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Medical formulas

For healthcare professions

Our policy

These medical formulas have been produced by South Thames Retrieval Service (STRS) at Evelina London for nurses, doctors and ambulance technician to refer to in the care of critically ill children.

The medical formulas calculate average requirements. It is the responsibility of the clinician to ensure drugs and equipment are used appropriately according to the clinical situation.

Medical formulas

General and respiratory

Estimated weight under 1 year (kg) 

= (age in months x 0.5) + 4

Estimated weight 1-5 years (kg)

= (age in years x 2) + 8

Estimated weight 6-12 years (kg)  

= (age in years x 3) +7

Estimated weight 13 years +

No standard formula once child is 13 years old. If child is 13+ years and post puberty then use adult dosing for drugs. If working out drug dosing by weight, cap estimated weight at 50kg or there is  risk of overdose for many drugs.

Estimated surface area (m2) = square root [height (cm) x weight (kg) / 3600]
Endotracheal tube inner diameter (mm)   = (>1 year) = age/4 + 4 (for croup or subglottic stenosis use 0.5 to 1 size smaller than above)
Endotracheal tube length at lip (cm)

= 12 + age/2 

Endotracheal tube length at nostril (cm) = 15 + age/2  (don't pre cut tubes before insertion otherwise they may be too short)
Oxygenation index (OI)   = MAP  x FiO2 (%) / PaO2 (mmHg)
Conversion from kPa to mmHg = 7.5 x kPa

Fluids, electrolytes and acid base

 

Glucose requirement (mg/kg/min) 

 = (mL/hr x % glucose infused)/(6 x wt) aim > 4 metabolic/liver patients

Osmolality serum

= 2 x Na + gluc + urea (mmol/L). Normal 270 – 295

Osmolar gap (mOsm/kg) 

= measured - calculate osmolality. Normal -3 to +10

 

Anion Gap (mEq/L) 

= (Na + K) – (Bicarb + Cl). Normal 8 - 16

Albumin-corrected anion gap (mEq/L)

= AG + (0.25 x [40 - albumin (g/L)])

Corrected sodium in DKA (mmol/L) =

= Plasma Na + (0.4 x (glucose – 5.5))

Chloride: sodium ratio

= plamsa Cl / Na. Normal 0.74 to 0.80, >0.80 = hyperchloarmia 

Base excess due to chloride (mEq/L)  

= Na - Cl - 32

Creatinine Clearance <1 year (ml/min/1.73m2) 

= (30 x Ht)/serum creatinine

Creatinine Clearance >1 year (ml/min/1.73m2) = (40 x Ht)/serum creatinine (all in mmol/L)
Fractional Excretion of Na (FENa)=  =(UNa x PCr)/(PNa x UCr) x 100(all in mmol/L)

  

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