THE EU PEAK FLOW METER

Transcription

1 01 THE EU PEAK FLOW METER 04 THE BUYERS GUIDE TO RESPIRATORY CARE PRODUCTS

2 THE EU PEAK FLOW METER 01 THE EU PEAK FLOW METER: HISTORICAL PERSPECTIVE, WHY CHANGE AND WHAT S AVAILABLE M.R. Miller Correspondence Dr Martin R Miller University Hospital Birmingham NHS Trust Selly Oak Hospital Birmingham B29 6JD UK martin.miller@uhb.nhs.uk The measurement of peak expiratory flow (PEF) was first introduced about 50 years ago [1, 2] as a simple and convenient measure to aid the diagnosis and management of patients with airflow obstruction. Its clinical use has waxed and waned in the intervening years and now its role in asthma management is better defined. The purpose of this article is not to argue the evidence concerning the use of PEF monitoring in the management of asthma [3], but to discuss how the measurement of PEF has changed recently and which types of meter may best suit certain applications. After covering the background to where PEF meters are today, the article will give an understanding of the characteristics of various types of meter for recording PEF, followed by a discussion concerning the clinical settings where PEF monitoring is used, so that the best match of device to purpose can be made. Background The first PEF meters [1, 2] represented a tremendous step forward in the ability to record aspects of lung function in a simple and repeatable way in order to help manage common lung disorders. The original calibration of these devices was undertaken to the best degree available at the time. Subsequent work found that when systems were available that could generate accurate and repeatable known flows, the readings from these meters were inaccurate in the midrange, with a significant overreading [4, 5]. While some believed this error was relatively unimportant for a device used to look at changes in PEF within individuals, in fact the measurement of within-subject PEF variability a hallmark of asthma is importantly distorted by this error such that clinical decisions based on PEF monitoring had in the past underestimated asthma severity [6]. New standards for accuracy were set [7] and have now been refined in a European Commission standard [8] that means all PEF meters sold in European Union countries now meet accuracy requirements and no longer need their results correcting for the known error in older instruments. Furthermore, the new standard covers aspects THE BUYERS GUIDE TO RESPIRATORY CARE PRODUCTS 05

3 01 THE EU PEAK FLOW METER Figure 1. Some of the devices available. (Left to right and top to bottom: MicroLife, Personal Best, PiKo-1, Pocket Peak, Microplus, Pulmolife, Micro, Mechanical Peak Flow Meters, Asma-1). concerning frequency response (vide infra) that previously had been ignored. PEF meter characteristics For a PEF meter to function correctly, it must be accurate within the required flow range, it should offer a suitably low resistance to flow and it must be able to record the types of flow input signals that patients produce, i.e. the frequency response characteristics must be adequate for purpose. In addition, the meter must be reliable and appropriately robust for its purpose. There are now many types of meter available that are of small size and practical to use (fig. 1). These use a range of transduction systems to convert the flow of gas through the device into a signal that can be measured. As the performance characteristics of meters are dependent on the type of transduction system, this aspect will be discussed first, followed by the performance requirements. Transduction systems For a PEF device to work, the rapid flow of gas through the instrument has to be converted into a signal that can be measured. The methods most commonly used in devices available today are described below. Variable-orifice devices Figure 2 shows the design principle for these devices. At the start, a 6 THE BUYERS GUIDE TO RESPIRATORY CARE PRODUCTS

4 THE EU PEAK FLOW METER 01 PRODUCT LISTINGS EU Standard EN13826 Designed specifically to meet EN13826 scale standard Movable asthma management colour zones that won t wash off In-built internal check valve, avoiding cross-infection concerns Available with either European or ATS scale Micro Medical Ltd PO Box 6, Rochester, Kent ME1 2AZ, United Kingdom Tel +44 (0) Fax +44 (0) MicroPeak Peak Flow Meter Micro DiaryCard Spirometer The new standard in Peak Flow measurement A new powerful and flexible replacement for paper diaries and mechanical PEF meters Designed specifically for European standard (EN13826) High visibility EU scale ATS scale version also available Wash-proof Colour-Zone asthma management system Built-in check valve Fully customisable for promotional purposes Easily cleaned Measures and stores PEF, FEV 1 and FVC, Flow/Volume curves and symptom responses All data electronically date and time stamped Easily configured for clinical trial protocols Simple to use for patients at home Secure audit path for regulatory clinical trials Phone: +44 (0) Phone:+44 (0) THE BUYERS GUIDE TO RESPIRATORY CARE PRODUCTS 7

