ABG

Question 1

Discuss osmolar gap, osmolality and osmolarity

Answer 1

Osmolality is measured in a lab
Osmolarity is calculated 2xNA +glucose + urea

Osmolar gab is the differnece between the above and identifies unmeasured solutets (toxic alcohols)

Elevated osmolar gap >10

Question 2

Discuss what a normal AA gradient is and how AA gradient is calculated

Answer 2

Alveolar gas equation - PAo2= Fio2 x (Atm-47) - (1.2 x pco2)

AA gradient = difference in PAo2 and pao2

A normal A–a gradient for a young adult non-smoker breathing air, is between 5–10 mmHg.

Increase with age so reasonable estimate is (age/4) +4

Question 3

Whate are the causes of normal AA and widened AA hypoxa

Answer 3

Normal A-a gradient

• Alveolar hypoventilation (elevated PACO2)
• Low PiO2 (FiO2 < 0.21 or barometric pressure < 760 mmHg)

Raised A-a gradient

• Diffusion defect (rare)
• V/Q mismatch
• Right-to-Left shunt (intrapulmonary or cardiac)
• Increased O2 extraction (CaO2-CvO2)