I’ve been reading Miller et al’s Laboratory evaluation of Pulmonary Function which was published in 1987. That was an interesting time since PFT equipment manufacturers had mostly transitioned to computerized systems but there were still a lot of manual systems in the field. For this reason the book’s instructions are still oriented mostly around manual pulmonary function testing and there are numerous warnings about double-checking the results from automated systems.

A friend is taking her father to a PFT lab (2500 miles away from where I am the moment so I couldn’t go along with them) because he has been short of breath for a couple of years, but oddly enough, only when lying on his side. I expect that despite these rather specific symptoms he will only get routine spirometry. I don’t necessarily fault the PFT Lab he’s going to for this, partly because physician orders often don’t include specifics, partly because they may not have the facilities to perform supine or lateral spirometry, and partly because its not clear lateral spirometry would show anything.

Turbine spirometers have been around in one form or another for well over a hundred years. The accuracy of the early versions of this type of spirometer was poor, partly because of the turbine designs weren’t terribly efficient and partly because these devices were mechanical in nature and the gear trains or other mechanical linkages added a lot of friction and resistance. From: Nouveaux éléments de pathologie générale, de séméiologie et de diagnostic. by Eugène Bouchut, 1875, page 865. ]

I’ve been thinking a bit about the shape of flow-volume loops lately. In part this has been about ways to accurately describe them in reports; in part speculation about the information that may be embedded in them that isn’t in any of the routinely reported spirometry values; and in part about how the human eye perceives and categorizes them in a way that is difficult to simplify and put into a computer algorithm. A couple days ago I found a recent article where a geometrical analysis was applied to flow-volume loops in individuals with COPD and this got me curious about what other graphical techniques have been used to analyze flow-volume loops.

A report came across my desk today and at first glance it looked fairly straightforward. There was a mildly reduced TLC and FVC, and although the SVC was slightly lower than the FVC it looked like this patient had mild restriction. Observed: %Predicted: Predicted: FVC: 1.73 68% 2.56 FEV1: 1.23 65% 1.89 FEV1/FVC: 71 97% 73 TLC: 3.58 73% 4.89 FRC: 2.07 75% 2.78 RV: 1.94 83% 2.33 RV/TLC: 54 114% 48 SVC: 1.69 66% 2.56 IC: 1.51 72% 2.11 ERV: 0.13 30% 0.45 In addition, the flow-volume loop looked fairly typical for restriction, with a normal peak flow and a reduced volume.

I was reading an article recently that made an off-hand reference to the 100% oxygen shunt fraction test. Results from the test were included in the data analysis but the equations the researchers used were not presented nor were they referenced, nor was the procedure described. This is probably because the shunt fraction test and its equations are very much old-school pulmonary physiology but even if the subject is probably covered at one time or another in physiology classes I suspect that some of the issues involved in the calculation are not as well understood as they should be.

I often find topics for this blog in a sideways fashion. Recently while searching for something else I ran across an article about the minimum clinically important difference (MCID) of the Residual Volume (RV) in patients with emphysema. I’ve come across the MCID concept before but I had never really followed up on it. This time I started researching MCID and immediately ran across a number of articles about the MCID of the 6-minute walk test (6MWT). This got me to review the articles I have on hand and I found that since I last wrote about the 6MWT I’ve accumulated quite a few new (or at least new to me) reference equations as well as a number of articles about performance issues. Given all this how could I not re-visit the 6MWT?

For the first dozen or so year that I worked in a pulmonary function lab it was with counter-weighted, volume-displacement water-seal spirometers more or less like this: Patients would do a series of tests and I’d end up with a bunch of pen traces on kymograph paper that I’d have to measure with a ruler and use a desktop calculator (it was about a foot square, weighed a couple of pounds and had a nixie tube digital display) to create a hand-written report. I’m not going to suggest that these spirometers were in any way better than what we’re using now but I have to say that I would have seen the following problems more or less immediately.

The patients whose reports I review have always been very accommodating. An issue of one kind or another catches my attention and before I know it I find several more reports that are similarly involved. Thanks to our patients I’ve had a number of reports come across my desk recently that showed a combination of restrictive and obstructive defects. This particular one may not be the best possible example but it seems to illustrate several points fairly well.

My hospital has an active liver transplantation program and all transplant candidates get a full panel of PFTs in my lab. The number of liver transplant candidates we get varies from week to week but probably averages between 150 and 200 a year. As with any population a certain number of them have COPD or other lung diseases but there are some that have normal spirometry and lung volumes but a reduced DLCO. This latter group of patients likely have hepatopulmonary syndrome (HPS).