V/Q inequality and its effect on PaO2 and PaCO2

"Compare the effect on arterial blood CO2 and O2 levels of ventilation/perfusion inequalities (1997)"

• Also refer to "Venous admixture"

Ventilation/perfusion (V/Q ratio)

V/Q ratio and O2-CO2 diagram

In an erect person,

• Ventilation increase from apex to base
  => 0.24 L/min to 0.82 L/min
• Perfusion increase from apex to base
  => 0.07 L/min to 1.29 L/min

Because the increase in perfusion is greater than that of ventilation

=> V/Q decreases from apex to base

=> 3.3 to 0.63

 

[O2-CO2 diagram - Graph of V/Q (X-PAO2, Y-PACO2) 20050118(1)]

 

Arterial partial pressure

Concentration/partial pressure of any gas in steady state is determined by V/Q ratio

• As V/Q -> infinity, P_Agas -> P_Igas
• As V/Q -> 0, P_Agas -> P_vgas

At apex (higher V/Q)

• PaO2 is higher (132mmHg > 89mmHg)
• PaCO2 is lower (28mmHg < 42mmHg)

(comparing with base (lower V/Q))

 

V/Q scatter and effects on PaO2

As V/Q inequality lowers PaO2 because:

1. more blood goes through areas of low V/Q (e.g. base)
2. shape of oxygen dissociation curve

Shape of oxygen dissociation curve

Alveoli with high V/Q ratio are on the flatter part of the haemoglobin dissociation curve than alveoli with a low V/Q.

=> the increase in O2 content associated with an increase in PaO2 is LESS than
the decrease in O2 content associated with a decrease in PaO2 of the same magnitude

=> the beneficial effect of high V/Q on oxygen content is not enough to compensate for the adverse effect of low V/Q on oxygen content.

 

V/Q scatter and effects on PaCO2

PaCO2 MAY increase, but not as much as the decrease in PaO2 because:

1. more linear CO2 dissociation curve
   => the CO2 content change due to an increase in PaCO2 and a decrease in PaCO2 of the same magnitude are not so different
   => high V/Q units can compensate better
2. steeper CO2 dissociation curve
   => CO2 content changes lead to much smaller change in its partial pressure (when compared with O2)

In practice, PaCO2 may decrease because:

=> Increase V/Q scatter

=> decreased PaO2

=> compensatory hyperventilation

=> increased PaCO2 elimination

=> PaCO2 often reduced rather than increased

Increases in PaCO2 are seldom caused by V/Q scatter.

Effects of increased FIO2

If oxygenation is impaired by V/Q scatter, increases in FIO2 will cause the PaO2 to approach the normal PaO2 value for that particular FIO2.

At FIO2 of 100%, V/Q scatter has almost no effect on PaO2.

=> This is not the case with true shunt. With significant true shunt, increased FIO2 would little effect on PaO2

Examiner's comment

• Require: what happens when blood from low V/Q mix with blood with high V/Q
• Need to refer to dissociation curve for CO2 and O2
• Relative volume of blood from low V/Q and high V/Q
• Compensatory hyperventilation
• Changes in inspired O2 in V/Q mismatch.