E6 - Clinical Oxygenation

Question 1

Name 5 possible causes that can lead to hypoxemia. Plus 1 bonus

Answer 1

1. Low partial pressure of inhaled oxygen
2. Hypoventilation
3. Absolute shunting
4. Relative shunting
5. Diffusion defects
   Bonus: Changes in cardiac output

Question 2

What are the 5 indices to monitor physiological shunt?

Answer 2

1. Classic shunt calculation
2. Modified shunt calculation
3. A-a gradient -> P(A-a)O2
4. PaO2/PAO2
5. P/F ratio

Question 3

What is a classic shunt equation?

Answer 3

Qs/Qt = (CcO2-CaO2)/(CcO2-CvO2)
The gold standard for measuring the efficiency of Oxygen uptake by lungs.
Takes the non-pulmonary factors into consideration
*Pulmonary artery catheter MUST be in place for this

Question 4

What is an estimated/modified shunt equation?

Answer 4

This is done when mixed venous blood is unavailable. 
Estimated C(a-v)O2: 3.5%
Qs/Qt = (CcO2-CaO2)/(CcO2-CaO2)+3.5

Question 5

What is the A-a gradient? What are the normal values?

Answer 5

Alveolar-arterial gradient. Useful to quantify the efficiency of Oxygen loading
Healthy adult: 10 mmHg up to 20 mmHg
Above 60: as high as 35 mmHg
Best calculated on Room Air

Question 6

What does a normal A-a gradient mean?

Answer 6

Hypoventilation, higher PaCO2. Caused by decreased inspired oxygen and decreased respiratory drive

Question 7

What does an increased A-a gradient mean?

Answer 7

If increased A-a with no response to supplemental O2: absolute shunting. (ex: atelectasis). Correct by increasing PEEP. (CPAP or BiPAP or VTR)
If increased A-a with significant response to supplemental O2: relative shunt OR diffusion defect (ex for relative shunt: pneumonia and pulmonary edema)
(ex for diffusion defect: COPD and pulmonary fibrosis)

Question 8

What is the 100% test?

Answer 8

Used to compare A-a on room air and then on 100% to differentiate between true capillary and relative capillary shunt.
On room air: A-a would be high. Once 100% given for 20 minutes, if A-a is still abnormally high= absolute shunt
If A-a is improving= capillary shunt
Normal on 100% is less than 50 mmHg
Higher than 50 mmHg = absolute shunt
If less than 50% after 100%= relative shunt