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
      

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)
    
• 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
    
• 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
• 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)