Hypoxia

Question 1

What are the classifications of hypoxia?

Answer 1

Hypoxic hypoxia
Anaemic
Circulatory
Histotoxic

Question 2

What is the oxygen delivery equation?

Answer 2

DO2 =
CO x (Hb x SaO2 x 1.34 + (PaO2 x 0.003))

Question 3

What are the causes of hypoxic hypoxia?

Answer 3

• Low PiO2
• Hypoventilation
• Diffusion defect
• Shunt
• V/Q mismatch

Question 4

How is inspired O2 calculated?

Answer 4

PiO2 = FiO2 x (Pb - PH2O)
Approx 20kPa

Question 5

What is the alveolar gas equation?

Answer 5

PAO2 = PiO2 - PaCO2 / R

Question 6

What is Fick’s law?

Answer 6

Rate of diffusion is proportional to surface area, pressure gradient and inversely proportional to distance flow has to travel

Question 7

What is a shunt?

Answer 7

Mixed venous blood mixes with arterial blood by:
- Completely bypasses lungs
- Passes through parts of lungs not involving gas exchange

Question 8

What are the causes of shunts?

Answer 8

• Extrapulmonary: Bronchial & thebesian veins, R-L cardiac defect
• Intrapulmonary: Alveolar full (pus, tumour, oedema, blood), atelectasis

Question 9

What is the shunt equation?

Answer 9

Qs / Qt = CcO2 - CaO2 / CCO2 - CvO2

Question 10

How do you collect mixed venous blood?

Answer 10

Pulmonary vein

Question 11

What are the values for PvO2 & PvCO2 before and after in shunt?

Answer 11

PvO2: 5.3
PcCO2: 6.0
Values same before & after

Question 12

What are the values for PvO2 & PvCO2 before and after in high V/Q or dead space?

Answer 12

Before:
- PvO2: 5.3
- PvCO2: 6.0
After (same as alveolar gas)
- PvO2: 20
- PvCO2: 0

Question 13

What does shunt blood and dead space blood look like?

Answer 13

Shunt = Venous gas
Dead = Inspired gas

Question 14

How does the volume change in the dependent & non-dependent parts of the lung?

Answer 14

D: Low vols
ND: High vols

Question 15

How does the ventilation change in the dependent & non-dependent parts of the lung?

Answer 15

D: Lower ventilation
ND: Greater ventilation

Question 16

How does the perfusion change in the dependent & non-dependent parts of the lung?

Answer 16

D: Higher
ND: Less

Question 17

In High V/Q or dead space, what are the effects on end tidal & arterial CO2?

Answer 17

• Alveolar gas same as inspired gas (CO2 basically 0) so on expiration will cause EtCO2 to drop
• No perfusion so no way for CO2 out of blood so PaCO2 will increase

Question 18

What is compliance?

Answer 18

Volume change per unit change in pressure

Question 19

Is compliance normally greater at the base of the lung in upright patients?

Answer 19

Yes
At the apex of the lung the alveoli are held open as gravity pulls the lung down.
As they commence inspiration (e.g. become inflated) they are closer to their elastic limit and less compliant.