Help me understand this respiratory concept!

[mdc1027]

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In my respiratory module, we are learning about respiratory function tests. One such test is the "Helium Dilution Technique" in which a spirometer is filled with helium to a known concentration and then a valve opened and the patient allowed to breath in the helium. After equilibration with the new volume, the helium concentration is measured again. Using the concentration volume ratio equation, C1*V1 = C2*V2, the volumes can be calculated. In the equation, V1 is the volume of the spirometer, C1 is the concentration of helium before beginning the test, C2 is the concentration after the test, and for some reason, V2 is the functional residual capacity of the lungs. I am confused as to why this volume is the FRC instead of the total lung capacity. Shouldn't the helium be able to be diluted into the volume of the whole lung and not just the FRC volume? I can't figure this out.

 

[xanthomondo]

In my respiratory module, we are learning about respiratory function tests. One such test is the "Helium Dilution Technique" in which a spirometer is filled with helium to a known concentration and then a valve opened and the patient allowed to breath in the helium. After equilibration with the new volume, the helium concentration is measured again. Using the concentration volume ratio equation, C1*V1 = C2*V2, the volumes can be calculated. In the equation, V1 is the volume of the spirometer, C1 is the concentration of helium before beginning the test, C2 is the concentration after the test, and for some reason, V2 is the functional residual capacity of the lungs. I am confused as to why this volume is the FRC instead of the total lung capacity. Shouldn't the helium be able to be diluted into the volume of the whole lung and not just the FRC volume? I can't figure this out.

Actually I think both are incorrect.

You're right about the V1, C1, and C2, but V2 is equal to the volume of the machine (V1) + the total lung capacity.

When you start breathing the helium in your lung the machine volume and your lung volume all become one big closed system.

 

[mdc1027]

...V2 is equal to the volume of the machine (V1) + the total lung capacity.

That's what I meant to say, it just didn't come out that way.

This is the argument I have presented to my professor who remains adamant that this volume is FRC, not TLC, but he can't adequately explain why (at least not in a way that I can understand).

 

[Inuranic]

The way I understand this test, the measurement is made AFTER a normal tidal volume is expired.

So:

Helium is put in the spirometer in a known amount, subject breathes it in and out for a little bit.

Helium diffuses throughout lung.

Measurement is taken after subject expires a normal tidal breath, hence, expiratory reserve volume and residual volume are measured, otherwise known as FRC.

If the measurement was taken after a maximal inspiration, then it would be TLC.

It all depends on when the measurement is taken.

 

[mdc1027]

The way I understand this test, the measurement is made AFTER a normal tidal volume is expired.

If the measurement was taken after a maximal inspiration, then it would be TLC.

It all depends on when the measurement is taken.

That clears it all up. Thanks a bunch!

 

[Downbytheicu]

If this is cleared up to you that's great, but from how I interpreted what was written, I'm not sure if the concept was explained correctly. Maybe I just misread it. Sorry if that's the case.

Essentially, at the point of system closure, i.e. after quiet expiration for FRC (or whenever you want; depending on what you want to measure) any increase in the volume of the lungs is EQUAL to a decrease in volume of the lung machine. So the total volume of the closed system stays the same.

If we have a patient exhale quietly to 3 L lung volume and plug him up to a machine with a piston of volume 4 L, then the total volume of the system is 7 L. As the patient inhales, expanding his lungs to 5 L, he got those two liters from the machine, where the volume is now 2 L. It still equals 7 L, thus there is no change in volume. After the patient inhales/exhales a bunch from the machine to reach steady-state we are still at 7 L. This allows us to use the equation you mentioned V1*C1=V2*C2 where V2 is equal to the volume of the machine + the initial volume of the lungs. Or, put another way. V2 = volume of machine + V1. That way we can solve for V1, and it can be whatever we want. But, it is not total lung capacity unless we start at maximal lung volume when we close the system.

 

[keregg228]

FRC is correct.