Article By: Alex Stenzler, Martin Stegenga
The gold standard considered by almost everyone who performs Forced Vital Capacity (FVC) spirometry measurements is, “Does the spirometer meet the ATS/ERS guidelines, and has it passed the 24 waveform tests the Task Force created in 1994”.1,2,3 Overwhelmingly, manufacturers tout their FDA clearance and the passing of the 24 waveforms when selling their spirometers. However, there are many spirometers sold in the US, that have all been cleared by the US Food and Drug Administration, that don’t really meet all of the requirements of the American Thoracic Society and European Thoracic Society (ATS/ERS) guidelines for spirometry measurements that specifically called out low flow measurements. These spirometers may produce significant errors when testing patients with severe obstructive lung disease. This is particularly important when testing patients with COPD.
We are not the first to point this out. In 2014, Lefebvre and Vandergoten, et al, questioned whether the standard curves of the American Thoracic Society are sufficient.4 They compared five office spirometers with a 3L syringe at three different flows and against the ATS/ERS waveforms, and then again with waveforms collected from patients and programmed into the same Hans Rudolph 1120 flow-volume simulator. Three of the patient waveforms were from patients with Stage IV COPD.
Testing with the 3L syringe, all spirometers measured the volumes accurately. In the nine different curves from the ATS/ERS waveform set, they reported biases in some of the spirometers, with some measurements falling outside the ATS/ ERS requirements. With the patient curves, they found significant errors in measurements where there were steep rise times or low expiratory flows, as seen in patients with severe COPD. They stated that “an ATS/ERS label may be provided to devices that do not deserve it”. They proposed that there should be new waveforms used to test spirometers that are more realistic than the original ones.
Alex Stenzler is the president of 12th Man Technologies, Inc. He was a Registered Pulmonary Function Technologist and has been working in the field of pulmonary function testing since 1968. He spent half of his career managing Respiratory Care Departments, COPD Clinics and Pulmonary Function Laboratories, or teaching pulmonary function technology as an Assistant Professor at the State University of New York at Stony Brook and twelve years at the Graduate School of Adelphi University. He has been the PI for two NIH and one DARPA grant and has extensive research experience. He has co-authored 16 peer-reviewed papers and one book chapter on pulmonary function testing. He holds 45 patents, most of which are related to pulmonary medicine. Martin Stegenga, B.Eng. is a computer engineer at 12th Man Technologies with seven years experience developing spirometry systems.
McCarthy, in his publication on selecting spirometers for home monitoring, reported that many of the spirometers on the market, do not meet the low flow requirements of the ATS/ERS standards.5 This low flow requirement, separate from passing the 24 waveforms, is that spirometers must be able to measure flows down to 0.025 L/sec. In the 2005 ATS/ERS publication, it specifically states, “The level of minimum detectable flow should be 0.025 L/sec”.2 In the just released 2019 ATS/ERS standard, the wording has changed in the section on determining whether patients reach a plateau to “There is less than a 0.025 L change in volume for at least 1 second (a “plateau”)”.3 Therefore, to detect a change of 0.025 in 1 second, the spirometer must be able to measure flows at 0.025 L/sec. Although worded differently, the requirements are unchanged. In McCarthy’s publication, there was a link to a video (https://respiratorytherapy.ca/videos/Kevin_ McCarthy_Spirometers.php) demonstrating that some turbine spirometers stopped recording at flows significantly above this ATS/ERS requirement.5
To understand why spirometers may pass the ATS/ERS waveforms and yet not meet the low flow requirements of the ATS/ERS and produce clinical results as reported by Lefebvre, an understanding of how the waveforms are generated is important. There are multiple manufacturers of waveform generators. These are program-controlled pistons that push air out of the piston in steps defined by a set of instructions that were provided by the ATS/ERS standards Task Force. Each volume step in a unit of time generates a precise flow. As can be seen in Figure 1 below, which is the flow versus time of waveform 17-1 from the set of 24 waveforms, when flows get to the low end of exhalation, there are spikes of flow separated by periods of no flow. The lowest spike of flow is 0.14 L/sec, 5.6 times higher than the minimum flow of the standard. When you average out the spikes of flow with the time periods of no flow, you can reach averages of 0.025 L/sec. However, patients don’t exhale with spikes of flow separated by periods of no flow. So, while spirometers that can measure down to 0.14 L/sec can pass the ATS/ERS waveforms, they may not be able to accurately test patients with severe airway obstruction with flows of 0.025 L/sec. This can be a serious deficiency in some spirometers.
