥10%) |
|---|---|---|---|
| HR | Normal | Increased | Markedly increased |
| BP | Normal | Normal/low | Low |
| Fontanelle | Flat | Sunken | Very sunken |
| Skin turgor | Normal | Reduced | Tenting |
| Mucous membranes | Slightly dry | Dry | Parched |
| Eyes | Normal | Sunken | Very sunken |
| Urine output | Slightly reduced | Reduced | Minimal/none |
| Capillary refill | <2s | 2-3s | >3s |
C. Maintenance Fluid Choice in Hospitalised Children
- Isotonic saline (0.9% NS) or Lactated Ringer's (LR) are now preferred for most hospitalised children (AAP recommendation)
- Isotonic fluids protect against hospital-acquired hyponatraemia
- Dextrose is still added (typically D5) for glucose support in children
Composition of Common IV Fluids:
| Fluid | Na (mEq/L) | K (mEq/L) | Cl (mEq/L) | Tonicity |
|---|---|---|---|---|
| 0.9% NS (normal saline) | 154 | 0 | 154 | Isotonic |
| 0.45% NS (1/2 NS) | 77 | 0 | 77 | Hypotonic |
| 0.225% NS (1/4 NS) | 38 | 0 | 38 | Hypotonic |
| Lactated Ringer's | 130 | 4 | 109 | Isotonic |
| D5W | 0 | 0 | 0 | Hypotonic (free water) |
D. Dehydration Management (Isonatremic)
Sample calculation - Isonatremic dehydration, 20 kg child, 10% dehydration:
- Maintenance: (100×10) + (50×10) = 1500 mL/day = 62.5 mL/hr
- Deficit: 10% × 20 kg × 10 = 2000 mL
- First 8 hours: replace 50% deficit = 1000 mL → 125 mL/hr + maintenance
- Next 16 hours: remaining 50% deficit = 1000 mL → 62.5 mL/hr + maintenance
The fluid replacement rate table (Table 11.3 in textbook) guides combining maintenance + deficit replacement.
III. FLUID REMOVAL
Fluid removal (e.g., diuresis) is a separate consideration and includes assessment for volume overload, especially in children with renal, cardiac, or hepatic disease.
IV. ELECTROLYTE MANAGEMENT
A. Serum Osmolality and Tonicity
Serum osmolality formula:
Serum Osm (mOsm/kg) = 2[Na] + Glucose/18 + BUN/2.8
- Normal = 285-295 mOsm/kg
- Tonicity = effective osmolality (excludes urea, which freely crosses membranes)
B. Sodium (Normal 135-145 mEq/L)
1. Hyponatraemia (Na <135 mEq/L)
Clinical Manifestations: Nausea, vomiting, headache, lethargy, seizures, coma (especially if acute or <125 mEq/L)
Approach - First assess osmolality:
| Serum Osm | Cause |
|---|---|
| Low (<280) | True hyponatraemia - assess volume status |
| Normal (280-295) | Pseudohyponatraemia (hyperlipidaemia, hyperproteinaemia) |
| High (>295) | Osmotic shift - hyperglycaemia, mannitol |
Volume-status approach to true hyponatraemia:
| Volume Status | Urine Na | Causes |
|---|---|---|
| Hypovolaemic | <20 mEq/L | GI losses (diarrhoea, vomiting), skin losses (burns, CF) |
| Hypovolaemic | >20 mEq/L | Renal losses: diuretics, salt-wasting nephropathy, adrenal insufficiency |
| Euvolaemic | <20 mEq/L | Primary polydipsia |
| Euvolaemic | >20 mEq/L | SIADH, hypothyroidism |
| Hypervolaemic | <20 mEq/L | CHF, cirrhosis, nephrotic syndrome |
| Hypervolaemic | >20 mEq/L | Renal failure |
Management of Hyponatremic Dehydration:
- Correct by no more than 10-12 mEq/L per 24 hours to avoid osmotic demyelination syndrome (ODS)
- Exception: witnessed acute onset (<48h) can be corrected faster
Na deficit (Equation 11.5):
Na deficit (mEq) = [Desired Na - Serum Na] × TBW (L)
Na content of infusate (Equation 11.6):
Na content (mEq/L) = [Na deficit + (14 mEq/100mL × maintenance volume)] / Volume deficit
Fluid rate (Equation 11.7):
Rate (mL/hr) = Na deficit (mEq) × 1000 / [infusate Na (mEq/L) × hours of infusion]
Symptomatic/Severe Hyponatraemia (CNS symptoms):
- Give Hypertonic Saline (3% NaCl) over 3-4 hours
- Goal: raise serum Na by ~5 mEq/L to stop seizures
- Then slow correction to stay within 10-12 mEq/24 hr rule
Sample case (Box 11.4): 15 kg child, 10% dehydrated, Na = 125 mEq/L, no CNS symptoms
- Maintenance = 1250 mL/day = 52 mL/hr
- Deficit = 10 × 15 × 10% = 1500 mL → 63 mL/hr
- Combined starting rate: ~115 mL/hr, then taper at 16 hours
2. Hypernatraemia (Na >145 mEq/L)
Clinical Manifestations: Lethargy, weakness, altered mental status, irritability, coma, seizures, high-pitched cry, thrombosis, brain haemorrhage, muscle cramps, hyperpnoea
Key point: Intravascular volume is better preserved in hypernatraemic dehydration vs. hyponatraemic dehydration (because water shifts into cells).
