Abnormal Arterial Blood Gases

Question 1

Hypoxaemia (low PO2) caused by: (1+2)

Answer 1

1. Diffusion Limitation
   - Limitation occurs when there is disease in the alveoli or pulmonary vessels
2. Right to Left Shunts
   - Some blood always flows past alveoli that are not being ventilated.
   - Our arterial PO2 is always slightly low due to shunting

Severe shunting occurs in:

• pneumonia + pulmonary oedema: some alveoli filled with fluid
• tissue trauma: alveolar wall swelling
• mucous plugging

Question 2

Hypoxaemia (low PO2) caused by: (3+4)

Answer 2

3. Alveolar Hypoventilation

4. Low Inspired FIO2 - fraction of oxygen in the inspired gas is < 21%

Question 3

Hypoxaemia (low PO2) caused by: (5)

Answer 3

5. V/Q Mismatch (ventilaton/ perfusion (blood flow))
   - For normal gas exchange to occur, ventilation + blood flow in the various regions of the lung must be matched
   - Some V/Q mismatch is normal
   - gravity pulls more blood into the lower regions of the lung = perfusion will be higher here = not a problem as is normal part of body
   - ventilation is higher in the upper regions
   - When we get an abnormal change in V or Q = either due to excessive decrease in perfusion or in ventilation

Question 4

The lung can be divided into three zones:

Answer 4

• Upper zone: higher V, lower Q
• Middle zone: V & Q are matched
• Lower zone: lower V, higher Q

Question 5

WITH HIGH V/Q: normal V low Q (e.g. pulmonary thromboembolism)
PAO2 is high; PACO2 is low

Answer 5

• Lower ventilation to match perfusion = through constriction + stop ventilation to an area that has little blood flow + redirect to other areas that have lots of blood flow
• When blood flow to an area decreases, airway resistance increases: gas is shunted away from the area due to low PCO2

Question 6

WITH LOW V/Q: low V normal Q (e.g. pulmonary fibrosis) PACO2 is high; PAO2 is low

Answer 6

• Lower perfusion to match ventilation = through constriction + stop sending blood to an area that is not being ventilated + redirect to other areas that are being ventilated
• When local hypoxia develops, vasoconstriction occurs: blood is shunted away from the hypoxic area: this may cause right sided heart failure

Question 7

HYPOCAPNIA + HYPERCAPNIA

Answer 7

HYPOCAPNIA (LOW PCO2)
- Due to alveolar hyperventilation

HYPERCAPNIA (HIGH PCO2)
- Due to alveolar hypoventilation