Nitrogen Washout Total volume expired x %N2 = Original volume of N2 in lungs Original volume of N2 in lungs x 1.25 = Original lung volume

Helium Dilution F V = F (V + V ) HEi SPi HEf SPf Lf Amount of solute (mg) = concentration of solute (mg/ml) x volume of solvent (ml) Total amount of He before test = total amount of He at end of test F V = F (V + V ) HEi SPi HEf SPf Lf

BODY PLETHYSMOGRAPH PBOX x VBOX = PBOX x (VBOX - V) i i f i where (VBOX - V) = VBOX i f PM x VL = PM x (VL + V) i i f i where (VL + V) = VL i f

. ANATOMIC DEAD SPACE VT = VD + VA VA = VT - VD n(VA) = n (VT) – n(VD) VA = VE - VD .

PHYSIOLOGIC DEAD SPACE Anatomic Dead space Alveolar Dead space = +

BOHR EQUATION DERIVATION FE x VT = Fi x VD + FA x VA CO2 CO2 CO2 CO2 The volume of CO2 in mixed expired air The volume of CO2 coming from dead space The volume of CO2 coming from alveoli Where F = fractional concentration E = mixed expired I = inspired A = alveolar FA = fractional concentration of CO2 in alveoli that are both ventilated and perfused CO2 VD = dead space for CO2 ( physiological dead space)

Substituting (VT – VD ) for VA : CO2 FE x VT = FA (VT – VD ) CO2 FE x VT = (FA x VT ) – (FA x VD ) CO2 VD x FA = VT (FA -FE ) CO2

FA - FE VD = VT FA Since F = P PTotal Pa - PE VD = VT Pa CO2 CO2 CO2

Bohr Equation Pa - PE VD CO2 CO2 CO2 = VT Pa CO2

Alveolar Ventilation and PA CO2 . . VE = VA x FA CO2 CO2 . V VA CO2 FA  . CO2 Since FA x (760 – 47) = PA CO2 CO2 . V VA CO2 Then PA  . CO2

ALVEOLAR AIR EQUATION 2 PACO2 PAO2 = PIO2 - + F R PAO2 = FIO2 ( PB - PH O) -

Nitrogen Concentration % Expired volume, L