Etiologies by urine osmolality:
| Urine Osm | Low Urine Na (<20) | High Urine Na (>20) |
|---|---|---|
| Low | Diabetes insipidus (central, nephrogenic), post-obstructive diuresis, CKD, diuretics | - |
| High | GI/skin/respiratory losses, increased insensible losses | Exogenous Na+ (meds, formula), mineralocorticoid excess |
Management:
- Correct Na by no more than 10 mEq/L per 24 hours
- Correct free water deficit over 48 hours - to prevent cerebral oedema (brain has produced idiogenic osmoles)
- Start with D5 1/2 NS (standard expert opinion)
- Witnessed acute hypernatraemia can be corrected faster (no idiogenic osmoles yet)
Free Water Deficit (Equation 11.8):
FWD (mL) = TBW (mL) × [1 - (Desired Na / Serum Na)]
Solute Fluid Deficit:
SFD = Total fluid deficit - FWD
Na required (Equation 11.10):
Na required (mEq) = [SFD (mL) + maintenance volume (mL)] × 14 mEq/100 mL
When Na >175 mEq/L (severe hypernatraemia): Use isotonic (NS) boluses first to restore perfusion before attempting free water correction - paradoxically, isotonic fluid lowers Na more safely than hypotonic fluid in extreme cases.
C. Potassium (Normal 3.5-5.0 mEq/L)
1. Hypokalemia (K <3.5 mEq/L)
Clinical manifestations appear at levels <2.5 mEq/L: skeletal muscle weakness, cramps, ileus, cardiac arrhythmias (flattened T waves, U waves on ECG), rhabdomyolysis
Etiologies (TTKG helps differentiate):
TTKG (Equation 11.14):
TTKG = [K]urine × (plasma osmolality / urine osmolality) (Valid only when urine osmolality > serum osmolality)
| TTKG | Interpretation |
|---|---|
| <2 | Extrarenal loss (GI, skin) |
| >4 | Renal loss (diuretics, hyperaldosteronism, RTA, vomiting with alkalosis) |
Management:
- Oral supplementation preferred when feasible
- IV potassium: do not exceed 1 mEq/kg/hr
- Always give K+ in a monitored setting when IV replacement is required
- Correct co-existing hypomagnesaemia (refractory hypokalaemia)
2. Hyperkalemia (K >5.5 mEq/L in children, >5.0 mEq/L in older children/adults)
Danger: Cardiac arrhythmias - peaked T waves, widened QRS, sine wave pattern, VF
Management algorithm (Fig. 11.1 in textbook):
| Step | Intervention | Mechanism | Onset |
|---|---|---|---|
| 1 | Calcium gluconate IV | Membrane stabilisation | Minutes |
| 2 | Sodium bicarbonate | Shifts K into cells | 15-30 min |
| 3 | Insulin + Glucose | Shifts K into cells | 15-30 min |
| 4 | Albuterol (nebulised) | Shifts K into cells | 30 min |
| 5 | Kayexalate (sodium polystyrene) | GI K removal | Hours |
| 6 | Dialysis | Definitive removal | Immediate |
D. Calcium (Normal total Ca: 8.5-10.5 mg/dL; ionised Ca: 1.12-1.32 mmol/L)
1. Hypocalcaemia
Symptoms: Tetany, Chvostek sign, Trousseau sign, seizures, prolonged QTc, laryngospasm
Key corrections:
- Albumin correction: For every 1 g/dL drop in albumin, total Ca drops 0.8 mg/dL (correct accordingly)
- pH: Acidosis increases ionised Ca; alkalosis decreases it
- Symptoms refractory to Ca supplementation - check magnesium (hypomagnesaemia causes refractory hypocalcaemia)
- If significant hyperphosphataemia present - correct it first (risk of calcification if Ca × PO4 product ≥ 70)
Etiologies: Hypoparathyroidism, vitamin D deficiency/resistance, neonatal hypocalcaemia, DiGeorge syndrome, pancreatitis, renal disease, hypomagnesaemia
2. Hypercalcaemia
Symptoms: "Bones, stones, groans, moans" - bone pain, nephrolithiasis, constipation, nausea, confusion
Causes: Hyperparathyroidism, vitamin D toxicity, malignancy, immobilisation, Williams syndrome, thiazide diuretics
Management: Saline hydration, furosemide, bisphosphonates (in severe/malignancy cases)
E. Magnesium (Normal 1.5-2.5 mg/dL)
Hypomagnesaemia:
- Causes: GI losses (chronic diarrhoea, short bowel), renal wasting (diuretics, cisplatin, aminoglycosides), malnutrition
- Effects: Neuromuscular irritability, arrhythmias, refractory hypokalaemia and hypocalcaemia
- Treatment: Oral or IV magnesium sulphate
Hypermagnesaemia:
- Causes: Excessive Mg supplementation, antacid overuse, renal failure
- Effects: Hyporeflexia, respiratory depression, cardiac arrest
