### 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 CO
_{2} - 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

## 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
- PO
_{2}decreases apex → base - PCO
_{2}increases apex → base, but much less cf. PO_{2} - ∴↓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*

**Dead space**: V_{T}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 ↓O
_{2}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 & P
_{A}O_{2}are not ↑O_{2}content of blood much further, because Hb here is fully saturated - ↑portion of dissolved blood is not going to ↑CaO
_{2}much - Alveoli with low V/Q which have O
_{2}content similar to mixed venous blood can ↓CaO_{2}significantly because it is on steep part of ODC

## Effect on PaCO2

- V/Q inequality does not have such drastic effect on PaCO
_{2}because of linear shape CO_{2}dissociation curve - There is immediate ↑MV with slight ↑PaCO
_{2}so that high V/Q compensate for low V/Q in terms of CO_{2}excretion

- ↑PCO
_{2}= ↑MV in linear fashion **Slope**of ↑depends on P_{A}O_{2}*(alveolar)*- ↓P
_{A}O_{2}= ventilation & slope of the line is**steeper**

- Author: Krisoula Zahariou