Ventilation Calculations

Vd/Vt is the only calculation that specifically relates to ventilation all other relate to oxygenation or circulation and perfusion
A percentage of the tidal volume tha is unavailable for gas exchange
acceptable range is (20-40%)

Vd/Vt=(Paco2-PECO2) x 100/ PaCo2

Acceptable up to 60% on a ventilator
If high usually relates to pulmonary emolus
PaCo2 value comes from blood gas
PeCo2 value comes from and end-tidal CO2 monitor or from a device called a Douglas Bag

PAO2=Pb-Ph20)FIO2)-PaCo2/0.8

A-aDO2=PAO2-PaO2

Must have the Alveolar oxygen equation solved first, unless given
Acceptable value is (25-65mmHg)
Above 65 but less than 300mmHg=V/Q mismatch
Above 300=shunt (also called venus admixture)
Best to get value on 100% O2
Improve by any therapy that improves alveolar space or recrietment, ie hyperinflation therapy, incentive spirometry

CaO2=(Hbx1.34xSaO2)+(PaO2x.003)
SHORTCUT
CaO2=(Hbx1.34)

Acceptable value is 17-20 vol%
has best relationship to tissues oxygenation

CvO2=(Hbx1.34xSvO2)+(PvO2x.003)
SHORTCUT
CvO2=(Hbx1.34xSvO2

Should be drown from the pulmonary artery
Decreasing values relate to decreasing cardiac output
Acceptable value (14-16 vol%)

C(a-v)O2=CaO2-CvO2

Directly measure how much oxygen is being consumed by tissues
Value increases as C.O. decrease(inverse relationship)
Acceptable value (4-5 vol%)

P/F ratio=PaO2/FIO2
Normal is 380 or greater
300 or less signifies Acute Lung Injury(ALI)
200 or less signifies ARDS Adult Respiratory distress Syndrome.

Qs/Qt=A-aDO2)x.003/((A-aDO2)x00.3)+C(a-v)O2
SHORTCUT
%shunt=take first digit of A-aDo2 add +1 and multiy by 5

Only works with PaO2 values between 40-60torr
Estimated SaO2=PaO2+30
If there is a large difference between measured and calculated SaO2 then it may be because of carbon nomoxide(CO)poisoning.