Skip to Main Content

Chapter 220. Important Formulae

Ruchir Gupta, MD

Allowable blood loss (ABL): weight (kg) × EBV × (Hinitial − Hfinal)/ Havg.

Total body deficit of HCO3: weight (kg) × (deviation from 24) × ECF.

Anion gap: Na + (Cl + HCO3).

  Normal 8–16.

image

|Download (.pdf)|Print
Perfusion to Various Body Compartments
CompartmentBody mass in adults (%)Cardiac output (%)
Vessel-rich group (brain, heart, liver, kidneys)1075
Muscles5019
Fat205
Vessel-poor group (bone, tendons)201
Table 220-1 Respiratory Formulae and Equations
View Table||Download (.pdf)
Table 220-1 Respiratory Formulae and Equations
Fick equation: VO2 (O2 consumption) (N: 250 mL/min)CO × (CaO2 − CvO2)
  • CO: cardiac output
  • CaO2: arterial oxygen content
  • CvO2: venous oxygen content
Alveolar gas equationPAO2 = (PB − 47)FiO2 − PaCO2/RQ
  • PB: barometric pressure
  • PA: alveolar pressure
  • RQ: respiratory quotient (0.8 on a typical diet)
  • Pa: arterial pressure
  • FiO2: fraction of inspired O2
A–a gradientPAO2 − PaO2
O2 content (N: 20 mL O2/dL)CaO2 = 1.39(Hb × SaO2) + (PaO2 × 0.003)
  • CaO2: arterial oxygen content
  • PaO2: arterial oxygen content
  • Hb: hemoglobin
Shunt fraction (N: 5%)
image
  • CćO2: end capillary
  • O2 content
  • CaO2: arterial oxygen content
  • image: mixed venous oxygen content
Oxygen delivery (N: 400–660 mL/min/m2)DO2 = CO × CaO2
Oxygen extraction(CaO2 − C
image)/CaO2
  • Bohr's equation (physiologic dead space)
  • Normal 25–30%
image
image: mixed expired CO2
Compliance
image
  • PAP: peak airway pressure
  • Pplat: plateau pressure
Time constant definitionTotal compliance × airway resistance
Fractional SaO2
image		SaO2: oxygen saturation of hemoglobin in the arterial blood
Laplace's lawT =PR/W

T: tension

W: wall thickness

Boyle's lawPfVf =P0V0, i.e., the PV product is constant
  • T: temperature
  • P: pressure
  • V: volume
Ideal gas lawPV =nRT
  • P: pressure
  • V: volume
  • n: number of moles of gas
  • R: universal gas constant = 8.31 J/moles/kg
  • T: temperature

Adapted from Schwartz's Principles of Surgery, Table 13-3.

|Download (.pdf)|Print
Cardiac Formulae
ParameterFormulaNormal range
Cardiac indexCO/BSA2.2–4.2 L/min/m2
Stroke volume indexSV/BSA40–70 mL/beats/m2
Left ventricular stroke work indexSI × 0.0136(MAP − PAOP)46–60 g m/beat/m2
Right ventricular stroke work indexSI × 0.0136(PA − CVP)30–65 g m/beat/m2
Stroke volumeCO × 1,000/HR40–80 mL
Pulmonary artery systolic pressureMeasured20–30 mm ...

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.

This site uses cookies to provide, maintain and improve your experience. MH Privacy Center Close