Respiratory Equations

Question 1

Partial Pressure of inspired oxygen (PiO2)

Answer 1

PiO2 = (PB - PH2O) x FiO2

PB = barometric pressure (760 mm Hg at sea level)

PH2O = water vapor pressure (47 mmHg)

Question 2

Partial pressure of oxygen in the alveolus (PAO2)

Answer 2

PAO2 = PiO2 - PaCO2/RQ

PiO2 = partial pressure of inspired oxygen

RQ = respiratory quotient (usually about 0.8)

Question 3

Delivery of oxygen (DO2)

Answer 3

CO x CaO2

CaO2 = oxygen content

Question 4

Oxygen extraction

Answer 4

CaO2 - CvO2/CaO2

Question 5

Oxygen consumption (VO2)

Answer 5

VO2 = (CaO2 - CvO2) x CO x 10 dL/L

CaO2 = arterial oxygen content 
CvO2 = venous oxygen content
CO = cardiac output

Question 6

Arterial oxygen content (CaO2)

Answer 6

CaO2 = 1.39 x Hb x SaO2 + (0.003 x PaO2)

Question 7

OI (oxygenation index)

Answer 7

[(MAP x FiO2) x 100]/PaO2

Question 8

Oxygen extraction ratio (O2ER)

And what is a normal ratio?

Answer 8

(CaO2 - (CvO2)/CaO2

Normal ratio 0.2-0.3 meaning excess of oxygen is delivered to the body’s tissues