Right, so. Bear with me, as it’s a fairly long read, and I don’t know that anyone will have any thoughts or suggestions anyway. I am going to speak to the physiologists again, who are as desperate to understand this as I am, but thought it’s worth a punt posting here all the same to see if any of the more clinically, technically minded members have any thoughts.
With lockdown, and because she tends to be less well/problematic, bod was one of the children sent a swanky nuvoair bluetooth home spirometer by her team. All the kids got a home spirometer, it’s just the more difficult children to manage got one with a few more included bells and whistles to better track what they were doing. The first thing we discovered is that it consistently overestimates her lung function by about 300mls in absolute values. A direct comparison between the ward vitalograph and the nuvoair gave values with a percent predicted variation of 15%: the nuvo had her fev1 at 76%, where the vitalograph had her at 61% with BTS criteria met. The team are more than happy with her technique on both machines, and her blows are reproducible. The graphs produced by the nuvoair for her blows would be deemed as unacceptable and the results binned as unreliable if they came off a pneumatic spirometer, but we’ve done a control test on our device and it came back as expected, so the issue does appear to be with the user. No one else has reported this issue anywhere in the world that we’re aware of, and the adult team at the RBH were one of the units that did the initial validation study and have been using these devices for 18 months with their cohort without coming across anything like it. The manufacturers themselves have no clue at all how this is happening. We then tried another spirometer from another manufacturer but using the same base, turbine driven technology, and found exactly the same outcome: the current hypothesis is that for some reason this tech can’t cope with the fluctuation in flow caused by her underlying tracheomalacia (and any possible impact of a potential connective tissue disorder) as she has a very distinctive and unusual graph pattern, and that rather than appropriately averaging the curve as the pneumatic devices do, it’s only taking the very top of the wobbles it’s detecting. Okay, fine, I can get with that, it makes sense in so far as how we’re getting the spectacular numbers we’re consistently getting, but it does raise a number of questions as to why it’s literally only happening to her when there are plenty of people out there with cf and TM/TBM, including other children. But hey-ho, they’re potentially going to do a paper, and it’s way beyond both my understanding and my pay grade.
What really doesn’t make sense to me, is that if you put today’s absolute values into the patient.info lung function calculator, it gives percent predicted values comparable to what I would reasonably expect her well lung function to be, rather than the bizarre values the nuvoair gives for the same absolute litreage. For example, the nuvoair claimed today that her absolute litres converted to 91% fev1 and 122% fvc. Yes, I am laughing as I type that. The patient calculator for the same litreage comes back with 71% and 101% respectively, which would be a reasonable well LF for her. If you then take off the 300ml it normally overestimates the absolute values by, you actually get something that makes sense for a child that hasn’t been consistently doing physio for a few weeks, is due IVs, is more breathless than usual and has been complaining she’s tight. Even if you say that today it only overestimated by 100ml, I still get values that make more sense for the circumstances.
If you put exactly the same figures into some of the other online calculators, you get the same results as the nuvoair. I know that percent predicteds are to some extent regional, but why are the calculators showing such a vast difference in normal range value, particularly when you bear in mind this particular child has chronic pseudo, fungal issues, an abysmal lung clearance index and a CT that shows extensive lung damage? Would they really be telling me in the US that in spite of all that she still has completely ‘normal’ lung function of 90% plus for both fev1 and fvc?!?! Even taking the 300ml off, with the calculators matching the nuvo values her pp came back as 80 and 101% respectively. In writing that, I did then suddenly wonder if maybe our nuvoair for some reason is using the wrong database for comparison, but even if it was, the other turbine driven spirometer did exactly the same thing and they both only ask for broad ethnic origin when you set them up, not country of origin...I also thought that the values had been jointly standardised to ethnicity by the ATS and ERS in the last few years, anyway?
Confused. And frustrated. The only thing the nuvoair does seem to appropriately track for the child is ratio and peak flow. But I just really want to understand what’s going on: even if the theory for the overestimated absolute values is correct, why are the percent predicted values so very different for the same figures?
results from my gp about my spirometry i had on tuesday
Spirometry results - help please!
Empheysema
Spirometry test results COPD?
