Monthly Archives: December 2012

Re-breathing DLCO, another almost unknown technique

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Re-breathing DLCO may not be well known because this technique has not been used in routine clinical testing. It has instead been primarily used to research diffusing capacity during exercise because it is able to do this without the need to interrupt or significantly alter a subject’s breathing pattern.

Re-breathing DLCO is probably best thought of as a hybrid between the single-breath DLCO and steady-state DLCO techniques. The gas mixture and calculations are from the single-breath world but the breathing pattern is more from the steady-state side of the family.

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It’s after midnight, do you know where your reports are?

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After the tests themselves, the second most important thing that a Pulmonary Function Lab needs to do is to report results. Like a tree falling in the woods, if a report hasn’t gotten in front of the person that ordered the tests, has anything happened? A successful PFT Lab needs to manage its reports effectively.

An obstacle that many labs will need to overcome is the reporting software that comes with their testing systems. I think that for many of the PFT equipment manufacturers reports are an afterthought at best. The ability to format reports to a specific lab’s needs is often limited and is just as often is a difficult and time-consuming process.

For example my lab maintains approximately eight different report formats, each tailored to the most common mix of tests we perform. If we need to change an item in patient demographics (and we have) then that means that this change needs to be made separately in each of the eight different formats. If this change requires more space on the report and that other report elements need to be moved to accommodate this, then this has to be done on each of the report formats as well. The software has no ability to copy or share changes between report formats (which I am sorry to say is ridiculous since I remember having reporting software in the DOS era where this was possible).

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Steady-State DLCO, an almost-forgotten technique

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Recently I have been reviewing a lot of early pulmonary function research. I’m not feeling nostalgic but I think that re-visiting some of the older methods and technology may be interesting. Almost all Pulmonary Function laboratories presently use the single-breath technique to measure diffusing capacity. There are a number of reasons why this is case but I suspect that many technicians and physicians aren’t aware that there are several alternative methods for measuring diffusing capacity and that there was a time when at least one of these, the steady-state DLCO, was routinely used in clinical labs. Continue reading →

Busted for speeding during an N2 washout

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Nitrogen washout lung volumes are still relatively new to my PFT Lab. The number of problems we’ve encountered has decreased substantially but we are still learning some of the idiosyncrasies of the system. Recently while trying to understand a test with odd results we were reminded by the manufacturer that during the washout period a patient’s inspiratory and expiratory flow rates should not exceed 1.5 liters/second. The reason this “speed limit” is necessary highlights some of the limitations of modern open-circuit lung volume measurements.

The basic concept behind nitrogen washout is relatively simple. The air we breathe contains 78% nitrogen which is a relatively inert, insoluble gas. If you have a patient breathe 100% oxygen and then collect their exhaled air you can calculate the volume of exhaled nitrogen by multiplying the concentration in the exhaled air by the total volume of air that was collected. Once you know the volume of nitrogen you can then calculate the lung volume.

Initially this was a laborious and cumbersome process. The patient’s exhaled breathing circuit and a Tissot Gasometer (a very large spirometer with a volume between 125 and 300 liters) are first flushed with oxygen several times to remove any nitrogen. Next, while breathing room air the patient exhales to RV and an end-expiratory gas sample is taken and used to estimate the patient’s alveolar nitrogen concentration. The patient is then switched to 100% oxygen and breathes for seven minutes. At the end of the washout period the nitrogen concentration of the exhaled gas in the gasometer is analyzed and the volume recorded.

 N2_Washout_System_JCI_v19_n4_p609_1940

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Plethysmography and “trapped air” demystified

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Recently I was reading the blog of someone who teaches Pulmonary Function testing and they stated:

“…in emphysema and air trapping, the VTG (thoracic gas volume) will be higher than a FRC (functional residual capacity) measured by Helium dilution and Nitrogen washout. This is because VTG is the volume of gas contained in the thorax, whether in communication with the airways or trapped in the thorax.”

This unfortunately is not correct, but it is a common misconception and since there was a time when I believed it myself I find it difficult to fault the author too much. I think it’s time to return to the basics of plethysmographic lung volumes however, and show why this is not true.

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