Changes to lung function equipment standards: Breath of fresh air

Regulatory requirements are constantly changing in line with European Union regulations, International Standards Organisation (ISO) measures and other guidelines and industry norms. The past two years have seen a large number of regulatory changes concerning spirometers and peak flow meters.


Peak flow meters


You probably use peak flow meters for detecting and monitoring occupational asthma by testing lung function. Whether you use simple mechanical peak flow meters or electronic types, new standards for peak flow meters mean that you may need to replace your old units. This is important for two reasons:




  • The scale may be different on new units


  • The design life of simple peak flow meters is only three years.

Electronic peak flow meters must have their calibration certified every year. This is good practice for mechanical peak flow meters as well and could give valid reason to extend the life of a unit for more than three years.


In 1994, the American Thoracic Society (ATS) published an update to a guideline called Standardization of Spirometry, which proposed a method for testing peak flow meters. This was the first standard to give a peak flow reading comparable with that measured by spirometers. Although widely adopted by the rest of the world, it took 10 years and the publication of the EN 13826:2003 standard before it was adopted in the UK.


The publication of the EN 13826:2003 standard for peak flow meters set a mandatory requirement for all CE marked medical devices in Europe. This standard stimulated the move from the old ‘Wright’ scale (named after Dr BM Wright, the inventor of the first peak flow meter) to a new ‘absolute’ scale that correlates with spirometer measurement of peak flows. Some manufacturers named this the ‘EU scale’ – a misnomer still used today.


The EN standard added to the ATS standard, which included a good test method for accuracy, by defining tolerances and definitions for many other aspects of peak flow meters, in particular the frequency response. A device with a poor frequency response will sometimes measure correctly, but can also grossly over-read or under-read a person’s peak flow, depending on the characteristics of the device.


In 2005, a joint working party of the European Respiratory Society (ERS) and the ATS published a series of five guidelines concerning lung function testing, but there was no significant change to the 1994 guidelines in regard to peak flow meters, except to remove the concept of a ‘low range’ peak flow meter.


In June 2007, the EN 13826:2003 standard was superseded by another standard, EN ISO 23747:2007, and perhaps the mis­named EU scale devices will now change to ISO scale. However, no differentiation of scale is required since the Wright scale products were withdrawn from the market by every manufacturer in 2004.


This international (ISO) standard brings a common standard into force worldwide, via the British Standards Institution (BSI), the Deutsches Institut für Normung (DIN), the American National Standards Institute (ANSI) and all the other national standards bodies. This standard applies equally to any device that measures and gives a reading of peak flow, regardless of whether the device is a peak flow meter or spirometer, and whether the units are L/min (litres per minute) or L/s (litres per second).


Users can now be sure their spirometers and peak flow meters will give the same ‘peak flow’ reading, except that spirometers are, of course, likely to be that bit more accurate. Look for EN ISO 23747 to be marked on all peak flow meters from early 2008.


Spirometers


The 1994 BTS/Association for Respiratory Technology and Physiology (ARTP) UK spirometry guidelines are in line with more up-to-date guidelines1 and no change appears to be imminent.


Major revisions to the previous ‘gold standard’ 1993 European Respiratory Society (ERS) standards for spirometers were published in December 2005 by the joint working party of the ERS and the ATS2. These revisions took the form of the following guidelines:




  • ATS/ERS Standardization of Lung Function Testing: General Considerations for Lung Function Testing


  • ATS/ERS Standardization of Lung Function Testing: Standardization of Spirometry


  • ATS/ERS Standardization of Lung Function Testing: Standardization of the measurement of lung volumes


  • ATS/ERS Standardization of the single breath determination of carbon monoxide uptake in the lung


  • ATS/ERS Standardization of Lung Function Testing: Interpretative Strategies for lung function tests.

The 2005 ATS/ERS guidelines also gave significantly updated information in the important area of ‘normal values’ for spirometry.


A major change in the ATS/ERS 2005 guidelines was to remove the former distinction between spirometers used for ‘monitoring’ (a lower requirement) and ‘screening’ (a higher requirement). All diagnostic spirometers must now conform to the same standards, whether they are used for monitoring, screening or any other purpose.


