ABG/VBG

Question 1

What is the Alveolar gas equation

Answer 1

PAO2 = FiO2 × (Patm − PH20) − PaCO2/ RQ
Barometric pressure at sea level is 760 mmHg and PH20=47
Resp Quotient is 0.8
Therefore
PAO2 = FiO2 x 713 - PaCO2/0.8

Question 2

What A-a gradient and the expected results

Answer 2

A-a gradient is difference between the alveolar PO2 (PAO2) and the measured arterial PaO2. It is a measurement of lung function. Best measure on room air.

A normal difference is between 5-10mmHg in young healthy non smoker

Increases with age and FiO2
A-a < (age/4) + 4
FiO2 0.21 - 7 in young and 14 in elderly
FiO2 1.0 - 31 in young and 56 in elderly

Question 3

What is the P/F ratio and some advantages/disadvantages

Answer 3

PaO2 = FiO2 x 500 (eg 0.21x500=105mmHg)
Therefore normal range 400-500mmHg

PaO2/FiO2 ratio quick, simple guide whether a significant A-a gradient may be present
More practical the a/A ratio as PAO2 calculations not required
Used in severity scoring (APACHE, SOFA, SMARTCOP and Berlin definition of ARDS)

Markedly dependent on FiO2 and barometric pressure
Does not account for PEEP or mean airway pressure
Dependent on tissue oxygen extraction
Can not distinguish hypoxia due to alveolar hypoventiltion

Should only be used is PaCO2 is normal and shunt not suspected

Example of use in Berlin ARDS severity (using PEEP 5)
Mild 200-300
Moderate 100-200
Severe <100

Question 4

How do you interpret the results of an A-a gradient in a patient with hypoxia

Answer 4

Normal A-a gradient
- Alveolar hypoventilation (elevated PACO2)
- Low PiO2 (FiO2<0.21 or barometric pressure<760)

Raised A-a gradient
- Diffusion defect
- V/Q mismatch
- Right to Left shunt (intrapulmonary or cardiac)
- Increased O2 extraction (CaO2-CvO2)