- Treatment: Stop Mg, calcium gluconate (antagonist), dialysis if severe
F. Phosphate (Normal 4.5-6.5 mg/dL in children; 2.5-4.5 mg/dL in adolescents)
Hypophosphataemia:
- Causes: Malnutrition, refeeding syndrome, DKA treatment, antacids, vitamin D deficiency
- Effects: Weakness, haemolytic anaemia, rhabdomyolysis, respiratory failure
- Treatment: Oral/IV phosphate replacement
Hyperphosphataemia:
- Causes: Renal failure, hypoparathyroidism, excessive intake, rhabdomyolysis, tumour lysis syndrome
- Risk: Ca × PO4 product >70 - calcification risk
- Treatment: Dietary restriction, phosphate binders, treat underlying cause, dialysis
V. ACID-BASE DISTURBANCES
Step-by-step approach:
Step 1: Determine pH
- pH <7.35 = Acidaemia
- pH >7.45 = Alkalaemia
- The body does not fully compensate - the pH always leans toward the primary disturbance
Step 2: Identify primary disturbance
| pH | HCO3 | PaCO2 | Primary disorder |
|---|---|---|---|
| Low | Low | Low (compensatory) | Metabolic acidosis |
| Low | High (compensatory) | High | Respiratory acidosis |
| High | High | High (compensatory) | Metabolic alkalosis |
| High | Low (compensatory) | Low | Respiratory alkalosis |
Step 3: Calculate expected compensation (Winter's formula for met. acidosis):
Expected PaCO2 = 1.5 × [HCO3] + 8 ± 2
If measured PaCO2 ≠ expected - mixed disorder is present
Step 4: Calculate anion gap (for metabolic acidosis)
AG = Na - (Cl + HCO3) | Normal = 8-12 mEq/L (or 12 ± 4)
High AG causes (MUDPILES):
- M - Methanol
- U - Uraemia
- D - DKA / Diabetic ketoacidosis
- P - Propylene glycol / Paracetamol
- I - Isoniazid / Iron
- L - Lactic acidosis
- E - Ethylene glycol
- S - Salicylates
Normal AG acidosis causes (HARDUPS):
- Hyperalimentation, Addison disease, Renal tubular acidosis, Diarrhoea, Ureteral diversions, Pancreatic fistula, Saline infusion
Step 5: Delta-delta ratio (if high AG present)
Delta-delta = (measured AG - normal AG) / (normal HCO3 - measured HCO3)
- <0.4 = pure normal AG acidosis
- 0.4-0.8 = mixed high AG + normal AG acidosis
- 1-2 = pure high AG metabolic acidosis
-
2 = high AG acidosis + concurrent metabolic alkalosis
QUICK REFERENCE SUMMARY TABLE
| Topic | Key Number |
|---|---|
| TBW (children) | 60% body weight |
| TBW (infants) | 75% body weight |
| Maintenance (first 10 kg) | 4 mL/kg/hr |
| Maintenance (second 10 kg) | 2 mL/kg/hr |
| Maintenance (>20 kg) | 1 mL/kg/hr |
| Max Na correction (hypo/hypernatraemia) | 10-12 mEq/L/24 hr |
| 3% NaCl for symptomatic hyponatraemia | Raise Na by ~5 mEq/L over 3-4 hr |
| Hypernatraemia correction time | Over 48 hours |
| K+ max IV infusion rate | 1 mEq/kg/hr |
| Osmotic demyelination risk | Rapid correction of hyponatraemia |
| Cerebral oedema risk | Rapid correction of hypernatraemia |
| Ca correction for albumin | 0.8 mg/dL per 1 g/dL albumin |
| Precipitation risk | Ca × PO4 ≥ 70 |
| Normal serum osmolality | 285-295 mOsm/kg |
CLINICAL PEARLS FOR PEDS PGY-1
- Always use isotonic fluids for maintenance in hospitalised children (not 1/4 NS anymore) - updated AAP guidance.
- Holliday-Segar = volume only, not electrolyte prescription. Add Na, K separately based on clinical context.
- Hyponatraemia correction: Slow is safe. >12 mEq/24 hr risks osmotic demyelination.
- Hypernatraemia: Correct FREE WATER deficit over 48 hr. Rapid correction = cerebral oedema.
- Acute onset (<48 hr) of either hypo- or hypernatraemia - brain hasn't adapted, so can correct faster.
- Hypokalaemia is refractory until you correct hypomagnesaemia.
- Hypocalcaemia is refractory until you correct hypomagnesaemia.
- In DKA - despite the hyperglycaemia and acidosis, total body K is depleted. Give K before insulin if K <3.5.
- Na >175: Use isotonic boluses first, not hypotonic fluids, to stabilise before gradual free water correction.
- Anion gap first in any unexplained acidosis - it directs your entire differential.
Based on: Harriet Lane Handbook / Nelson Paediatric Reference (ISBN 9780323876988), Chapter 11 - Fluids and Electrolytes
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