In 1844 John Hutchinson published his first paper describing his spirometer and his research on the Vital Capacity. He was the first person to use the word “spirometer” to describe his instrument and the first to use the term “vital capacity” to designate the maximum amount of air an individual can exhale after a maximal inhalation. Although he is remembered as the inventor of the spirometer, he was not the first person to use a gasometer to measure lung volumes nor was he the first to measure the vital capacity. What made his research different from those that came before him was partly the prodigious number of individuals whose vital capacity he measured but far more importantly that he was able to show a clear relationship between standing height, age and vital capacity which had not been previously apparent. This finding galvanized researchers in England, Europe and the United States and in many ways helped set the course of research into lung function for many decades to come.
This clear relationship between standing height and vital capacity has been taken as scientific fact since that time despite inconsistencies not only in Hutchinson’s data but in almost all population studies since that time. The problem is that the relationship between standing height and vital capacity is not precise but only approximate. In order to explain the range of results that appeared in his data Hutchinson and other researchers of his time divided their study population into groups by their occupation. This approach may appear to be quaint to us now but at the time they were very serious both about the utility of doing this and what it told them about the different classes of society.
The first studies on vital capacity that divided the population by race were done in the United States. The reasons that this was done are both simple and complex, and overall there’s not a lot we can look back and be proud of. At that time there was an overwhelming societal concern with the races in general and not only the recently freed black slaves and the Amerindians but also about the different “races” of Europe that were emigrating to the United States. There was much public talk and private thought about the concepts of racial degeneracy, racial mongrelization and racial vitality, and unfortunately the vital capacity was taken as a way of measuring these things. Despite incredibly significant errors in both the methods and conclusions of these studies this approach spread to Europe during the second half of the 19th century and dividing study populations by race has become standard practice ever since.
When I first started doing pulmonary function testing I was taught to decrease the predicted vital capacity by 15% for Blacks and 10% for Asians. Decades later ethnicity-based population studies replaced these fractions. I always took this as the correct way to approach predicted values (and it is embedded in the ATS/ERS standards) but at the same time I’ve always had patients where it was either difficult to assign ethnicity or where their results significantly exceeded their ethnicity-based reference values. Over the last several years I have had the opportunity to study the issues surrounding reference equations extensively and I have become somewhat disenchanted with the notion of ethnicity-based reference equations.