Why does pulmonary embolism give normal C02 level?

[sameeralord]

Hello everyone,

This has bothered me for sometime now.

Ok Pulmonary embolism , now decreased perfusion into a part of lung. No blood available for oxygen from lung to diffuse into. Blood has decreased oxygen. That is fine but since no blood is reaching the lungs, the carbon dioxide should also be retained in blood, shouldn't this elevate carbon dioxide in blood. Ok I understand body now goes into relfex tachypnoea to get more oxygen in mean time, and this will also push carbon dioxide out. But how can this reduce carbon dioxide level, if the lung is not receiving carbon dioxide from blood anyway, due to decrease perfusion. Thanks :)

 

[Bran]

Hello, Sameeralord - you might find the answer to your question here, on the American Heart Association's website, in a writing from Samuel Z. Goldhaber, MD, and C. Gregory Elliott, MD, titled Acute Pulmonary Embolism: Part I, that discusses acute PE's - the section that might be most relevant to your question is titled, Other Gas Exchange Abnormalities. In this section is written the following:

"Increased dead space impairs the efficient elimination of carbon dioxide. However, medullary chemoreceptors sense any increase in arterial Pco2, and they will increase the total minute ventilation, thereby lowering the arterial Pco2 to normal and often below normal. Thus, most patients with PE present with a lower than normal arterial Pco2 and respiratory alkalosis because of an increased total minute ventilation."

I hope this helps!