Vd/Vt, how accurate is it really?

My lab stopped inserting A-lines to get arterial blood samples during exercise testing well over 10 years ago. Our decision was partly based on the fact that we didn’t do them often enough to be good at it and partly based on the fact that we didn’t think that we were getting enough extra information from ABG’s to be worth the effort. Since another local hospital (a competitor but part of the same medical school network so we share pulmonary fellows with them) routinely performs level II and level III exercise tests we felt we could refer any patients that really needed ABG’s to the lab there. We don’t regret the decision and don’t feel that it has compromised the quality of our exercise testing.

Because we don’t obtain ABG’s, one of the values we don’t calculate is the deadspace/tidal volume ratio (Vd/Vt). Recently I was reading an article that related Ve-VCO2 to Vd/Vt and I was reminded of some the issues I had with Vd/Vt when I calculated it in the past. We’ve gone through two different exercise test systems since that time so I’m not sure if some of these problems still exist but I thought it would be a good idea to review both the problems and the literature on Vd/Vt to see if I could make some sense of them.

As a reminder, the original Bohr equation for Vd/Vt was:

Original Bohr Equation

The first problem I had run into was that mixed-expired CO2 (PECO2) is routinely calculated from CPET data as:

PECO2 Equation

But it was also reported as a separate value by the test system’s software and the two values did not match.

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It wasn’t a leak

The most common problem we have with helium dilution FRC tests are leaks. Although the system tubing and spirometer bell leak occasionally, we do have valve failures relatively frequently. Valve failures are usually obvious but they sometimes only fail partially so leak checks are regularly performed on these test systems. We can’t perform leak checks on patients except while they are being tested however, and patient leaks are far more common than system leaks.

A technician asked me to look at a patient’s helium dilution FRC test because it had an odd helium tracing. The technician was sure the patient had been leaking but the FRC from this test was was actually the lowest of three tests and they weren’t sure why that was the case.

Once I saw it I was immediately able to tell the technician that it wasn’t a leak and that it was probably okay to report the results. I was able to say this because when there is a leak during a helium FRC test the helium constantly decreases and never plateaus. The rate of decrease may change but the most pertinent point is that the helium concentration never plateaus and even more importantly, it never increases.

Helium Tracing

I’ve seen this particular type of helium tracing before but to be sure I could properly explain what caused it I downloaded a table of system readings from the test software and they verified that what I thought happened was probably correct.

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Pneumotach accuracy

The first reasonably accurate flow-measuring device was the Fleisch pneumotachograph which was developed in 1925. Originally the Fleisch pneumotach bounced a light beam off a mirror mounted on a diaphragm and from there onto photographic film, all of which made it difficult to use. World War II saw the development of sensitive pressure transducers, amplifiers and recorders and by 1950 the pneumotachograph went totally electronic and began to be commonly used in routine pulmonary research.

The first spirometers that used a flow sensor came onto the marketplace around 1970. Since that time, flow sensors of one kind or another have made steady inroads and now the majority of test systems use flow sensors and there are only a small handful of volume-displacement spirometer systems still being manufactured.

There are a variety of difficulties involved with measuring gas flow rates and this has driven the development of a number of different flow measurement techniques. As well as the pneumotachograph, there are now Pitot tube, hot wire, turbine and ultrasonic flow sensors. Some of these techniques are more linear than others but none of them are perfectly linear.

The pneumotachograph however, is inherently the most linear method for measuring gas flow rates and has been more completely characterized than all other techniques. For this reason it is probably used in pulmonary function equipment more frequently than any of the other type of flow sensor and is also far more likely to be used in research.

Gas flow through a pneumotach is measured from the difference in pressure across a resistance. There are a variety of ways of creating this resistance but there are only two methods that are relatively linear, the Fleisch and the screen (aka Lilly) pneumotach.

The resistance in the Fleisch pneumotach consists of a set of narrow capillary tubes, parallel to the direction of flow.

Fleisch Pneumotach

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Student spirometers, what can they teach?

It’s been several decades since I last saw water-seal bell spirometers being used in a Pulmonary Function lab. They have been displaced mostly by systems that use flow sensors of one type of another and the small handful of equipment manufacturers that still make volume-displacement spirometers use rolling seals. This isn’t to say that this kind of spirometer isn’t being manufactured any more and in fact there appears to be a modestly thriving market in water-seal spirometers intended for use by students.

In the low-end of the market (under $1000) there are several different systems that would likely never make it into a PFT Lab. None of the manufacturers or distributors provide any claims about their accuracy and considering they are mostly made of injection-molded plastic it is hard to see what level of accuracy they could ever offer. Moreover, the volume readout for these spirometers is a dial that is moved by the chain attached between the bell and the counter-weight. The gradation on these dials allows you to measure (somewhat optimistically, I’d say) differences in exhaled volume of 0.1 liter.


Sierra Exif JPEG

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