alveolar ventilation/gas equations

Alveolar ventilation equation

VA = (VCO2 x K)/PACO2

VA = alveolar ventilation (ml/min)
V¬CO2¬¬ = rate of CO2 production (mL/min)
PACO2 = alveolar PCO2 (mm Hg)
K = constant = 836 mm Hg @ BTPS (body temp {310 K}, ambient pressure {760 mm Hg}, and gas saturated with H2O vapor)
-equation describes the inverse relationship between ventilation and alveolar PCO2

alveolar ventilation can also be calculated using:
V¬A = f(VT – VPD)
f = frequency of breathing (/min)
VT = tidal volume (mL)
VPD = physiological deadspace (mL)

Alveolar gas equation

PAO2 = PIO2 – PACO2/R + F

PIO2 = PO2 of inspired air (mm Hg)
Ft = correction factor
R = respiratory exchange ratio = VCO2/VO2

-PIO2 = FIO2(PB – PH2O), where FIO2 = fraction of O2 in inspired gas, PB = barometric pressure, PH2O = water partial pressure in inspired gas due to humidification
-equation describes the inverse relationship of PA
-F is small and usually ignored

Normal #’s:
PAO2 = 100 mm Hg
PACO2 = 40 mm Hg
VT = 450-500 mL
f = 12/min
VCO2 = 200-250 mL/min
R = 0.8

Sources:
West Pulmonary Pathophysiology
Costanzo Physiology
Access Medicine