louismarelise
Forum Senior
Topics: 36
Posts: 56
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Hi,
I can't understand the logic behind the equation for phys. dead space (Vd).
Vd=Vt (PaCO2-PACO2)
PaCO2
b) In this equation it also looks like Vd should be equal to Vt and that doesn't make sense to me. I hope someone has time to explain this equation to me please.
Thanks
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| cyra
Moderator
Topics: 29
Posts: 844
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The equation is Bohr Equation....you will be able to understand it better if you look at it this way...(write it down on a piece of paper...couldn't manage the horizontal div line...have to make do with the forward slash!)
Vd/Vt = PaCO2-PeCo2/PaCO2
Vd=dead space Vt=tidal volume
PaCO2=arterial PCO2 PeCO2=expiratory artial pressure of CO2
Physiologic dead space is anatomic dead space(conducting airways from mouth or nose to the terminal bronchioles) plus alveoli which are well ventilated but poorly perfused and therefore less efficient in gas exachange.Because atmospheric PCO2 is practically zero, all the CO2 expired in a breath can be assumed to come from the communicating alveoli and none from the dead space. By measuring the PCO2 in the communicating alveoli (which is the same as that in the arterial blood) and the PCO2 in the expired air, one can use the Bohr Equation to compute the "diluting," non-CO2 containing volume, the physiologic dead space.
Therefore....Vd = PaCO2 - PeCO2 and Vt = PaCO2...and if you want to figure the diluting effect out,you just divide the two...and end up with the equation...
In healthy individuals, the anatomic and physiologic dead spaces are roughly equal since all areas of the lung are well perfused. However, in disease states where portions of the lung are poorly perfused, the physiologic dead space may be considerably larger than the anatomic dead space.That is why physiologic dead space is a more clinically useful concept than is anatomic dead space.
Hope this helps!
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| ssrpk
Forum Fanatic
Topics: 154
Posts: 2,819
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nice explanation cyra, keep it up 
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| pups
Forum Junior
Topics: 3
Posts: 14
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that is an awesome explanation thanks a lott Cyra!
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| cyra
Moderator
Topics: 29
Posts: 844
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Thanks guys!
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| louismarelise
Forum Senior
Topics: 36
Posts: 56
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Thanks for going the extra mile to explain this equation to me.
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