5 01 THE EU PEAK FLOW METER Flow Figure 2. Variable-orifice device such as the mini-wright meter. Flow Flow P1 > P2 Figure 3. The Venturi or fixed-orifice principle. Figure 4. The stator/rotor device. a) b) P1 > P2 Figure 5. a) Fleisch and b) Lilly pneumotachographs. P1 > P2 baffle occludes the orifice of the mouthpiece. When the subject blows through the meter, the baffle moves forward, uncoiling a spring. As the baffle progressively moves up the meter it exposes more and more of an exhaust orifice (shaded grey in the figure). The area of the orifice exposed is proportional to the peak flow. Some devices have a bending piece of metal (Pocket Peak), rather than a baffle attached to a coiled spring, that opens up the area of the orifice (mini-wright, Micro Peak, Vitalograph mechanical meter). The reading from these devices is dependent on gas density and so readings are affected by marked changes in atmospheric pressure such as at high altitude. The usual ambient weather variations in barometric pressure and temperature are not important in this respect. Fixed-orifice or Venturi devices Figure 3 shows the principles for devices where the pressure drop across a fixed orifice is measured. This drop is proportional to the square of the flow. The flow is turbulent and so the signal is dependent on gas density (i.e. altitude). These devices require pressure transducers to be very accurate in the low-pressure range, as the signal is very small at low flows, such as when recording at the end of the manoeuvre. The nonlinearity of these devices can be corrected by electronics holding an array of calibration factors for the range of flows. Stator/rotor devices Figure 4 shows the principle of devices where the flow of gas is turned into a vortex, usually by passing it through a fixed set of angled blades. This vortex then rotates a finely balanced vane that is very light with low inertia and set with precision bearings. The movement of the vane can be measured by its rorotation, which causes the path of a visible light or infra-red beam to be interrupted. These interruptions are counted and the frequency of the counts is related to flow. These devices have a stable calibration and the output is not altered by changes in gas composition which makes them suitable for recordings at altitude. The engineering of the bearings is critical to prevent the vane from either stopping too early or overrunning, either of which would affect the registration of low flows. Pneumotachographs Pneumotachographs were the first truly accurate flow meters to be used in respiratory practice. They are essentially of two designs: the Fleisch flow head (fig. 5a) and the Lilly mesh screen (fig. 5b). The pressure drop recorded is dependent on the viscosity of the gas passing through and not its density, so the signal is not altered by altitude but is sensitive to temperature and condensation building up in the flow head. The Lilly has less thermal mass but is intrinsically not quite as linear as the Fleisch [9]. The latter has considerable thermal mass and the flow head can change in temperature over a period of testing, which will influence the results. To avoid condensation and keep the flow-sensing element at a stable temperature, it is best practice to place these devices in the flow stream of a fan between blows [10]. Heating of the flow head can be undertaken to help prevent condensation and maintain thermal stability, but the heating controller must be extremely sophisticated to avoid temperature changes [11], so using flow heads unheated when recording single expiratory manoeuvres is more accurate. These devices need accurate and regular calibration and so are essentially only suitable for a laboratory, workplace or primary care setting when used by technical staff adequately trained in their maintenance. 8 THE BUYERS GUIDE TO RESPIRATORY CARE PRODUCTS

6 THE EU PEAK FLOW METER 01 Table 1. A list of manufacturers, their devices with characteristics and estimated cost. Manufacturer / Supplier Website Device Transduction PEF only Price Logging Clement Clarke mini Wright variable orifice yes 15 no International airzone variable orifice yes 15 no Edinburgh Way mini Wright digital fixed orifice no 35 yes Harlow one flow tester fixed orifice no 535 yes Essex CM20 2TT, UK Micro Medical MicroPeak variable orifice yes 10 no PO BOX 6 PulmoLife stator/rotor no 165 no Rochester Micro Spirometers stator/rotor no 335 yes Kent. ME1 2AZ United Kingdom (part of VIASys Healthcare Inc.) nspire Health Pocket Peak variable orifice yes 25 no Louisville, Colorado, PiKo-1 fixed orifice no 25 yes USA KoKo fixed orifice no 25 yes (formerly Ferraris Respiratory) ndd Medizintechnik Easyone ultrasonic no 1875 Technoparkstrasse 1 CH-8005 Zürich, Switzerland Respironics peakflowfamily.respironics.com Assess variable orifice yes 15 no 1010 Murry Ridge Lane AsthmaCheck/Mentor variable orifice yes 15 no Murrysville, Personal best variable orifice yes 15 no PA , USA Vitalograph mechanical PEF meters variable orifice yes 10 no Maids Moreton, Asma-1 stator/rotor no 28 yes Buckingham Vitalograph Micro pneumotacho- no 500 yes MK18 1SW, UK graph Table 2.. Clinical settings for recording PEF and instrument requirements Setting Accident and emergency, outpatient dept, ward Home, workplace Lung function laboratory, primary care Requirements to consider Low cost, robust, multi-patient use, fixed calibration Low cost, robust, fixed calibration, data logging Absolute accuracy, multi-patient use Flow Figure 6. The ultrasonic flow meter. Ultrasonic Figure 6 shows the design for devices where an ultrasonic beam is passed diagonally across a flow of gas and the transit of the beam is affected by the gas flow according to the Doppler effect. By sending beams in both directions, various corrections can be made to render this an extremely accurate and low-resistance device. The calibration is stable and the output is affected by properties that determine the speed of sound, such as gas density. The crosssectional area of the beam needs to be adequately matched to the diameter of the tube through which the gas flow profile is passing. For example, measuring the Doppler effect on a very narrow beam passing across the flow in a much wider-bore tube would not give a signal representative of the whole of the flow profile. These devices are technically sophisticated and offer promise for many applications. Their calibration is stable, which has advantages for non-laboratory applications. Accuracy All PEF meters in the EU must meet the CE standard PrEN13826 [8], which means the accuracy of readings from handheld PEF meters will now agree with the standards met by larger laboratory-based equipment. In addition, the meters have to meet a drop test requirement to show they are suitably robust for patient selfadministered use. PEF meters in use in the USA do not have to meet THE BUYERS GUIDE TO RESPIRATORY CARE PRODUCTS 9

