Describe the effects of V/Q inequality on the partial pressure of oxygen (PaO2) and carbon dioxide (PaCO2) in arterial blood.
Very few candidates demonstrated understanding of this core topic. Candidates did not
accurately define V/Q inequality and the physiological factors causing this phenomenon. V/Q
scatter as well as true shunt (V/Q=0) and dead space (V/Q=∞) needed to be considered. The
inability of high V/Q areas to compensate for low V/Q zones owing to the relatively small
contribution of blood flow from these high V/Q units was not discussed. The differential effect of
FiO2 on true shunt versus V/Q scatter was seldom explained. The shape of the oxy-Hb
dissociation curve and CO2-dissociation curve were sometimes mentioned but their effect on
arterial gas tensions not well explained.
Often graphs were reproduced inaccurately and contradictory statements made, leaving the
impression that candidates did not understand the basic concepts. It is core knowledge for Intensive Care Specialists managing respiratory failure. A sophisticated knowledge based on the
chapter in Nunn is a minimum standard expected for this topic
In case anybody cares, the chapter in Nunn's being referred to is Chapter 7 (p.109 of the 8th edition).
- V/Q inequality:
- Any ratio of ventilation (V) and perfusion (Q) which is not "ideal", i.e. 1.0
- V/Q ratios throughout the lung are "scattered" between 0 and ∞ :
- The upright lung has a V/Q gradient from top to bottom:
- V/Q = ∞ : "dead space", where there is no perfusion
- V/Q > 1.0 : lung apices
- V/Q = 1.0 : ideal V/Q ratio: midzones (around the 3rd rib)
V/Q < 1.0 : lung bases
- V/Q = 0 : or "true" shunt - collapsed regions of lung
- The range of V/Q ratios in a normal healthy young person's upright lung is around 0.6-3.0
- This gradient virtually disappears when the subject is supine and the lung is horizontal.
- The effect of changing V/Q ratio on gas exchange:
- The lower the V/Q ratio, the closer the effluent blood composition gets to mixed venous blood, i.e. to "true" shunt.
- The higher the V/Q ratio, the closer the effluent blood composition gets to alveolar gas.
- The relationship between PaO2 and V/Q is steeper and more sigmoid than the relationship between PaCO2 and V/Q.
- The effect of low V/Q ratio on oxygenation:
- Low V/Q values (V/Q ratios between 0 and 1) result in hypoxia
- The hypoxia due to low V/Q ratio is reversible with increased FiO2
- "True" shunt where V/Q = 0 does not improve with increased FiO2
- The effect of low V/Q ratio on CO2 removal:
- The same change in V/Q (from 1.0 to 0.1) has a significant effect on oxygenation, but a minimal effect on CO2 removal
- This is because the relationship of CO2 clearance to V/Q ratio is more flat and linear than the relationship of O2 uptake
- High V/Q ratio units have excellent gas exchange but minimal blood flow
- Only about 15% of the cardiac output circulates through lung units with a V/Q ratio of 5 and above
- Therefore, these units cannot contribute enough oxygenated blood to compensate for the poor gas exchange occurring in low V/Q units