Viva F6(iii)

What is the V/Q ratio?

• V is ventilation and Q is blood flow.
• V/Q matching refers to the coordination of blood flow and gas flow into a lung unit.
• A V/Q ratio of 1.0 is perfectly matched 
• Theoretically possible V/Q ratios span from zero (at the collapsed bases where V̇ = 0) to infinity (in the completely anaemic apices where Q̇ = 0).
• This spread of V/Q values is described as "V/Q scatter". 
• If this lung is relatively healthy, there should be minimal scatter and the bulk of the lung will have near-ideal matching (close to 1.0)
• The typical V̇ would usually be about 4L/min of air, and the typical Q̇ would be about 5L/min of blood. As such, the "global V/Q ratio" would be about 0.8, if the results from the whole lung were averaged.

How would you measure V/Q matching?

• Multiple Inert Gas Elimination Technique (MIGET)
  • Several gases are prepared, which have different blood solubility
  • Saline or dextrose with these dissolved gases in it is infused into the subject.
  • Gas levels in the arterial  blood are measured
  • Gas levels in the mixed venous blood are also measured
  • For each gas, the ratio of arterial to mixed venous partial pressure can therefore be determined.
    • This ratio falls as V/Q rises (i.e. as perfusion decreases, more inert gas gets left in the blood)  
• Three-compartment model (Riley method)
  • From  alveolar O₂, alveolar CO₂, arterial O₂ and arterial CO₂ it is possible to determine:
    • The magnitude of shunt (as a fraction of cardiac output)
    • The magnitude of dead space (as a fraction of tidal volume)
• Imaging
  • MRi
  • SPECT
  • PET

How do V/Q ratios vary across the lung?

• The upright lung has a V/Q gradient from top to bottom:
  • The lung bases have a low V/Q ratio (~ 0.6)
  • V/Q ratio reaches 1.0 at around the 3rd rib
  • Lung apices have a high V/Q ratio (~ 3.0)

How does low V/Q ratio affect gas exchange?

• Oxygen content is decreased
• CO₂ content is increased
• The lower the V/Q ratio, the closer the effluent blood composition gets to mixed venous blood, i.e. to "true" shunt.

How does high V/Q ratio affect gas exchange?

• Oxygen content is increased
• CO₂ content is decreased
• The higher the V/Q ratio, the closer the effluent blood composition gets to alveolar gas.

What is the difference in the effect of V/Q matching on O₂ and CO₂ exchange? 

• The relationship between PaO₂ and V/Q is steeper and more sigmoid than the relationship between  PaCO₂ and V/Q.
• A small change in V/Q ratio can produce a large change in PaO₂, whereas a relatively large change in V/Q ratio is required to change the PaCO₂

• The greatest useful improvement in gas exchange occurs in the V/Q range of 0.1-1.0
• This is the range generally encountered in the bases of lungs.
• Here, a small change in V/Q yields a substantial improvement in oxygenation.
• Comparatively, improving the V/Q ratio of the severely under-ventilated regions from crap (0.01) to slightly less crap (0.1) does little to improve the efficiency of the gas exchange.  
• For CO₂ clearance, the relationship is flatter and more linear, which means that a tenfold change in V/Q ratio from 0.1 to 1.0 produces a relatively modest change in the CO₂ of effluent blood.

What is the effect of low V/Q ratio on O₂ exchange?

• Well, it gets worse.
• The greatest change is in the V/Q range of 0.1-1.0
•  Here, a small decrease in V/Q significantly degrades the oxygen content of effluent blood

What is the difference between a low V/Q and shunt?

• Increasing the FiO₂ will reverse the hypoxia when it is due to a low V/Q ratio (V/Q scatter), but not when it is due to shunt.
• "True" shunt where V/Q = 0 does not improve with increased FiO₂

What is the effect of low V/Q ratio on CO₂ clearance?

• Well, it gets worse, but not by much.

• In the V/Q range of 0.1-1.0, the CO₂ clearance decreases trivially.
• Decreasing the V/Q ratio does not significantly affect CO₂ clearance, until a massive amount of "true" shunt has developed.
• Because CO₂ and the control of ventilation are so closely linked, it is quite difficult to develop hypercapnia purely as the consequence of shunt and V/Q mismatch. 

What is the effect of high V/Q ratio on gas exchange?

• Gas exchange will occur according to the laws of diffusion, which means arterial oxygen and CO₂ will trend to equilibrate with alveolar
• Therefore effluent blood closely resembles alveolar gas in its dissolved gas composition
• What little blood flows through these regions is therefore maximally ventilated
• Increasing the ventilation of these lung regions, therefore, has minimal effect on increasing the gas exchange
• Increasing the FiO₂ delivered to these units will also not be fruitful, as their oxygenation is usually already maximal on room air (i.e. all the haemoglobin is oxygenated)

How much do high V/Q lung units contribute to gas exchange?

• Their total contribution to the gas exchange of the whole respiratory system is minimal, because blood flow to these segments is also minimal
• A rabbit study by Lamm et al (1995) demonstrated that overall, only about 15% of the cardiac output goes to the "Zone 1" segments of the lung, with most of the ventilation there being in lung units with a V/Q ratio around 5.
• Low V/Q units are responsible for most (85%) of the blood flow returning to the left atrium.
• Thus, high V/Q units cannot compensate for the failure of low V/Q units

How do V/Q relationships differ in different lung pathologies?

• Prompt: tell me about asthma, COPD, pneumonia, heart failure, etc.
• COPD, PE, asthma: There are a large number of high V/Q units, which means a large minute volume but poor gas exchange as all that ventilation is wasted on lung regions with a high V/Q.
• Pulmonary oedema, pneumonia, atelectasis:  large number of high V/Q units, as well as true shunt.