Monthly Archives: August 2014

It isn’t a Raw without a TGV

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One of the key reasons to perform spirometry is to measure expiratory flow rates. The flow rate of air through any system is primarily a function of driving pressure and resistance. Since there are limits to anybody’s ability to increase driving pressure (and physiological reasons why airflow does not continue to increase when driving pressure exceeds a certain threshold value) FEV1 is largely related to the amount of resistance in the airways.

The gold standard for measuring airway resistance (Raw) has been plethysmography. Like many other pulmonary function tests measuring Raw depends on a series of assumptions and a standardized approach to assessing the results. One of the standardizations is that a Raw maneuver must always be paired with a TGV maneuver. Although the knowledge of lung volume allows values like specific resistance (sRaw) and specific conductance (sGaw) to be calculated, this is not the reason for the TGV maneuver at all.

Resistance is calculated from:

 Resistance Formula

Inspiratory and expiratory flow rates are relatively easy to measure but the driving pressure, which in this case is alveolar pressure, is not. For this reason an indirect approach is needed to estimate alveolar pressure.

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Looking for help with an orphaned system

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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.

If you have version 4 of the Spirotech S600 software (most likely on 5-1/4″ floppy disks), or if you have a manual or schematic for this system please contact Gabriel at gabriel_mendiondo@hotmail.com.

I don’t know if Spirotech was absorbed by a larger company or went out of business but the last Spirotech spirometers that I know of were manufactured around 20 years ago. Many of us are fortunate enough to be able to occasionally replace older equipment (although this is usually only when the older equipment does not work and can’t be repaired, and even then you have to jump through hoops to justify a capital purchase).  I have been in the position of trying to keep an older system functioning numerous times. The problem has always been that once a given test system has been replaced by a manufacturer’s newer model finding parts for the older model becomes difficult at best. More often it becomes impossible.

I don’t blame our equipment manufacturers for this.  I’ve watched pulmonary function equipment evolve for over forty years and every model was built with the technology available at the time.  But time moves on and trying to keep an older technology alive is always a lot more expensive than adapting to newer technology (and that’s even presuming that it was worthwhile to keep the older technology alive in the first place).  Still, there is a fair amount of older equipment out there that is at least potentially capable of functioning.  3D printing may be a partial solution to missing parts but I think the bigger problem is not so much physical parts but computer software.  Computers and computer software have been evolving incredibly rapidly and even if the software for an older test system was located more than likely the computer hardware the software was written for may no longer exist.  But there’s always Ebay so even this problem can potentially be overcome.

So, hold onto the disks and manuals that came with your test systems.  Even if you no longer have the test systems they were intended for they may still be able to help somebody else who is trying to make the best they can of a nonexistent budget.

Assessing FEV1 trends, re-visited

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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.

Trend Table 1

In the 14 years between these two tests the FEV1 has decreased by 0.56 L or -12.6%. By the 10% threshold criteria this is a significant change but I think that 14 years is a reasonably long period of time and the age-adjusted change is only 5.1% which indicates this change is not significant.

Trend Table 2

In the year between these two tests the FEV1 has decreased by 0.22 L or 7.0%, which doesn’t meet either criteria for a significant change.

But what has changed between these tests is that in both instances the spirometry went from normal to showing mild obstruction. This is a qualitative change and I think it is likely significant.

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What’s abnormal about FRC?

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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?

The answers to that question range from “a whole bunch” to “not much” to “darned if I know”. When you measure lung volumes TLC is really the only clinically important result. RV can be useful at times but although the other lung volume subdivisions may play a role in the measurement process they have only a limited diagnostic value. All lung volume measurements start with FRC, however, and if you don’t know you have an accurate FRC how do you know that TLC is accurate?

FRC is a balance point of opposing forces in the lung and thorax. Lung tissue wants to collapse, the rib cage wants to spring open and the diaphragm wants to do whatever muscle tone, gravity and the abdomen allows it to do. All of these forces are to one extent or another dynamic and can change over time. These changes can occur both slowly and rapidly, and are the primary reason why isolated changes in FRC don’t tend to have a lot of clinical significance. For all lung volume measurements however, one primary assumption is that FRC does not change during the test and this isn’t necessarily true.

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