Why is predicted FVC calculated on height only (and not weight)?

Question

(please forgive me if this is a stupid question, as I have very little medical knowledge)

I've been found to have an unusally low vital capacity (about 65% of the predicted value) so I'm up for a series of tests. Naturally I'm worried.

However, it would seem that most methods for calculating the predicted VC take into account the age, gender and height, but not weight. Given that I have a very tall and slim build, it seems natural that I would have a lower VC if weight is not factored in.

What I mean is that most men that are as tall as me also have a wider ribcage. Many weigh as much as 20 kg more than me without being overweight (and that means they also need more oxygen).

So, is chest width meaningfully related to vital capacity? And if so, why isn't it taken into consideration when calculating predicted VC?

Answer 1

Predicted Normal Values

Many studies have published lung function reference values for a variety of race/ ethnic groups, countries, and age ranges. Populations of interest are sampled and spriometric studies were done and statistical analyis was used to determine "normal" results for each parameter of interest.

It was found that general equations could be made to describe the variation of each predicted value by e.g., age, sex, etc.

http://dynamicmt.com/dataform3

So, some calculators may use weight and you can experiment with them to see the effect if any on the predicted lung capacity. But you may find little difference indicating that weight is not a major variable, and is adequately accounted for in the standard deviation of the result.