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Jan10-13, 09:12 AM
This is a bit long, so only read on if you have too much time on your hands...
Clinically its important to know how large someones lungs are. Asking them to breathe out as much as they can, will not tell us the lung volume, as there is always a residual volume of gas left in the lungs.
One method of measuring Lung Volumes is to ask the patient to sit in a sealed perspex cabin (a body plethysmograph). It uses Boyles law to calculate the lung volume.
At the end of a breath out, the mouth/lungs are occluded: we have a fixed ammount of gas and calculate its volume.
The patient tries to breathe in and out, there is no flow as their mouth is occluded.
Changes in lung pressure are meaured in the mouth
Changes in lung volume are meaured as a volume discplacement in the cabin (or rather the proportional pressure change).
As the patient tries to breathe in against the occluded mouthpiece, we see decreasing pressure in the lungs p1, and increasing lung volume V1
As the patient tried to breathe out against the occluded mouthpiece, we see increasing lung pressure p2 and decreasing lung loume V2.
We can now calculate the volume in the lungs when p=1atm
I hope that makes sense....
My question is, can you think of another way of measuring lung volumes without a perspex cabin, eg by applying a pressure, changing the volume etc? (there are other ways using gas dilutiuon, but I wanted a more mechanical process). If I use a syringe to push a few mls of air into the patients mouth/lungs, the pressure change will not reflect this volume as the patient will react/relax, I do not know what the patient is doing, they may push or pull, the pressure will not refelct the volume change....
Thanks in advance
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