Gas solubility and why it matters

I have been searching through Pulmonary Function videos on YouTube in order to find ones I thought would be useful for technician education. So far what I’ve found have been intended either for medical students or for patients and not, in my opinion, particularly suitable for training technicians. Lately I’ve been looking at videos about lung volumes and have seen a half dozen presenters describe lung volume subdivisions using the same graph we’ve come to know and love with varying degrees of effectiveness and obfuscation.

From "Standardisation of the measurements of lung volumes", pg 512

From “Standardisation of the measurements of lung volumes”, pg 512

In a discussion of helium dilution lung volume measurements one of the presenters made an interesting statement and that was that “helium does not pass the alveolar-capillary barrier which means it stays inside the lungs during the test”. This is wrong on multiple levels. First, the alveolar-capillary membrane evolved for gas exchange and does not discriminate against individual gases so there is no barrier. Second, the reason that gases can be used as tracer gases or as probes of pulmonary circulation has entirely to do with gas solubility. Third, since it was a university-sponsored video with other egregious errors (for example did you know that lung volumes are measured in ml/kg?) what the heck are they teaching their medical students?

Gases can and will be absorbed by blood and tissue. The quantity of gas that can be absorbed is determined by the gas’s solubility and the Bunsen solubility coefficient is a measure of how much gas is absorbed (usually in milliliters of gas per milliliter of liquid) when the gas is at 1 atmosphere of pressure. When there is a multi-gas mixture, the quantity of gas absorbed for individual gas is calculated by:

Gas Content Calculation

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How long should patient test data be retained?

 In a recent post on the AARC Diagnostics forum a PFT Lab manager asked how long they need to keep database records. The ostensible reason for this was that they had too many years of records and had been having database problems.

The poster wasn’t specific about what kinds of problems they were having. Database problems can be hard to diagnose particularly when a database is networked but with a modern SQL database the number of records shouldn’t be an issue. SQL databases containing millions of records are routinely used in demanding multi-user applications. If this was thirty years ago when computers first started to be commonplace in the PFT Lab I could understand since PC-based databases were still in their infancy then. It was at least partly for this reason that a number of PFT equipment manufacturers developed their own proprietary databases. This is no longer the case and I have difficulty believing that there are any manufacturers at this time that don’t use a commercial SQL database of one kind or another.

I am not suggesting the poster wasn’t having problems. Even though SQL databases tend to be very robust that doesn’t mean that incorrect settings or bugs in the software accessing the database can’t cause problems. Equipment manufacturers and hospital IT departments may not have the expertise or the patience (or even the desire) to diagnose and fix these kinds of problems either. What I found curious however, was that almost everybody responding to the original post seemed to be eager to get rid of their “old” patient data as soon as they possibly could.

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What’s normal about FEV1 and how much does ethnicity matter?

When it comes to spirometry, it’s really all about FEV1. FVC and the FEV1/FVC ratio are also important of course, but because FVC is more likely to be underestimated than FEV1 they are less reliable.

Changes in FEV1 are critical in monitoring airway disease. The recent ATS guidelines on Occupational Spirometry indicate that a 15% decrease (adjusted for changes in age) is significant and cause for concern. For diagnosing airways disease however, it is important to know what a normal FEV1 is.

I have been able to find twenty-four different reference equations for FEV1. That’s good in one sense but that quantity also makes it that much more difficult to determine which reference equations should be used. When I graph results it often becomes more apparent what the equations are trying to tell us but in this case I came away a bit more confused instead.

Female FEV1 165 cm non-clustered

Male FEV1 175 cm Age non clustered

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Personal spirometers, under $500

Pulmonary patients have been using personal peak flow meters for several decades and I started to seeing patients that had their own oximeter over 10 years ago. Within the last couple of years a number of different spirometers intended for personal use have become available. These devices have significant limitations when compared to regular office or lab spirometers but because there are many individuals with asthma or COPD for whom FEV1, and possibly FVC, are significantly better indicators of lung health than Peak Flow (PEF) alone, I think they can serve a useful purpose.

There is no official definition of what constitutes a personal spirometer but I’m going to go with the ability to measure FEV1 as a minimum which means that there is a bit of crossover with some of the electronic peak flow meters. Because the majority of personal spirometers I have been able to find sell for less than $500 for the time being I’m going to keep the discussion to spirometers that I believe are under that price point.

There is, of course, a correlation with price and the number of features. Not surprisingly, the least expensive units measure the fewest values and have the least memory. More is not necessarily better, however, and I think that the first factor that should be considered when comparison shopping is what needs to be measured?

Company: Product: Measures:
Carefusion Micro1 FEV1, FEV6, FEV1/FEV6, PEF, FEF25, FEF75, FEF25-75, Pre/Post-Bronchodilator, %predicteds
Carefusion Precison Diary FEV1, FEV6, PEF
Carefusion PulmoLife FEV1, FEV1% predicted, Lung age
Contec SP10 FEV1, FVC, FVC, PEF, FEF25, FEF75, FEF25-75, %Predicted.
Ganseman PC-Spiro FEV1, FVC, PEF, FEV1/FVC ratio, %predicteds
MDSpiro PulmoLife FEV1, FEV1% predicted, Lung age
MDSpiro SpiroCheck FEV1, FEV1% predicted, Lung age
MDSpiro SpiroCheck Home Monitor FEV1, FEV6
Nspire Piko-6 FEV1, FEV6, FEV1/FEV6

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