# Formulas

## Acid/Base

- Winter's equation:

- Expected pCO2 = 1.5 * [HCO3-] + 8 (+/-2)

- If the measured PaCO2 is less than the expected → concurrent respiratory alkalosis
- If the measured PaCO2 is more than the expected → concurrent respiratory acidosis

- If the measured PaCO2 is less than the expected → concurrent respiratory alkalosis

- Expected pCO2 = 1.5 * [HCO3-] + 8 (+/-2)

## Burn

- Parkland formula: Fluid Requirements = TBSA burned(%) x Wt (kg) x 4mL
- Give 1/2 of total requirements in 1st 8 hours, then give 2nd half over next 16 hours.

## Cardiovascular

- O2 content = (Hb x 1.34 x SaO2) + (PaO2 x 0.003)
- Mean Arterial Pressure = 1/3(SBP) + 2/3(DBP)

## Endocrinology

- Estimated average glucose from Hgb A1C = (28.7 X A1C
_{US units}) - 46.7

## FEN

- Anion gap = Na – (Cl + HCO3-)

- Corrected anion gap, mEq/L = Anion gap, mEq/L + [ 2.5 × (4 – albumin, g/dL) ]
- Delta gap = Anion gap – 12 (normal anion gap)

- Corrected anion gap, mEq/L = Anion gap, mEq/L + [ 2.5 × (4 – albumin, g/dL) ]
- Bicarbonate deficit = 0.4 × body weight in kg × (24 – bicarbonate level)
- Calculated serum osmolality = 2 (Na) + Urea + Glucose

- Normal = 280 - 300 mmol/l

- Fractional Excretion of Sodium (FENa)% = 100 × (SCr × UNa ) / (SNa × UCr)
- Maintenance fluids, calculated using the “4-2-1” Rule:

- For 0-10 kg: + 4 mL/kg/hr
- For 10-20 kg: + 2 mL/kg/hr
- For >20 kg: + 1 mL/kg/hr

- For 0-10 kg: + 4 mL/kg/hr
- Corrected Sodium (Hillier, 1999) = Measured sodium + 0.024 * (Serum glucose - 100)
- Corrected Calcium = (0.8 * (Normal Albumin - Pt's Albumin)) + Serum Ca
- Free Water Deficit in Hypernatremia
- FW Deficit (Adult Male) = 0.6 x weight (kg) x (current NA / 140-1)
- FW Deficit (Adult Female) = 0.5 x weight (kg) x (current NA / 140-1)
- FW Deficit (Elderly Male) = 0.5 x weight (kg) x (current NA / 140-1)
- FW Deficit (Elderly Female) = 0.45 x weight (kg) x (current NA / 140-1)
- FW Deficit (Child) = 0.6 x weight (kg) x (current NA / 140-1)

## General

- Ideal/Predicted Body Weight:
- IBW (men) = 50kg + 2.3kg*( Height(in) - 60 )
- IBW (women) = 45.5kg + 2.3kg *( Height(in) - 60 )

Note: this formula is only an approximation, and is generally only applicable for people 60 inches (5 foot) tall or greater.

## Neurology

Cerebral Perfusion Pressure = MAP – ICP

## Pulmonary

- Alveolar-arterial O2 gradient, pATM at sea level 760 mmHg
- PAO2 - paO2
- A-a O2 gradient = [FiO2 x (pATM - pH2O)] - (paCO2/0.8) - paO2
- =[FiO2 x (760 - 47)] - (paCO2/0.8) - paO2
- Normal for young adult non-smoker < 15 mmHg at sea level at RA
- Normal estimate by age < (age/4) + 4
^{[1]}

- Expected Peak Flow:
- PEFR, children = [(Height in cm - 100) × 5] + 100
- PEFR, adult men = {[(Height in m × 5.48) + 1.58] - [Age × 0.041]} × 60
- PEFR, adult women = {[(Height in m × 3.72) + 2.24] - [Age × 0.03]} × 60

## Renal

- Cockcroft-Gault CrCl, mL/min = (140 – age) × (weight, kg) × (0.85 if female) / (72 × Cr)

## References

- ↑ Nickson C. Lifeinthefastlane. A-a gradient. http://lifeinthefastlane.com/ccc/a-a-gradient/.