What is the difference between vital capacity forced vital capacity?

[cooldude89]

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What is the difference between vital capacity forced vital capacity?

 

[Phloston]

What is the difference between vital capacity forced vital capacity?

VC = IRV + TV + ERV; it exists whether person tries to demonstrate it or not (i.e. measurable, but not measured)

FVC = that which the person actively demonstrates (i.e. measured)

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That's just my assumption.

 

[mayn]

The only thing that I can think of, besides what Phloston has said, is generally FVC is a little less than true VC. This is because during forced expiration the enormous positive pressure inside the chest compresses the bronchioles a little, leading to less than absolute maximum expulsion of air.

 

[Phloston]

The only thing that I can think of, besides what Phloston has said, is generally FVC is a little less than true VC. This is because during forced expiration the enormous positive pressure inside the chest compresses the bronchioles a little, leading to less than absolute maximum expulsion of air.

Are you sure about that? Although, yes, you'd be increasing intrathoracic and -abdominal pressure during FVC, I wouldn't think the pressures are great enough to induce early-dynamic small airway closure. I would think that's only in pathologic states.

 

[mayn]

Yeah I definitely remember my physio prof saying that, just double checked the class slides too. However, I think he might've just been trying to lay down a principle, and point out a theoretical difference. I doubt it would have much practical implications.

Kind of like using creatinine clinically to estimate GFR, though its bound to be a little over-estimate (compared to inulin).

 

[cooldude89]

so on the step exam can we consider them as the same value? 4.8 liters is the volume of VC is 4.8 liters also the volume of FVC?

 

[mayn]

so on the step exam can we consider them as the same value? 4.8 liters is the volume of VC is 4.8 liters also the volume of FVC?

I would assume FVC = VC, for all practical purposes.

Unless it was a really tricky question trying to specifically test that concept of excessive force used in forcefully expiring leading to some amount of air remaining in the lung. The only time I saw a question on this though was on my physio respiratory block exam, by my prof lol.

 

[tk0102]

Are you sure about that? Although, yes, you'd be increasing intrathoracic and -abdominal pressure during FVC, I wouldn't think the pressures are great enough to induce early-dynamic small airway closure. I would think that's only in pathologic states.

That's right. FVC is significantly different from VC (or "SVC''--slow vital capacity--as it's sometimes called) only in pathological states. Very likely due to dynamic airway obstruction.

http://www.ncbi.nlm.nih.gov/pubmed/17890480
http://www.ncbi.nlm.nih.gov/pubmed/9669830

 

[WarriorMD]

Nevermind - brain freeeze 🙁