Factors Impacting Lung function predicted values

As you suggest trunk-to-leg ratio does impact predicted spirometry values. However, this is already incorporated in predicted values according to variation by race/ethnicity. Approximately 35-40% in the difference in FEV1 predicted values in African American vs. Caucasian adults can be explained by differences in the trunk-to-leg ratio. (1) I was not able to identify additional equations to specifically apply to your patient. However, it is certainly important to be certain that you are using appropriate predicted equations for your patient. The GLI equations published in 2012 are robust across ages and multiple races/ethnicities. (2)

There are a number of other considerations for your patient. Based upon the spirometry values you provided, the FEV1/FVC ratio is 0.73, so depending upon the age of your patient, he likely has some degree of obstruction. It would be helpful to have full lung volumes in order to assess whether he has a component of restrictive lung disease. Further, a formal exercise challenge could also provide objective evidence of EIB vs. lead you to try to identify some other underlying lung disease leading to exercise symptoms.

1. Harik-Khan RI, Fleg JL, Muller DC, et al. The effect of anthropometric and socioeconomic factors on the racial difference in lung function. Am J Respir Crit Care Med 2001;164:1647–54.
2. Quanjer, P. H., Stanojevic, S., Cole, T. J., Baur, X., Hall, G. L., Culver, B. H., . . . Initiative, E. R. S. G. L. F. (2012). Multi-ethnic reference values for spirometry for the 3-95-yr age range: the global lung function 2012 equations. Eur Respir J, 40(6), 1324-1343.

I hope you find this information helpful.
Regards,
Daniel Jackson, MD