Seeing shouldn’t always be believing

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.

VT_Curve_waver_redacted

The end-exhalation portion of the flow-volume loop looked a lot like cough artifacts but coughs usually look different on the volume-time curve so out of curiosity I measured the rate at which the volume oscillated and it was about 1 hz. Oscillations are not that uncommon in flow-volume loops but they usually occur throughout the loop and the oscillation frequencies usually start around 4 hz and can go up to 20 hz or higher. Lower frequencies are usually related to neuromuscular issues but a 1 hz oscillation was a bit on the unusual side and there was nothing in the patient’s diagnosis or history that indicated this might be an issue.

Interesting as they might be, there wasn’t anything overtly diagnostic about the oscillations so I went back to the report. I gave it another quick look-over and was about to put in my Out box when I noticed that the oscillations weren’t visible on the report.

VT_Curve_waver_2_redacted

When we configure a report for our testing software there are always certain design considerations. The hospital’s medical records department has very strict requirements for fonts and font sizes; for the placement and size of the hospital’s logo, and for the order and placement of the certain pieces of patient and test identification. This means that up to 2 inches of each page of the report have been spoken for before we even start to fit in the rest of the patient’s demographics, test results, technician notes, interpreting physician notes, graphics and the trends. Theoretically there is no reason we couldn’t make separate pages for flow-volume and volume-time graphics in every report, more realistically this is wasteful of paper and in addition would probably make the report harder, not easier, to read.

For every report format we’ve created we’ve had to make trade-offs in the size of the graphics. The spirometry reports are able to devote almost an entire page to the flow-volume loop and volume-time curve, but on all of the other reports, these graphics have had to share space with other report elements and this has limited how large they can be printed.

When graphics are displayed on a computer screen or printed on a report, the lines that make them up can never be smaller than a single pixel. This means that how much detail a line can show will depend on what scale is used display or print it. When you squeeze a graphic down to a smaller size, something will always be lost along the way.

The volume-time and flow-volume loop graphics on the report are about 1/4 the size as those on my computer screen. In order to make the graphics fit the size available for them on the report, the software simplified them. This removed some of the details from the curves, which in this case included the oscillations.

I’ve been involved with computers and graphics for close to 40 years and in the early years screen and printer resolution was always a significant issue so it’s not like this is a new problem. Computer screens and printers have improved dramatically and we are probably inclined to believe this kind of problem is not an issue any more, but the fact is that when we configure reports (or use the reports configured for us by the manufacturer) the problem never really went away.

There is no particular reason that high resolution graphics need to be on a final report any more than every single patient effort needs to be reported. This is okay because some simplification is always going to be needed to create a readable report. What is important however, is that whoever is responsible for reviewing and interpreting test results must be able to review them at a high enough resolution that test quality is easily evident and this is where I think there may be a problem.

I have a 24” LCD display on my computer with 1920 x 1200 pixels. That’s about as large a display as is commonly available (yes, they make larger displays with more pixels but the price curve gets very steep very rapidly and only a few graphics cards will support them anyway). This works well when I review results on my computer but I usually see reports as PDF’s which is more-or-less the same as their printed version. It’s only when I pull test results up in the lab’s testing software that I can see anything at a higher resolution and although I do this frequently I don’t do this for all reports and all tests. Moreover I suspect that PFT’s are most often reviewed and interpreted as paper reports, not on a computer screen, and that most reviewers don’t have access to higher resolution graphics. This makes it important that we be aware of the consequences of the resolution for the graphics we choose to place on our reports. Small graphics save space but only at the cost of losing information.

Single page reports for spirometry seem to be popular, particularly for office spirometry systems. I think that the limited quality of the graphics on these reports is often overlooked. I understand that handling multiple-page reports is likely a burden and therefore a problem for clinics and physician offices but at the same time a certain implication of this seems to be that those involved with office spirometry do not have the need (or the ability?) to assess test quality.

Many equipment manufacturers are moving towards the web-based on-line review and electronic signing of PFT reports. I’d like to think that this will make high-resolution graphics (and raw test data) available to a reviewer, and will be simple and straightforward to use. My lab is moving in this direction but the details we’ve received so far indicate that it will be the PDF version of the report that will be accessed via a web browser, not the raw data or the original graphics. I am disappointed about this and if this is actually the case we will probably have to re-configure some of our reports in order to show higher-resolution graphics, even if it moves us to longer reports.

Looking well down the road, I can see a time when all PFT reports regardless of where they are performed and regardless of which manufacturer’s equipment they come from are on-line and come with full graphics and the raw data imbedded, and can be pulled up and displayed (and trended) whenever needed. I see the beginning of this in the establishment of the inter-hospital communication mandates and standards but since I see no movement towards this goal either by the standards organizations that govern our field or any apparent desire for collaboration between PFT equipment manufacturers for the time being this will have to remain a pipe dream on my part.

The resolution and size of the graphics that are part of PFT test results needs to be a consideration when configuring reports. There needs to be balance between the amount of space they are allotted and the amount of information that needs to be conveyed. A picture may be worth a thousand words but without some care the message it provides can be misleading.

2 thoughts on “Seeing shouldn’t always be believing

  1. Hi Richard, I’m just starting out in PFT’s.
    Came across your blog and had a patient with similar results at end exhalation of FVC and SVC tests. I’m wondering what you think could be the cause of these oscillations.

    My patient indicated they were feeling out of breath. My take is they probably couldn’t maintain the exhalation to RV and started to cough/inhale small volumes to alleviate the perceived breathlessness.

    • My thought is that if these oscillations were due to cough or SOB, they would have a higher amplitude and would be more irregular. Cough and SOB are also fairly common events in spirometry and this tracing stood out for me as being unusual. Since the oscillations were relatively slow and regular my best guess is that they could be due to a low frequency neuromuscular tremor.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.