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Alveolar Air Equation

calculates the partial pressure of oxygen (oxygen tension) in the alveoli (Alveolar PO2 or PAO2)

PAO2 =

(PB - PH2O)FIO2 - (PCO2/0.8)

shortcut for PAO2 equation

(7 x FI02 as a whole #) - (PaCO2 + 10)

normal value for PAO2

varies directly with pt's FIO2 and PB

A-aDO2 (A-a Gradient)

measures the difference(D) or gradient between alveolar and arterial PO2 (best done after pt on 100% for 20 minutes)

A-aDO2 equation

A-aDO2 = PAO2 (use Alveolar Air Equation)-PaO2 (use ABG)

normal value for A-a Gradient

25- 65 mmHg

V/Q mismatch shown in A-a Gradient

66-300 mmHg

Shunting (shunt refers to perfusion without ventilation) shown in A-a Gradient

> 300 mmHg

causes of pulmonary shunting

A pulmonary shunt often occurs when the alveoli fill with fluid, causing parts of the lung to be unventilated although they are still perfused

why is increasing the FIO2 unable to help increase the PaO2 when shunting is present?

Because shunt represents areas where gas exchange does not occur

how do you improve oxygenation when their is shunting

PEEP or positive pressure therapy

how do you improve oxygenation with a V/Q mismatch

increase the FIO2

CaO2= Arterial oxygen content

measures the oxygen carried by the hemoglobin as well as that dissolved in the plasma

where is the majority of the arterial oxygen content of the blood found

dissolved in the plasma

what is the best index of oxygen transport


CaO2 =

Hb x 1.34 x SaO2 (if SaO2 and Hb in normal range, then CaO2 will be in normal range)

normal value for CaO2

17-20 vol% (mL/dL)

CvO2 = mixed venous oxygen content

monitors tissue oxygenation

CvO2 =

Hb x 1.34 x SvO2

normal value for CvO2

12-16 vol%

what is CvO2's relation to Qt

direct (Qt decreases, CvO2 decreases)

C(a-v)O2 arterial-venous oxygen content difference

measures the oxygen consumption of the tissues

C(a-v)O2 =


normal value for C9a-v)O2

4-5 vol%

C(a-v)O2 relationship to CvO2 and Qt

inverse C(a-v)O2 increases as CvO2 decreases

PaO2/FIO2 ratio or P/F ratio

used in determination of acute lung injury or acute respiratory distress syndrome

normal P/F ratio

380 mmHg or more

P/F ratio that signifies acute lung injury

<300 mmHg

P/F ratio that signifies ARDS

< 200 mmHg


VO2 / C(a-v)O2 x 10

normal Qt

4-8 L/min

Qs/Qt (shunt equation)

the portion of cardiac output that is shunted (perfusion without ventilation)

Qs/Qt =

(shortcut) for every 100 T (mmHg) of my A-a graident I add %5 shunt after starting with 5% normal physiological shunt. (if A-a gradient is 300 mmHg then Qs/Qt would be (5% x 3) +5% = 20% shunt

normal Qs/Qt



the % of hemoglobin that is bound by O2

how can actual SaO2 be measured

oximeter or co-ox (ABG only calculates)

estimating SaO2 based on PaO2

if PaO2 between 40 and 60 add 30 to get SaO2 (if PaO2 50 then SaO2 is about 80)

at what temperature are ABGs typically reported

normal body temp of 37 degrees C

if pt has fever how will blood gas values be affected

lower PaCO2, PaO2 and higher pH (higher pH=patient Has fever)

VD/VT ratio

% of tidal volume doesn't participate in gas exchange. Dead-space to tidal volume ratio-ventilation without perfusion (the life is in the blood-so dead-space represents areas of the lung that are being ventilated but not perfused)

normal value for VD/VT ratio

20-40%, up to 60% for ventilator patients

what VD/VT ratio would suggest that a vent pt. could be weaned

< 60%

Increase in VD/VT ratio would indicate what

dead-space producing disease-i.e. pulmonary embolus

VD/VT ratio=

{(PaCO2 - PECO2) / PaCO2} x 100

desired VE =

(VE x PaCO2) = (VE x PaCO2)

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