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.

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.

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.

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.

Spirometry is the most commonly performed (and mis-performed) pulmonary function test around the world. The apparent simplicity of spirometry is misleading since there are numerous subtleties that have a significant effect on the results. I suspect that when the FVC is thought about it is most often considered to be an index towards the total capacity of the lung. That’s certainly true in it’s own way, but the FVC is actually a critically important factor when determining airway obstruction. I’ve had a number of reports across my desk lately where the patient had a reasonably large change in FVC when compared to their last visit but little change in FEV1, and this has made a difference in how the results are interpreted. For example:

Recently a rather eminent reader commented on an older blog entry. He finished his comment with a paragraph on another topic, however. Specifically: By the way, it is also high time that we scuttle the habit of expressing a measurement as percent of predicted. As Sobol wrote [5]: “It implies that all functions in pulmonary physiology have a variance around the predicted, which is a fixed per cent of predicted. Nowhere else in medicine is such a naive view taken of the limit of normal.”

There has been a fair amount of confusion about PFT lab staff licensure requirements. This information is not available on the AARC website, nor on any of the AARC state society websites. A month or so ago I reached out to all of the AARC state societies but received responses from only a handful of them. I was recently able to complete this research however, by visiting the websites of the remaining state licensing boards and state legislatures.

Outside the pulmonary lab there is this notion that spirometry is supposed to be so easy that anyone can do it. You just tell the patient to take a deep breath in and blow out fast and to keep blowing until they’re empty. What’s so hard about that? Sheesh. GIGO. I keep finding ways in which the patient, their physiology and our equipment can conspire in ways to produce errors that even experienced technicians can miss. I’ve been paying a lot of attention to flow-volume loops lately and maybe it’s for this reason that I’ve seen a steady stream of spirometry tests that have something wrong with the FVC volume.

When Tiffeneau described the FEV1 in 1947 and Gaensler the timed FVC and FEV1/FVC ratio in 1950 this opened up an entirely new territory for pulmonary investigators to explore. Numerous new measurements were rapidly mapped out with often conflicting titles. The situation became confusing enough that the British Thoracic Society met in 1956 specifically to standardize terminology. At this time only the volume-time curve was available for measurement purposes (usually a pen trace on kymograph paper) and this fact helped determine what measurements it was possible for researchers to make.

I’ve discussed the issue of inserting a predicted FVC into the predicted lung volumes several times now. At the risk of beating this issue to death I’d like to put to rest the notion that an FVC and an SVC are the same thing. A Forced Vital Capacity (FVC) maneuver is designed to measure the maximum expiratory flow rates, in particular the expired volume in 1 second (FEV1). It has long been recognized that the effort involved in the FVC maneuver can cause early airway closure, even in individuals with normal lungs, and that for this reason the vital capacity can be underestimated due to gas trapping. This effect is usually magnified with increasing age and in individuals with obstructive lung disease.