Question about Dead Space

[JiPo]

Hey everyone,

I have question about dead space.

In a healthy patient, the number I read is that the dead space is about 25-30% of the tidal volume. But, when you actually calculate the ratio of dead space to tidal volume (Vd/Vt) in a healthy patient, you get a much smaller number. The formula I am using is (Vd/Vt) = (PaCO2 - EtCO2)/PaCO2.

In a healthy patient, we say the difference between End tidal CO2 and PaCO2 is about 3-5. Assuming EtCO2 is 35, and PaCO2 is 40, you get (40-35)/40, which is 12.5%.

Clearly, I am missing something. Why is the calculated value of dead space so different from the traditional number quoted in textbook?

Thanks in advance everyone.

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[ucladoc2b]

EtCO2 35 seems a bit high for a healthy person with a PaCO2 of 40. Usually it’s closer to 32. Dead space is usually around 7-8. That gets you pretty close to the correct number.

 

[Choo]

anatomic dead space is around .3; The equation you have is alveolar dead space over alveolar tidal volume which would add to the physiologic dead space. A good explanation is in Yao under hypoxia and equipment failure.

 

[vector2]

EtCO2 35 seems a bit high for a healthy person with a PaCO2 of 40. Usually it’s closer to 32. Dead space is usually around 7-8. That gets you pretty close to the correct number.

What? No. As the post above me says, the equation in the OP is for alveolar dead space, not physiologic dead space (which is anatomic + alveolar). In a healthy person, alveolar dead space is essentially <3-5 since all alveoli are presumably well-ventilated and perfused.

To the OP, physiologic dead space can be calculated with that equation but to do so you don’t use EtCO2, you use something called a mixed expired CO2 which is usually a much smaller number. You can google it to learn more.