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.
The distance attained during a 6-minute walk (6MWD) has been used extensively to assess the functional capacity of patients with a variety of diseases and conditions. It is relatively easy to perform and requires a minimum of equipment. Changes in 6MWD before and after rehabilitation, surgery, or medications are often used to signal the success or failure of these therapies. For the last dozen years every drug and device research study my lab has been involved with has used the 6MWD as one of their outcomes.
The 6-minute walk distance has been noted to depend on the age, gender, height and weight of the individual. There are, however, relatively few studies to choose from when it comes to selecting normal values for the 6MWD and each of these studies differs not only in the degree of importance it assigns to these variables but in the predicted 6MWD. Despite the clinical significance of changes in the 6-minute walk distance it is far from clear what a normal 6-minute walk distance actually is.
Although the authors of the Global Lung Function Initiative (GLFI) study acknowledge the effect of height on their reference equations the range and distribution of heights in its study populations was not included in the report. This was a similar problem for the NHANESIII reference equations since the height range was never reported within the text of the original report however it did include scatter graphs showing the range of heights. These graphs imply the height range was 162 to 194 cm (64” to 76”) for caucasian males and 145 to 180 cm (57” to 71”) for caucasian females. Using the extremes of these height ranges it is interesting to see how the GLFI reference equations compare to the NHANESIII reference equations.
The PFT Lab I am associated with has been making a point of having the technicians re-measure patient height with each visit. Part of the reason for this is that several years ago the medical assistants in the pulmonary outpatient clinic were tasked with obtaining patient heights, weights and blood pressures. For a period of time the technicians used these heights when entering the patient demographic information but it was soon noticed that patient heights often changed by several inches from visit to visit. For this reason we have asked the technicians to re-measure patient height instead.
One possible cause for the fluctuation in heights was that the medical assistants were measuring patient height using the height rod attached to the scale while also taking their weight. The PFT Lab has wall-mounted stadiometers in or near all of the lab’s testing rooms so that patient height can be taken with their back against the wall rather than free-standing.
Another reason to regularly re-measure patient height is that the lab’s population has a significant number of patients that have routinely been seen by the lab and the pulmonary physicians for years. The lab’s patient and results database now goes back over twenty years and patients that were seen 15 and 20 years ago have been referred again for pulmonary function testing and their height has changed in the meantime.
The PFT Lab I work with has recently gone through a major software and hardware upgrade. As part of this process we made the decision to switch our spirometry predicted equations to NHANESIII. The lab has been using the Morris predicteds for at least the last 25 years and this switch has led us to re-visit some of the issues involved in interpreting spirometry results.
More than one person that I’ve known and respected has said that spirometry is all about FEV1 and I think this is a true statement. There is a lot of other information you can get from a Forced Vital Capacity but it always comes back to FEV1.
Stepping aside from the mechanical and patient issues involved in obtaining an FEV1, once you have an acceptable FEV1 measurement how do you assess it? There is always the percent predicted and the lower limit of normal (LLN) but a reduced or normal FEV1 by itself cannot differentiate between an obstructive, restrictive or normal pattern. This is where the FEV1/FVC ratio comes in and an interesting question is where the predicted values for this ratio come from.