Other changes in the 2005 standard guidelines include:




  • MVV – The measurement of direct maximum voluntary ventilation (MVV) is dropped by the standard, preferring a FEV1 (forced expiratory volume in one second) factor (indirect MVV is usually taken as 37.5 x FEV1).


  • Slow VC – VC (vital capacity) measurement is recommended prior to FVC (forced vital capacity) measurement – surprisingly, this is a new recommendation.


  • 3L Precision Syringe – A three-litre syringe is recommended for the routine daily accuracy check (confusingly called ‘calibration’ in the guideline) – three pumps of a one-litre precision syringe is no longer acceptable.


  • Linearity – Linearity checking is a new item. This is detailed for both volume displacement spirometers and flow sensing spirometers and is intended as a periodic check, not a daily check. (Linearity assessment should be a part of the routine annual planned preventive maintenance service.)


  • Back pressure – The term for the maximum allowable flow impedance (called total resistance to airflow) must be <0.15kPa/L/s (1.5 cmH2O/L/s). The total resistance measurement must include all tubing, valves, pre-filter etc, that may be inserted between the subject and the spirometer. Some spirometry machines work with a turbine fixture leading to a rotating vane which receives the flow of our rotations at a speed proportional to the flow delivered. The vane interrupts a light source and detector arrangement, and the number of revolutions measures the flow (usually via a calibration factor set at the factory). Almost all spirometers using moving vanes will fail this criteria.


  • Disposable noseclip – A noseclip or manual nose occlusion is recommended for most spirometry examination types- for example, FVC and FEV1 – and a noseclip is essential where inspiratory manoeuvres are involved.


  • Posture – A sitting posture is now recommended for all spirometry testing. A chair with arms and without wheels is recommended. If a different posture is used – for example, standing – this should be noted on the spirometry report. It is good practice to always note testing posture on spirometry reports.


  • Curve plateau – A ‘plateau’ in the volume/time curve must be achieved at the end of the blow for the test to be deemed ‘acceptable’. This acceptable plateau used to be defined as <0.03 L over 1s but has now been tightened up to a lower flow rate of <0.025L in 1s – extremely difficult for ‘turbine’ type spirometers to achieve.


  • Six-second FVC – A test duration (FET) of six seconds now defines a good end of test – this means that the subject has tried to exhale for six seconds. The spirometer device must be capable of accumulating volume for at least 15 seconds – proving the 1963 Vitalograph had it right after all.


  • Quantifying bronchodilator responsiveness – The improvement in FEV1 used to be expressed as ‘[Post – Pre] x 100 / predicted value’, but the ATS/ERS now recommend ‘[Post-Pre] x 100 / Pre’ – the increase expressed as a percentage of the initial value. There is a second criterion, the increase in mL: ‘[Post-Pre]’.

In 2008, a new ISO standard covering spirometry is due to be published. This new standard will have the same influence as the ISO peak flow meter standard has had and it will become mandatory.


Spirometer test methods are to be tightened up considerably with new test methods to prevent ‘programming’ of the test waveforms to ensure a test ‘pass’. The effect of this will be to make spirometers give much more repeatable readings. Spirometers that cannot measure low flows will fail the test as will devices that have a wide scatter in their readings – typical in ultrasonic measuring technologies.


The advancement in standards for pulmonary function test equipment over the past decade has been considerable, but it has not quite come to the ultimate conclusion. The benefit of this to the user is considerable. Providing that the operator carries out the designated routine daily procedures correctly, their equipment will be safer, more hygienic and more accurate. The role of the spirometer as a source of error will be virtually eliminated, so poor test quality will be down to the operator alone. However, this too is being addressed by manufacturers and service providers.


References




  • Recommendations of the British Thoracic Society and the Association of Respiratory Technicians and Physiologists. Guidelines for the Measurement of Respiratory Function. Respiratory Medicine 1994 38, pp 165-194


  • Mr Miller et al, Standardisation of Spirometry, European Respiratory Journal, April 2005




 

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