F6iii / 17B06: Explain the effect of VQ mismatch on O2 transfer & CO2 elimination

17B06: Exam Report

Describe the effects of Ventilation/Perfusion (V/Q) inequality on the partial pressure of oxygen (PaO2) in arterial blood.

48% of candidates passed this question.

Overall answers lacked sufficient detail on a core area of respiratory physiology. Answers expected included a description of V/Q ratios throughout the lungs and an explanation of how V/Q inequality lowers PaO2.

F6iii 17B06: Explain the effect of V/Q mismatch on O2 transfer & CO2 elimination

V/Q Importance

  • 1° function of lung → oxygenate blood & remove CO2
  • V/Q ratio determines partial pressure of gases in lungs
  • Alveolar ventilation = 4L/min
  • Pulmonary BF = 5L/min
  • ∴if uniform, every alveolus would have an ideal ratio 0.8 → not the case!

Regional V/Q Differences

  • In upright lung
  • Ventilation increases apex → base due to ↑compliance
  • Perfusion increases apex → base due to ↑hydrostatic pressure
  • But not uniformly, perfusion ↑↑↑much more
Ventilation Perfusion

V

Q

V/Q

PaO2

PaCO2

Apex

V

0.24

Q

0.07

V/Q

3.3

PaO2

132

PaCO2

28

Base

V

0.82

Q

1.29

V/Q

0.67

PaO2

89

PaCO2

42

Therefore:

  • High V/Q apex, low V/Q base
  • PO2 decreases apex → base
  • PCO2 increases apex → base, but much less cf. PO2
  • ∴↓pH apex → base
  • Volume less in apex

Definitions

  • V/Q scatter: not all alveoli have the same V/Q ratio. V/Q scatter is responsible for the natural A – a gradient.

NB: less V/Q scatter supine because the vertical gradient is much less

Scatter
  • Dead space: VT that does not partake in gas exchange
    • V/Q = ∞
  • Shunt = blood that enters the arterial circulation without passing through ventilated lung
    • V/Q = 0

Effect on PaO2

  • V/Q inequality ↓O2 content of arterial blood
  • Alveoli @ base have greater blood flow
  • The high V/Q units at Apex cannot compensate
  • ODC is sigmoid shaped
  • So alveoli with high V/Q & PAO2 are not ↑O2 content of blood much further, because Hb here is fully saturated
  • ↑portion of dissolved blood is not going to ↑CaO2 much
  • Alveoli with low V/Q which have O2 content similar to mixed venous blood can ↓CaO2 significantly because it is on steep part of ODC
Saturation

Effect on PaCO2

  • V/Q inequality does not have such drastic effect on PaCO2 because of linear shape CO2 dissociation curve
  • There is immediate ↑MV with slight ↑PaCO2 so that high V/Q compensate for low V/Q in terms of CO2 excretion
  • ↑PCO2 = ↑MV in linear fashion
  • Slope of ↑depends on PAO2 (alveolar)
  • ↓PAO2 = ventilation & slope of the line is steeper
Slope