Ultrasonic spirometers

There are at least a half dozen companies that use an ultrasonic flowmeter in their spirometer. The first patent for an ultrasonic flowmeter was made in the 1970’s but it wasn’t until the 1990’s that the first ultrasonic spirometers came to market. The basic idea is fairly simple and that is to measure the transit time of ultrasonic pulses through flowing gas. Pulses that travel in the same direction the gas is flowing will take less time to travel a given distance, while pulses traveling against the direction of gas flow take a longer time.

This particular measurement process is called time-of-flight (as opposed to doppler shift) and has a relatively flat flow/signal curve and frequency response. An early design of this kind of flowmeter had the ultrasonic transducers sitting in the flow of gas, but this both impedes the flow of gas and is hard to clean. A transverse design was developed that put the transducers outside the path of gas flow and this configuration has been used in all ultrasonic spirometers.

Ultrasonic flowmeter

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Temperature (in)correction

Sooner or later we all get lucky and find ourselves able to replace older equipment. When you have equipment that’s so old it can’t be repaired (either because the manufacturer no longer supports it or because the manufacturer no longer exists), you’d think this would be a no-brainer but money is always in short supply. I’ve often had to try to keep equipment running long past its expected life time and was only allowed to replace it when it finally broke beyond all hope of repair.

One of the reasons to perform biological QC is so that you can recognize changes in the equipment that don’t appear during a calibration. It also a useful (and recommended) way to assess new test equipment. So what happens when you finally get that new test system and your results are substantially different from what they were before?

I was recently contacted by the manager of an employee health service that had replaced their 18 year old spirometer with a brand-new one. When using their new spirometer they had found their biological QC results coming out noticeably lower (-9%) than they had gotten from their old spirometer and I was asked if I could help them determine why.

My first question was whether or not they were using the same 3 liter syringe to calibrate the different spirometers. Once I found out this was the case I then asked them to use the 3 liter syringe in test mode. The results from this were actually very informative. The old spirometer showed an average FVC of 3.24 liters and the new spirometer showed an average FVC of 3.06 liters.

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Filter FUD

A lab manager recently emailed me and asked my opinion about whether it was okay to use generic mouthpiece filters on their test systems. They had asked the same question of their equipment manufacturer and received the following statement (parts of which have been redacted by me):

“The [model number] PFT system was designed/tested/certified using the [manufacturer’s] filter. While other “off-label” filters may fit our devices, they have never been tested or approved for use by [the manufacturer]. The precision and accuracy of our devices could be compromised by using different type filters. It is our recommendation that you continue to use the [manufacturer’s] approved filters with your PFT equipment.”

Since I doubt the manufacturer has tested their equipment with any other mouthpiece filters than those they sell this is in some ways a true statement. Having said that, it is also a statement designed to sow fear, uncertainty and doubt (FUD) in the minds of their customers about a subject that is relatively straightforward.

The human respiratory tract is a potential source of particles in the 0.1 to 20 micron range, particularly when coughing but even to some extent during quiet breathing. Mouthpiece filters are barrier filters and intended to prevent these particles from getting into PFT equipment. Filter manufacturer’s claims are very similar and usually state a “Bacterial filtration efficiency: > 99.999% and Viral filtration efficiency: > 99.99%”. In one sense this statement is somewhat disingenuous because mouthpiece filters are not tested with bacteria or viruses (which have diameters as small as 0.03 microns) directly, but are instead tested with aerosols generated by a nebulizer.

A HEPA (High Efficiency Particle Absorption) filter is a true bacterial filter and to meet standards it must filter out 99.97% of all particles 0.3 microns or larger. Mouthpiece filters are not HEPA filters, partly because of cost but far more importantly because HEPA filters have a lot of resistance to air flow. A HEPA filter is a sieve mouthpiece with opening sizes that prevent particles above a specific size from passing through. Mouthpiece filters instead work by impaction and electrostatic attraction. Larger particles are captured by impacting or otherwise being intercepted by the filter fibers and the fibers usually also have an electrostatic charge that attracts smaller particles.

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Hyperventilation Syndrome

While reviewing a CPET I noticed the patient had a low PETCO2 throughout exercise and an elevated Ve-VCO2 slope. In addition the patient’s minute ventilation was on the high side (75% of predicted) at peak exercise. This is something you might expect to see in association with pulmonary vascular disease but the subject had a normal DLCO; normal spirometry; their oxygen saturation was normal at all times; and they had a normal maximum VO2 and a normal VO2 at anaerobic threshold. Since there didn’t seem to be any clinical reason for the low PETCO2 I had to wonder whether it was due to hyperventilation syndrome (HVS).

Hyperventilation syndrome is something that everybody “knows” about but is still somewhat ill-defined and this is at least partly because it is most often diagnosed solely by patient-reported symptoms. My lab does not have any diagnostic criteria for hyperventilation syndrome and for this reason I decided to review the literature on the subject.

Hyperventilation syndrome is usually suspected when a patient has rapid, shallow breathing with an irregular breathing frequency and with frequent sigh breaths. Common complaints are dizziness, dry mouth, tingling sensations in the hands and feet and often in combination with chest pain. These symptoms may raise the suspicion that a patient has hyperventilation syndrome and the classic way to diagnose HVS is has to have the patient perform a Hyperventilation Provocation Test (HVPT). During this test a patient voluntarily hyperventilates for three minutes and is then asked whether they felt the symptoms they had been complaining of occurred while they were hyperventilating.

The causes of HVS are considered to be primarily psychosomatic and the majority of articles written on the subject primarily explore this aspect. There are surprisingly few articles on the physiology of HVS and for this reason the physiological causes and consequences of HVS are poorly understood. Of note, I reviewed a couple dozen textbooks on pulmonary function testing and pulmonary diseases that I have on hand and found hyperventilation syndrome to be mentioned in only one (Cotes) where it merited one relatively small paragraph.

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