Eye was fooled

A couple of days ago I was reviewing (triaging, actually) the spirometry portion of a full panel of PFTs performed with pretty terrible test quality and was trying to decide if the technician responsible for performing the tests had made the right selections from the patient’s test results. I noticed that the FEV1 that had been selected was actually the lowest FEV1 from the all the spirometry efforts the patient made, and was trying to decide whether this was really the correct choice. We use peak flow to help determine which FEV1 to select and that particular spirometry effort appeared to have the highest and sharpest peak flow by a large margin:

particularly when compared to the other spirometry efforts:

But this was hard to reconcile given how low the FEV1 was relative to the others:

Test #1 Test #2 Test #3
Observed: %Predicted: Observed: %Predicted: Observed: %Predicted:
FVC (L): 1.71 41% 2.46 59% 2.39 58%
FEV1 (L): 1.24 39% 1.81 57% 1.77 55%
FEV1/FVC: 73 95% 74 96% 74 97%

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PFTs on YouTube

A friend recently sent me the links to several YouTube videos on pulmonary function testing. I’ve spent some time off and on over the last year looking at YouTube videos and in particular I’ve been looking for ones that can be used as part of technician education. Maybe I’ve set the bar too high but all too often I’ve been disappointed and frustrated with what I’ve found. One reason for this is that many videos are aimed at other audiences than technicians (i.e. medical students, physicians, patients). Another reason is that too often only simple concepts are presented, often in rote fashion and often without good visual explanations (c’mon, these are videos after all, not podcasts). A final reason is that sometimes they’re outdated, misleading or just plain wrong.

Still, even the flawed videos can be useful. Sometimes this is because they occasionally explain some concepts well; sometimes despite being simplistic they present a good overview; and sometimes because their mistakes can serve as points for discussion. I’ve tried to select videos that have at least some potential for use in technician education.

John B. West Respiratory Physiology Lectures

Based primarily on his classic textbook, ‘Respiratory Physiology’ (which should be on everybody’s bookshelf). Not 100% perfect but this is what many of the other videos should aspire to be. Many complex concepts explained using simple examples. Lots of interesting pictures and illustrations. Should be part of every technician’s education.

  1. Structure and Function
  2. Ventilation
  3. Blood Gas Transport
  4. Acid-Base Balance
  5. Diffusion
  6. Pulmonary Blood Flow
  7. Pulmonary Gas Exchange, Part 1
  8. Pulmonary Gas Exchange, Part 2
  9. Mechanics of Breathing, Part 1
  10. Mechanics of Breathing, Part 2
  11. Control of Ventilation
  12. Defense Systems of the Lung
  13. Respiration under Stress
  14. Respiration at the Limit

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Z Score to remember is -1.645

The use of Z scores to report PFT results, both clinically and for research is occurring more and more frequently. Both the Z score and the Lower Limit of Normal (LLN) come from the same roots and in that sense can be said to be saying much the same thing. The difference between the two however, is in the emphasis each places on how results are analyzed. The LLN primarily emphasizes only whether a result is normal or abnormal. The Z score is instead a description of how far a result is from the mean value and therefore emphasizes the probability that a result is normal or abnormal.

Reference equations are developed from population studies and the measurements that come from these studies almost always fall into what’s called a normal distribution (also known as a bell-shaped curve).

A normal distribution has two important properties: the mean value and the standard deviation. The mean value is essentially the average of the results while the standard deviation describes whether the distribution of results around the mean is narrow or broad.

The simple definition of the Z score for a particular result is that it is the number of standard deviations that a result is away from the mean. It is calculated as:

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A couple weeks ago I was asked whether it was safe for a patient with an abdominal aortic aneurysm (AAA) to have pulmonary function testing. My first thought was that it was probably unsafe but after a moment or two of thought I realized that I hadn’t reviewed the subject for a long time. When I checked the 2005 ATS/ERS general testing guidelines (there are no contraindications in the 2005 spirometry guidelines) I found that AAA wasn’t mentioned at all. In fact, the only absolute contraindication mentioned was that patients with a recent myocardial infarction (<1 month) should not be tested. Some relative contraindications were mentioned:

  • chest or abdominal pain
  • oral or facial pain
  • stress incontinence
  • dementia or confusional state

and activities that should be avoided prior to testing include:

  • smoking within 1 hour of testing
  • consuming alcohol within 4 hours of testing
  • performing vigorous exercise within 30 minutes of testing
  • wearing clothing that restricts the chest or abdomen
  • eating a large meal with 2 hours of testing

but these were factors where test results were likely to be suboptimal and not actually contraindications.

This got me curious since I thought that pulmonary function testing was contraindicated for more conditions than just an MI. I reviewed the 1994 and and then the 1987 ATS statements on spirometry but again found no mention of contraindications. Ditto on the 1993 ERS statement on spirometry and lung volumes. Finally, in the 1996 AARC clinical guidelines for spirometry I found a much longer list of contraindications:

  • hemoptysis of unknown origin
  • pneumothorax
  • recent mycardial infarction
  • recent pulmonary embolus
  • thoracic, abdominal or cerebral aneuysms
  • recent eye surgery
  • presence of an acute disease process that might interfere with test performance (e.g. nausea, vomiting)
  • recent surgery of thorax or abdomen

So where did the AARC’s list of contraindications come from? And why is there such a discrepancy between the ATS/ERS and the AARC guidelines?

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Assessing post-BD improvement in FEV1 and FVC as a percent of the predicted

The 2005 ATS/ERS standards for assessing post-bronchodilator changes in FVC and FEV1 have been criticized numerous times. A recent article in the May issue of Chest (Quanjer et al) has taken it to task on two specific points:

  • the change in FVC and FEV1 has to be at least 200 ml
  • the change is assessed based on the percent change (≥12%) from the baseline value

The article points out that the 200 ml minimum change requires a proportionally larger change for a positive bronchodilator response in the short and the elderly. Additionally, by basing the post-BD change on the baseline value it lowers the threshold (in terms of an absolute change) for a positive bronchodilator response as airway obstruction become more severe. As a way of mitigating these problems the article recommends looking at the post-bronchodilator change as a percent of predicted rather than as a percent of baseline.

The article is notable (and its authors are to be commended) because it studied 31,528 pre- and post-spirometry records from both clinical and epidemiological sources from around the world. For the post-bronchodilator FEV1 and FVC:

  • the actual change in L
  • the percent change from baseline
  • the change in percentage of predicted
  • the Z-score

were determined.

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