1
Q
Discuss osmolar gap, osmolality and osmolarity
A
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
2
Q
Discuss what a normal AA gradient is and how AA gradient is calculated
A
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
3
Q
Whate are the causes of normal AA and widened AA hypoxa
A
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)
