7th February 2024

Effect of PaCO2 and PaO2 on ventilation

PaO2

This can be represented by the graph below.

First, look at the line with normocapnia. Minute ventilation (MV) is unchanged when PaO2 is above 8kPa. When PaO2 falls below 8kPa, there is a steep rise in MV.

Now, compare the lines between normocapnia and hypercapnia. A rise in PaCO2 will also stimulate the chemoreceptors (both central and peripheral). Hence, the line is shifted rightwards and upwards, representing higher MV for every PaO2.

PaCO2

This can again be represented in the graph below:

First, consider the left curve. MV will remain relatively the same below PaCO2 of 5kPa. With PaCO2 above 5kPa, there is a linear rise in MV up to approximately 11-12kPa. Above 11-12kPa, MV starts falling off due to hypercapnic respiratory depression.

A rightward shift of the line can occur in a few situations (i.e., use of opioids – blunting response to rise in PaCO2, COPD – chronic hypercapnia)