Looking at the past, looking to the future

As New Year’s Day approaches it is a tradition for people look back to see what has happened during the last year and then look forward and guess what will happen during the next year. I’ve never done a New Year’s blog before but I’ve been mulling over a number of ideas for a while and this looks like a good place to explore them.

I’ve had the opportunity over the last several years to research the history of pulmonary function testing. There are a couple of interesting lessons from the past that may be useful, particularly when we are trying to guess what direction pulmonary function testing is heading towards in the future.

The spirometer as we know it and the measurement of the Vital Capacity began with John Hutchinson in 1846. In a sense there was really nothing new in what he did. His spirometer was a modified gasometer that had been invented by James Watt in 1790 and used by other researchers (notably Humphrey Davy who was the first person to measure the Residual Volume). The Vital Capacity had also been measured previously by many individuals. The remarkable thing that Hutchinson did however, was to present the first true population study and to clearly show the relationship between age, height and the Vital Capacity.

Measuring the Vital Capacity took off like a rocket and researchers all across Europe and the United States studied it in many different diseases and locations. An incredibly wide variety of spirometer technologies were developed as well, some of which are still in use. Over and over again researchers tried to show the value of the Vital Capacity (particularly in Tuberculosis) but the reality is that the clinical value of the Vital Capacity is quite limited. This is because when you only look at the volume of the Vital Capacity there are many reason why it can be reduced and so the finding of a reduced Vital Capacity is non-specific. The clinical use of spirometers languished for decades and the biggest use of spirometers wasn’t clinical at all, they were instead mostly used in schools, gymnasiums and penny arcades to measure lung “power”.

It wasn’t until over a hundred years after Hutchinson presented his first paper on spirometry that Tiffeneau in 1948 and Gaensler in 1950 presented their papers on the FEV1 and FEV1/FVital Capacity ratio and an entire paradigm shift occurred almost overnight. Today spirometry is mostly about flow rates and only partly about volume. It is the flow rates (Peak flow, FEV1) that are so incredibly clinically relevant and in just a few years after Tiffeneau’s and Gaensler’s papers spirometry became an invaluable and critical tool in the diagnosis and monitoring of many different lung diseases.

Why did it take so long for expiratory flow rates to be appreciated? This was in part because spirometers of the time were not designed to measure flow but only to measure volume. Water seal spirometer bells were often quite massive and the breathing tube and mouthpiece were quite narrow. Lightweight spirometer bells and wide bore tubing could have been manufactured almost any time after the late 1800’s but nobody tried because spirometers were only supposed to measure volume.

The first viable flow sensor, the Fleisch pneumotach, may have been invented in 1925 but it used a mirror and photographic paper that had to be developed in order to measure flow rates and this made it suited for only minor research. What actually brought expiratory flow rates to the forefront of pulmonary medicine was the Maximum Voluntary Ventilation test which itself was an attempt to mimic ventilation during exercise. The MVV test fostered an appreciation of spirometer frequency response and flow rates. Tiffeneau himself explained the FEV1 as a substitute for the MVV test.

So my first thought and question about the future of pulmonary function testing is: What test(s) are we performing now that need to be looked at from a completely different perspective?

There were a couple dozen companies manufacturing spirometers in the late 1800’s and early 1900’s. Have you heard of the Narragansett Machine Company, National Spirometer Company, Spalding, Simplex, Shepard, Lewis, Barnes or Brown? No? These companies made tens of thousands of spirometers but none of them survived more than a short time into the 20th century. So where did all our PFT equipment come from? It came from Basal metabolism testing, not spirometry.

The last half of the 19th century saw numerous researchers measuring exhaled CO2 production and oxygen consumption. CO2 absorbants like caustic soda (potassium hydroxide, KOH), baryatra water (barium hydroxide, BOH) and eventually, soda lime (calcium hydroxide, CaOH) were used for both purposes. Much of this research centered on metabolism and both open-circuit and closed-circuit techniques were developed and refined. Somewhere along the way, it was realized that if you filled a closed-circuit spirometer with oxygen and included a CO2 absorbant, that you could cheaply and reproducibly measure oxygen consumption by measuring the change in circuit volume over time. The science (and industry) of Basal Metabolism measurements was born.

