British Lung Foundation
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Fev1 ?

Hi All hope you are all ok. I have a question regarding Fev1. It is calculated on what is predicted, although what is that. For instance if your lung capacity is supposed to be 4 liters would Fev 1 be an amount lower than that. or are you expected to empty your lungs in 1 second?


9 Replies

We never empty our lungs David. They would collapse if that happened. FEV1 is a measure of the volume of air you can expel in one second so it would always be less than the overall capacity, and the FVC - forced vital capacity - which measures the entire blowing out, when they say 'keep going, keep going', gives a nearer measure of your lungs' capacity. Someone will probably come on and give a better explanation though :)


thank you that was my thinking although was interested to know


It should be 70% of your full capacity


So there seems to be a difference here - but probably not an exact science.


Simply put your lungs are measured against the 'average' lung performance by healthy people of a similar age, height, gender and ethnicity. The FEV1 is a measurement of forcibly expelled breath within the first second of exhaling. 'Normal', healthy lungs would expel at least 80% of the lungs vital capacity in that first second.


Hidden Eh? It's a riddle isn't it? :) So, a 20% filled cup is empty?


Not quite right Hidden - see my explanation above. And a few members on here have lived and even worked with a FEV1 of less than 20% for years and are still going strong. :)


Thanks everyone


Fev1 (Forced Expiry Volume) is the volume of air that you can blow out in 1 second.

FVC (Forced vital capacity) is the total volume of air that can forcibly be blown out after full inspiration in however long, but usually stopped at 12 seconds.

Normal = should be able to blow 70% ( arguably 80% dependant on your country's Health Dept view) ) of your total capacity (FVC) in 1 second.

Fev 1 is measured in litres, so 2 or 3 lires, or thereabout. If big number like 25, 40, 60, 80 etc that will be % of predicted - that which expected from 'normal lungs' of a person your height, age & gender.

Men have significantly bigger lungs, and so different norms, than women. So:

Predicted numbers is an average of normals of your gender, that include big people with small lungs and small people with big lungs.

So taken by itself Fev1 could look 'obstucted' because your blow is less than average... but you could have smaller than average lungs thus actually be normal..

Someone else could blow 90% predicted but have bigger than average lungs and actually be obstructed.

So they need to take into account the size of your own lungs hence the ratio of blow(FEV1) to your volume (FVC) where Fev1 divided by FVC (FEV1/FVC) = 75%, arguably, is bottom of the normal range, or top of the obstructed.. depending on age, symptoms and depending on history, (ex)smokers with symptoms very likely to be deemed obstructed.

There is another measure that tells where the obstruction occurs. I fail on the last quarter of the FVC blow (FEF 25-75) which indicates small airway obstruction. See link


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