Spirometry: quantification of the shape of the maximal expiratory flow-volume curve

The relationship between Forced Expiratory Volume in one second (FEV1) and the Forced Vital Capacity (FVC) is intensively used in diagnosing because of its quantitative description with the index FEV1/FVC. There is not such index for the relationship between airflows (Peak Expiratory Flow (PEF) and Maximal Expiratory Flow at xx% of FVC (MEFxx, where xx = 75, 50, 25%)). Our aim was to describe that relationship quantitatively with the angle alfa between the 4-dimensional vector F = [PEF, MEF75, MEF50, MEF25] and the plane determined by two reference vectors: the mean of such vectors for the young and elderly. The reference vectors were found with data for 1,120 males and 1,625 females - Polish (white) population, healthy, never-smoking, aged 18 - 85 yrs, who performed a technically adequate spirometry maneuver. Upper Limit of Normal (ULN) was determined with the linear regression of the 5th percentiles of alfa on age (0.074*age + 2.76). Such an ULN seems to well indicate both improper examination and non-healthy persons.