Central and Peripheral Chemoreceptors
The central chemo receptors monitor carbon dioxide levels in the body. When those carbon dioxide levels are high a signal is sent to speed up the drive to breathe to blow off the excess carbon dioxide. So the levels of carbon dioxide dictate how fast we will breathe.
The peripheral chemo receptors are sensitive to the levels of oxygen in the body. They will send a signal to breathe when the partial pressure of oxygen begins to fall. This is referred to as the hypoxic drive but this drive has a much more minor role in breathing.
So increased oxygen slows down respiration right?……
COPD patients tend to have chronically elevated levels of carbon dioxide due to the nature of their illness. The theory goes then that because of this chronically elevated level of carbon dioxide in the chemo receptors become tolerant of these high levels and therefore the carbon dioxide ceases to be that person’s drive to breathe. What therefore drives them to breathe is the hypoxic drive, or the lower levels of oxygen.
The hypoxic drive theory then goes on to say that if you give these patients too much oxygen you blunt their hypoxic drive. As their chemo receptors are already tolerant of high levels of carbon dioxide, and therefore they have also lost that drive, their respirations will begin to slow causing a further rise in carbon dioxide levels, and a consequent acidosis.
Aubier and colleagues demonstrated that reduction in respiratory drive is not a major contributor to oxygen induced hypercapnia in patients with acute exacerbation of COPD.
….NO…its more likely the Haldane effect…..
The Haldane effect describes the property of haemoglobin in the blood whereby the deoxygenation of the blood will increase its ability to carry carbon dioxide. Conversely oxygenated blood has a reduced capacity for carbon dioxide.
So if you increase oxygen in the blood by giving them supplemental oxygen then via the Haldane effect carbon dioxide molecules will be displaced in favour of the oxygen.
This then causes an increase of carbon dioxide in the blood or a rising PaCO2. This rise in carbon dioxide is normally excreted through increased minute ventilation however those with chronic COPD cannot increase their minute ventilation.
In the Aubier study above the Haldane effect accounted for 25% of the increase in PaCO2
