I was contacted recently by a reader who is trying to resurrect an older test system. It is a Spirotech S600, manufactured by Graseby-Andersen and outfitted for spirometry, helium dilution lung volumes and single-breath DLCO. It seems to be in reasonably good shape but he cannot test the components or use it in any way because he does not have the software for it. He is working on a shoestring budget and does not have funds to purchase a newer system.

There are a couple of different ways to assess changes in FEV1 from one patient visit to another. For several decades my lab has used a change of >=10% and >=200 ml as the threshold for a significant change. Recently the ATS released standards for occupational spirometry that included an age-adjusted change in FEV1 of >=15% as the threshold for significant change. For the time being we have continued to use the 10% threshold when comparing results that are relatively close in time and are using the 15% threshold when they are separated by a much longer period. Since we haven’t actually gotten around to defining what is recent and what isn’t there is still a bit of uncertainty in how we apply this but even though there are differences in thresholds and how the numbers are calculated both approaches are essentially numerical. Recently a couple of reports crossed my desk that have caused me to wonder whether a qualitative change should also be a consideration.

I’ve had a number of reports across my desk in the last couple of weeks with both elevated and reduced FRC’s that were associated with a more-or-less normal TLC. I reviewed the raw data from all of these tests (I review the raw data from all lung volume tests) and in only a few instances did I make any corrections to the report. This made me think however, about what, if anything, is an abnormal FRC trying to tell us?

Roughton and Forester’s seminal paper from the 1950’s showed that DLCO was a function of two resistances: the alveolar-capillary membrane and the rate of CO uptake by red blood cells. This relationship is shown by: Roughton and Forster also showed that the membrane diffusing capacity (DMCO) and pulmonary capillary blood volume (Vc) could be calculated by performing the DLCO test at different oxygen concentrations and then plotting the results.

A spirometry report that looked very questionable came across my desk recently. The flow-volume loop was misshapen and the technician’s notes indicated that the results had been highly variable and to “interpret with caution”. I pulled up the raw test results and saw a series of test efforts with flow-volume loops that were all somewhat flattened and with no consistency in either the loops or the numerical results. This kind of inconsistency can be an indication of poor patient effort but can also occur because of airway problems. The cardio-thoracic surgeons at my hospital have an active airway stenting program and so we see a fair number of patients with trachemalacia. One hallmark of tracheomalacia is that there is usually a flow limitation and that this means that there is usually a flat expiratory plateau in the flow-volume loops. These loops had peak flow-ish humps, but the humps seemed to appear in different locations in every loop and they seemed to have a relatively high frequency flutter.

Recently I was reviewing a spirometry report and noticed that the FVC was below normal. A low FVC can suggest a restrictive lung disease but the reported expiratory time was only about 4 seconds. I took a look at the graphics included with the report and the volume-time curve showed the effort ended well before 6 seconds so my first thought was that the reduced FVC was more likely because of suboptimal patient effort than anything else.

Although the numerical results are of course important, visual inspection of the volume-time and flow-volume loop graphs from a spirometry test are a critical part of interpretation. Spirometry quality and performance issues that don’t show up in the numbers are often highly evident in the graphs. Choices we make in creating and configuring reports however, can hide important visual details and have the potential to decrease interpretation quality. Recently I was inspecting the results for a spirometry test. There wasn’t anything particularly unusual about the numbers or the graphics on the report, I just like to make spot-checks on spirometry quality and wanted to make sure the best results had been selected. When I pulled up the raw test date on my computer screen I noticed an unusual wavering pattern in the volume-time curve. I don’t remember seeing a volume-time curve like this before and when I checked all of the patient’s efforts were similar and all showed similar oscillations.

The single-breath DLCO maneuver can rightly be criticized as being an artificial maneuver that bears little resemblance to normal breathing. It is only by standardizing the maneuver that clinically relevant and reproducible results can be obtained. One important aspect of this standardization is the breath-holding period. The single-breath DLCO maneuver begins with a subject exhaling to RV, followed by an inhalation of the test gas mixture to TLC and then a 10-second breath-holding period, ending with an exhalation during which a sample of alveolar air is collected. The initial choice of a 10-second breath-hold period was largely arbitrary and was selected in order to strike a balance between being a short enough period that for most patients to hold their breath, long enough to minimize the inspiratory and expiratory phases and long enough to allow for a sufficiently measurable amount of carbon monoxide to be taken up.

Because our lab database goes back 24 years, we’ve started to see a certain number of patients who had last been seen ten or even twenty years ago fairly often. For this reason I’ve been thinking about what is a clinically significant change over that long a time period. The guidelines my lab uses for interpreting change in test results came about from a consensus among the department’s pulmonary physicians close to twenty years ago. As usual there are some discrepancies between our guidelines and those the ATS-ERS have published.

There are a number of simple observations that can be made during a cardio-pulmonary exercise test (CPET) that can point you immediately in a specific diagnostic direction. Recently I was reminded of this while reviewing the CPET results on patient with a complicated medical history whose test had been requested as part of a pre-operative assessment. Most patients that are candidates for cardio-thoracic surgery do not need to have a CPET and that’s because it is usually straightforward to determine who is high risk and who is low risk from other routine tests. When risk is hard to determine or equivocal, the cardio-thoracic surgeons will order a CPET. They are primarily interested in the VO2 max and Ve-VCO2 slope since there are a number of widely accepted pre-op assessment algorithms that use these values. Even if the CPET results indicate the patient is high risk, the test details can help determine whether there is anything that can be done to improve the patient’s odds.