We decided to test a range of spirometers for their ability to accurately measure flows at the required low flow threshold.
Using a precision driven piston, which is used to test inhaled drug delivery systems (Copley Scientific Model BRS3000, UK) and a fixed flow of 0.025 L/sec (1.5 L/min) with a critical orifice (Figure 2), we compared the spirometers’ reported FVC without and with the bias flow. Measurements were performed in triplicate and the average values reported. When the feature existed in the spirometer, we removed the BTPS correction factor.
For the baseline measurement, we set the piston to deliver a volume with a sinusoidal flow pattern of approximately 3 liters within 2 seconds. This produced a peak flow of 2.4 L/sec. We had each spirometer measure this volume 3 times. We then added the bias flow of 0.025 L/sec, teed into the piston flow, and let the low bias flow continue out to 15 seconds. We chose 15 seconds as this is the requirement of the new ATS/ ERS standard.3 The addition of 15 seconds of 0.025 L/sec flow will increase the volume by 0.375 Liters. The actual bias flow for each measurement was measured with a calibrated TSI (TSI Incorporated, MN) hot-wire laboratory flow meter.
We tested eight different spirometers, one of which was tested with its two different flow sensor options (disposable and reusable). They included three ultrasonic spirometers, three turbine spirometers and three pneumotach based spirometers. All spirometers claim to be approved to measure FVC.
Table 1 below presents the results from these tests. Because there may be some bias by each spirometer and because some spirometers have built-in BTPS corrections that we couldn’t change for testing with room air, to determine the ability to accurately record the low flow additional volume, we calculated the difference between the baseline measurement and the measurement with the known added flow. The bias flow across all the measurements ranged from 0.0257 L/sec to 0.0262 L/sec for the 15 seconds, accounting for a small potential difference in added volume of 0.007 L (7 mL).
Six of the nine spirometers failed to measure the low flow at 0.025 Liters per second as required by the ATS/ERS standard. The ability to measure the low flow was not technology specific as the spirometers that passed included two ultrasound and one vertical turbine. All of the spirometers seemed to read the 0.025 L/sec bias flow when it was a component of the piston flow, resulting in a higher volume during the bias flow even while failing to measure the flow when it was the only flow component.
With an increased interest in monitoring patients in their homes with measurements of FVC, there has been an explosion of new spirometers that supposedly meet the ATS/ERS requirements for FVC measurements. We decided to evaluate both the new spirometers as well as spirometers that have been on the market for some time to determine if they met the ATS/ERS low flow requirements.
We found that the majority of spirometers being sold for the measurement of Forced Vital Capacity, are actually unable to measure the low flows required by the ATS/ERS standard and that this has important implications, particularly when testing patients with severe obstructive disease. It’s not just the error of the measurement that is the concern, but the fact that many of these spirometers stop measuring when flows near the low limit required to identify the End Of Forced Exhalation (i.e., the plateau).
We agree with the conclusion of Lefebvre that many spirometers that have the ATS/ERS label do not deserve it. More importantly, since the introduction of the new ISO 26782 waveform set with more representative waveforms, even spirometers that can pass those tests, do not appear to meet the low flow requirements of the standards.6 Perhaps spirometer manufacturers need to revisit their technology with a determination to make them useful for patients with severe lung disease so that the healthcare personnel, or the patients who monitor themselves get actionable data and not just numbers.
Physicians and healthcare providers who monitor patients with severe obstructive lung disease should carefully evaluate which spirometers they select for monitoring their patients.
- Standardization of Spirometry, 1994 Update. American Thoracic Society. Am J Respir Crit Care Med 1995; 152(3):1107-1136.
- Brusasco V, Crapo R, Viegi G, et al. No. 5 in SERIES: ATS/ERS Task Force: Standardization of Lung Function Testing, Standardization of Spirometry Eur Respir J, 2005, Vol 26, pp. 319-338.
- Graham BL, et al. Standardization of Spirometry 2019 Update, An official American Thoracic Society and European Society Technical Statement. 2019; 200(8):e70-e88.
- Lefebvre Q, Vandergoten T, Derom E, Marchandise E, LiistroG. Testing spirometers: are the standard curves of the american thoracic society sufficient? Respir Care. 2014 Dec;59(12):1895-904.
- McCarthy K. Selecting spirometers for home testing. Respiratory Therapy 2017; 12(4):14-17.
- International Organization for Standardization. ISO 26782. Anaesthetic and respiratory equipment: spirometers intended for the measurement of time forced expired volumes in humans. Geneva.