This idea originates, as far as I know, from Michael Sims, president and CEO of NspireHealth. I got it second hand and suspect that it is a small part of a larger presentation but this one point is worth discussing by itself. Specifically, we call the places we work “Pulmonary Function Laboratories” and this is at best an outdated and somewhat obscure term that doesn’t do much to make it clear what we do.
What’s wrong with calling it a Pulmonary Function Lab?
Well “Pulmonary” is okay since it’s a rather dignified and erudite term for the part of the body we’re primarily concerned with. “Function” however, is a somewhat vague or ambiguous term. The dictionary definition (or at least one of them since the other is mathematical) is “an activity or purpose natural to or intended for a person or thing”. That sort of applies to what we do but not with any particular precision or clarity.
I think that it’s the words “Lab” or “Laboratory” are the biggest problem since they conjure up images of Bunsen burners, test tubes and white-coated scientists engaged in research (the buzzing electrical arcs climbing up Jacobs’ Ladders and cries of “it’s alive!” are optional). The dictionary definition is “a room or building equipped for scientific experiments, research, or teaching, or for the manufacture of drugs or chemicals.” Not particularly specific to what we do and not necessarily a place you’d want to have any tests performed.
Michael Sim’s suggestion was that we re-brand our place of work by calling it “Pulmonary Diagnostic Services” instead. This is an unambiguous title that clearly identifies what we do. More than that, it gives us an opportunity to re-imagine and re-define exactly what it is we do.
Determining whether a subject has a ventilatory limitation to exercise used to be fairly simple since it was based solely on the maximum minute ventilation (Ve) as a percent of predicted. There has been some mild controversy about how the predicted maximum ventilation is derived (FEV1 x 35, FEV1 x 40 or measured MVV) but these don’t affect the overall approach. Several decades ago however, it was realized that subjects with COPD tended to hyperinflate when their ventilation increased and that this hyperinflation could act to limit their maximum ventilation at levels below that predicted by minute ventilation alone.
The fact that FRC could change during exercise was hypothesized by numerous investigators but the ability to measure FRC under these conditions is technically difficult and this led to somewhat contradictory results. About 25 years ago it was realized that it wasn’t necessary to measure FRC, just the change in FRC and that this could be done with an Inspiratory Capacity (IC) measurement.
The maximum ventilatory capacity for any given individual is generally limited by their maximum flow-volume loop envelope. When a person with normal lungs exercises both their tidal volume and their inspiratory and expiratory flow rates increase.
We have a number of patients who have spirometry and DLCO testing performed at regular intervals. I’ve noticed that every so often DLCO results change significantly without a change in spirometry (or lung volumes) or there’s a modest change in spirometry and a marked change in DLCO. I’ve been concerned that this may be a symptom of problems with our DLCO (CO/CH4) gas analyzers and at least once recently this kind of discrepancy did lead to having an analyzer being serviced. Realistically though, the gas analyzers are routinely passing their calibrations and when I look at the trends in calibration there hasn’t been any systematic drift. This doesn’t rule out intermittent problems however, so in order to find out whether these changes in DLCO are “real” or an artifact of our testing systems I decided to see if taking a closer look at the results would help resolve this.
First, what constitutes a significant change in DLCO?
My lab’s current working definition is an increase or decrease in DLCO that is ≥2.0 ml/min/mmHg or ≥10%, whichever is greater. This is slightly different from the ATS/ERS DLCO intra-session repeatability requirements (≥3.0 ml/min/mmHg or ≥10%) and may mean that we’re setting the bar too low but there’s a difference between intra-session and inter-session variability. Specifically, we average the two closest results (assuming there are at least two tests of good quality) from one testing session to another and it is the inter-session average we are comparing, not individual tests and for this reason we feel that a smaller change can be relevant.
Note: The ATS/ERS statement on interpretation does discuss inter-session DLCO variability but there it is expressed as >7% within the same day and >10% year to year without setting an upper limit. The year to year value is based solely on a study from 1989 on eight individuals using a manually operated testing system (Collins Modular Lung Analyzer) that used a semi-automated alveolar sampling bag and for this reason it’s hard to be sure it is still relevant.
Second, which test parameters have the greatest effect on calculated DLCO?
As a reminder, the DLCO formula is:
For a variety of reasons my wife recently had a full panel of PFTs (spiro+BD, lung volumes, DLCO) at a different hospital than the one I work at. I went with her and was pleased to see the technician perform the tests pleasantly, competently and thoroughly. I was able to glance at the results as the testing proceeded so I had a fairly good idea what the overall picture looked like by the time she was done.
The difficulty came later when my wife asked me to print out her results so we could go over them together. Many hospitals and medical centers have websites that let patients email their doctor, review their appointments and access their medical test results. They go by a variety of names such as MyChart, MyHealth, Patient Gateway, PatientSite, PatientConnect etc., etc. My hospital first implemented something like this over a dozen years ago so I had thought that by now they were fairly universal but conversations with a couple of friends from around the country have let me know that this isn’t really the case.
Regardless of this, the hospital where my wife had her PFTs does have a website for patients and her PFT results showed up about a week later. When I went to look at them however, I was completely taken aback. Not because the results were wrong but because they were presented in a way that made them incredibly difficult to read and understand.
Here’s the report (and yes, this is exactly what it looked like on the patient website):
Airway hyper-responsiveness is a primary feature of asthma. There are a number of bronchial challenge tests designed to evoke and measure this factor, the most common of which require the inhalation of one or another bronchoconstrictive agent such as methacholine, histamine, mannitol or hypertonic saline.
An elevated ventilation can cause many asthmatics to bronchoconstrict and this is often the cause of Exercise-Induced Bronchospasm (EIB). There are two competing theories as to why this happens. A number of researchers have suggested that the mechanism is a drying of the airway mucosa which changes the osmolarity of the respiratory tract fluid which in turn causes some cells to releases mediators that cause bronchoconstriction. Other researchers assert that it is the cooling of the airways during hyperventilation and an increased blood flow and edema during subsequent re-warming that causes the bronchoconstriction. There is evidence to support both interpretations and it is likely that both mechanisms coexist, with one or the other being more predominant in any given individual.
Although the inhalation challenge tests are reasonably sensitive not all patients with EIB have a positive reaction. When a patient’s primary complaint is exercise-related or when they have had a negative inhalation challenge test and are still symptomatic, a ventilatory challenge test should be considered. There are several ventilatory challenge tests that are specifically oriented towards evoking and characterizing EIB. These are the Cold Air challenge, Eucapnic Voluntary Hyperventilation and Exercise Challenge. There are a number of similarities between these tests.
Cold Air Challenge
A Cold Air Challenge (CACh) test consists of having a patient hyperventilate while breathing air that has been cooled to a temperature of between -10°C and -20°C. It is usually performed using a mixture of 5% CO2, 21% O2, 74% N2 in order to prevent dizziness from hypocapnia.