"Compare the effect on arterial blood CO2 and O2 levels of ventilation/perfusion inequalities (1997)"
In an erect person,
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)]
Concentration/partial pressure of any gas in steady state is determined by V/Q ratio
At apex (higher V/Q)
(comparing with base (lower V/Q))
As V/Q inequality lowers PaO2 because:
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.
PaCO2 MAY increase, but not as much as the decrease in PaO2 because:
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.
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
Increased age increases V/Q scatter
Anaesthesia increases V/Q scatter
* Partly from inhibiting hypoxic pulmonary vasoconstriction by inhalational agents
PEEP increases V/Q scatter
* Alveoli with high V/Q gets preferentially ventilated
About | |
Created | 20050227 |
Updated | 20050227 |