see also: [[Ventilator strategies]]
> [!references]-
> - Nunn's Applied Respiratory Physiology Chpt 7
> - West's Resiraptory Physiology Chpt 5
> - [Airway Jedi - VQ mismatch](https://airwayjedi.com/2017/01/06/ventilation-perfusion-mismatch/)
> - [deranged physiology - dead space](https://derangedphysiology.com/main/cicm-primary-exam/respiratory-system/Chapter-074/dead-space-and-its-components)
>[!key points]
> - COPD is classic disease of V/Q mismatch
> - PE is classic disease of dead space
> - Severe pneumonia is classical disease of shunt
Recall that there are four causes of [[Blood gas#2. oxygenation (A-a gradient)|↑ A-a gradient]] hypoxia:
1. V/Q mismatch
2. Right-left shunt
3. Increased O2 extraction
4. Diffusion defect (rare)
And note that normal A-a gradient hypoxia is usually caused by hypoventilation.
| | A-a gradient | Response to O2 |
| --------------- | ------------ | -------------- |
| Hypoventilation | normal | decent |
| Shunt | ↑ | Small |
| V/Q mismatch | ↑ | Good |
What has always confused me is that phonetically, a "mismatch" in ventilation and perfusion always strikes me as being consistent with the problem in shunt: low ventilation and high perfusion. Indeed, when I was in medical school, I understood V/Q mismatch to be due to either dead space or shunt. From the best I can glean, V/Q mismatch is essentially a synonym for dead space effect (PE rarely becomes hypercapneic because they ventilate off the CO2).
## Definitions
| Phrase | Meaning | Causes |
| ------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Shunt | Blood passes through the lungs without being oxygenated, either because alveoli not ventilated, or there is a direct bypass eg a cardiac shunt. | - atelectasis<br>- VSD<br>- [[Pneumonia]]<br>- [[Pulmonary oedema\|APO]]<br>- mucous plugging<br>- pulm AV fistula |
| Dead Space | - Fraction of tidal volume not participating in gas exchange<br>- ==Ventilation occurs, but no blood flow available for gas exchange==<br>- Composed of apparatus dead space (eg from ventilator circuit) and **physiological dead space**, which is made up of *alveolar dead space* from gas filling lungs that are poorly perfused and *anatomical dead space* in the conducting airways. | - [[Pulmonary Embolism\|PE]]<br>- haemorrhagic shock (↓ blood flow to lungs)<br>- cardiac failure<br>- emphysema (enlarged alveoli with less surface area and fewer capillaries) |
| V/Q mismatch | Occurs when there is an imbalance between ventilation (V) and perfusion (Q) in different areas of the lungs.<br>- in any lung unit, the PO2 and PCO2 is determined by the ratio of ventilation to blood flow.<br>- **low V/Q** ratio is a shunt-like effect<br>- **high V/Q** ratio is a dead space effect<br>- high VQ inequality can lead to CO2 retention, although many COPDers compensate with ventilation (they are still hypoxic because CO2 diffuses better than O2, so the increase in ventilation does more to offload CO2 than to oxygenate) | - [[COPD]]<br>- [[Pneumonia]] |
## V/Q Mismatch
- A term that encompasses both [[#Shunt|Shunts]] and [[#Dead Space]]
- Represents disparity between ventilation and perfusion ; ==generally the term is invoked when there is a decrease in ventilation compared to perfusion, but not a complete shunt OR a decrease in perfusion compared to ventilation but not abject dead space==
- Over the lung, the balance between ventilation and perfusion is unbalanced
- eg: in COPD, some alveoli are over-ventilated but under-perfused, while others may be well-perfused but poorly ventilated, leading to a V/Q mismatch
## Shunt
*Shunt* refers to blood that enters the arterial system without going through ventilated areas of the lung.
- Physiological shunt is a reason why there is a normal A-a gradient of 5-10 mmHg between the alveolar PO2 and arterial PO2 (or less than \[age in years/4] + 4 ).
- O2-depleted bronchial artery blood is collected by pulm veins after it perfuses the bronchi, bypassing alveoli. Other source is coronary venous blood that drains directly into LV through thebesian veins.
- Some pts have an abnormal vascular connection between small pulm artery and vein (pulm AV malformation)
- VSD is another cause of right-to-left shunting
> An important feature of a shunt is that the hypoxemia cannot be abolished by giving 100% O2.
> - This is because the shunted blood that bypasses ventilating alveoli is never exposed to the higher alveolar PO2, so it continues ot depress the arterial PO2.
> When 100% O2 is inspired, the arterial PO2 does not rise to the expected level
## Dead Space
***Anatomical dead space***
- Anatomical dead space is the airway volume with **ventilation and no blood flow**.
- the conducting airways take **no part in gas exchange**
- volume approximately 150mL
Anatomical dead space is determined by the morphology of the airways and the lung.
***Physiological dead space***
In contrast, **physiological dead space** is the volume of airways and lung that does not eliminate CO2.
- in normal people, **volumes are almost the same**, but in many diseases, **physiological dead space increased** due to inequality of blood lfow and ventilation of the lung (VQ mismatch).
![[Pasted image 20250507220249.png]]
> [!caption] Components of a single breath of expired gas. The rectangle is an idealized representation of a single expired breath. The physiological dead space equals the sum of the anatomical and alveolar dead spaces. The alveolar dead space does not equal the volume of unperfused spaces at alveolar level but only the part of their contents that is exhaled. This varies with tidal volume.
## Dead space vs shunt
![[Pasted image 20250507212429.png]]
## Hypoxic pulmonary vasoconstriction
- Regional hypoxic pulmonary vasoconstriction is beneficial as a way of diverting the pulmonary blood flow away from regions in which the oxygen partial pressure is low and is an important factor in the optimisation of ventilation/ perfusion relationships
- is also important in the foetus to minimise perfusion of the unventilated lung.
- long-term continuous or intermittent HPV leads to remodelling of the pulmonary vasculature and pulmonary hypertension, and this response is disadvantageous in a range of clinical conditions