By the 1930’s closed circuit systems intended for basal metabolism measurements were modified to measure both CO2 production and oxygen consumption (again using chemicals) and the science of spiroergometry (exercise testing) came into existence. Researchers also found that the standard closed-circuit basal metabolism equipment could be modified quite easily to measure gas dilution lung volumes and techniques for measuring FRC, TLC and RV using nitrogen, hydrogen and helium were developed. Open circuit basal metabolism systems were modified to measure lung volumes by nitrogen washout.

Around 1920 a number of companies came were formed for the purpose of manufacturing and selling Basal Metabolism systems; Collins, Sanborn, McKesson and Jones. These companies thrived and grew selling this type of equipment and were among the first companies in the 1950’s and 1960’s to design and sell test equipment for pulmonary function labs. Not all of these companies survived to the 21st century (at least by solely manufacturing PFT equipment) but they did determine to a large extent what tests were performed and how they were performed in the years when clinical pulmonary function labs came into existence.

My next thought and question is: What companies and products that aren’t being used in pulmonary function testing now will be the core of future PFT Labs?

We tend to think of medicine and technology as a linear process that progresses logically. This is only marginally true and new discoveries and paradigm shifts cannot be predicted. They may look obvious after the fact, but that just means that we think we have 20-20 hindsight.

So what do I think will be important or at least happening in the future?

I think that oscillometry (FOT and Impulse) holds promise but there needs to be a clearer understanding of what oscillometry is really measuring and what clinical relevance the measurements have.

I think that measurements of ventilation inhomogeneity (Phase III slope of the single-breath nitrogen washout and the Lung Clearance Index) hold promise for better monitoring of the status of many (not all) obstructive lung diseases. Again there needs to be a better correlation between changes in these values and clinical status and this will require longitudinal testing.

I think that cardio-pulmonary exercise testing is under-utilized but this is largely due to the complexity and expense of the test equipment and the need for multiple staff members to be present. Advances in measurement technologies and in expert systems to monitor the tests in real-time could reduce the cost of performing CPETs greatly while keeping them safe to perform as well.

I think that measurements of trace exhaled gases and exhaled breath condensates has great promise, not necessarily for pulmonary function, but for disease diagnosis and monitoring cellular metabolic processes. Notably, however, despite this being an exhaled air measurement, other than NO, these tests are not being performed by pulmonary function labs.

I think that computerized interpretation of PFT results (still) has potential and could be quite useful where it is needed the most; doctor’s offices and clinics, but that it needs to include the ability to assess test quality, testing errors and probabilities before it will ever be more than a toy, which it has been since PUFF was first written in the 1970’s.

I think that the trend towards personal health monitoring is going to mean that more people, both those in good health and those with lung disease, will be performing their own spirometry. This will actually mean that more testing will be performed in pulmonary function labs in the long run, not less, and that monitoring personal spirometry results and trends will become part of routine clinical care.

I think that economic and regulatory forces will continue drive the development of inter-hospital communication standards and that eventually a patient’s medical records will be completely transportable. This means that when you perform PFTs on a patient, their trend report will include all their PFTs, no matter when or where they were done.

Finally, I think that we tend to be overly focused on running our PFT Labs and on performing quality pulmonary function testing as best we can according to ATS/ERS standards (at least that’s what I hope everybody is doing). This is a good thing, but it also loses sight of our real purpose and that is to diagnose and monitor lung disease. How we go about fulfilling this purpose must change as technology changes, our understanding of lung physiology and disease processes change, and our ability to treat lung disease changes.

Along the way we need to keep our perspective and our humility. We can look back at the patent nostrums and medical procedures of the 19th century say, “What were they thinking? That was totally wrong. How did anybody survive that?” In the far future I suspect that anybody will be able to wave a tri-corder wand over themself and their personal medi-bot will say, “Hmmm, some of the peripheral airways and acini in your left upper lobe look a bit off. Here, inhale these nanobots I’ve just programmed to repair them. Let’s check again tomorrow to see how they’re doing”. That future person will look back at the 21st century and say “What were they thinking? That was totally wrong. How did anybody survive that?” And you know what?  They will be completely right.

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PFT Blog by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License

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