7 01 THE EU PEAK FLOW METER PRODUCT LISTINGS PiKo Monitors Mini-Wright Digital nspire Health introduces PiKo Lung Health Monitors, the world s smallest electronic PEF, FEV1, and FEV6 meters. All PiKo monitors provide automatic test quality alerts, electronic data storage of 96 patient tests, single button review, and offer optional PiKoNET software for extended tracking and trending. PiKo monitors are downloadable to a personal computer. Revolutionary in their design, these miniaturized devices exceed all published accuracy standards, and remarkably, is priced at a similar cost to mechanical peak flow meters. For more information call +44 (0) or visit THE NAME SPEAKS VOLUMES It s Intelligent. It s Simple. It s Accurate. It s Compact. It s Easy to Clean. It s a Mini-Wright. Not only the name speaks volumes, so does the product. Accurate PEF and FEV1 measurements for treating both asthma and COPD patients at the touch of a button. Easily customised, intelligent patient monitoring in the palm of your hand. A water tight seal for simple sterilisation required under industry standards. And a name synonymous with accuracy, reliability and performance. Visit to find out more about the digital peak flow meter that speaks volumes or for the distributor nearest you. Phone: +44 (0) Phone: Telehealthcare SIBEL DATOSPIR PEAK-10 A modern medicine technique with definitive solutions Remote Patient Monitoring (RPM) Effective monitoring of chronic disease Effective therapy control Cost efficient Remote diagnostic possibilities Selective emergency management The DATOSPIR PEAK-10 peak flow meter provides an objective measurement of your Peak expiratory Flow (PEF) and is valuable information of your lung functions. It includes a colour zone asthma management. The correct use of this instrument will help you and your doctor to follow your condition closely and will enable him/her to provide the most beneficial treatment for you. It is important to keep a record of your peak flow. Readings should be taken twice a day; first thing in the morning and at bed time, or as directed by your physician. You should also measure and note your peak flow if you suddenly become wheezy or breathless at any time of the day or night. The peak flow meter DATOSPIR PEAK-10 has been designed and manufactured according to the Medical Device Directive 93/42/CEE. This device meets new EN scale standard. This device has been verified and calibrated at SIBEL, S.A., according to the verification and adjustment procedure. Phone: +49 (0) Phone: THE BUYERS GUIDE TO RESPIRATORY CARE PRODUCTS

8 THE EU PEAK FLOW METER 01 this standard but are tested against a less demanding standard from the American Thoracic Society [7]. The readings from old devices based on the original Wright meter scale [1, 2] can be clinically misleading [6], depending on where across their range the reading is being made [4, 5]. The CE standard has addressed these accuracy issues and also addressed the frequency characteristics of these devices for the first time. It had been noted by paediatric clinicians that astute children could generate falsely high PEF readings by tongueing the delivery of the blow a bit like spitting into the meter which in under-damped meters would give a high flow. This new aspect of PEF meter testing found that some meters available in the USA were severely under-damped [12], that is they over-read with flow profiles that have a rapid rise up to and fall off from the peak. These meters do not meet the new EC standard, which is the only standard to test this aspect of performance. Calibration Most hand held PEF meters do not need day-to-day calibration. This is ideal for unsupervised patient use of the meter. The more sophisticated flow measuring devices common in lung function laboratories do need frequent calibration, since their transduction systems for converting flow into a measurement signal are usually not so stable and are influenced by ambient conditions. Although devices are marketed as not requiring calibration, it is possible with misuse or accident that the device no longer reads correctly. Most manufacturers state that if a device gives surprising readings then a calibration check procedure is required [13]. For handheld devices that are only giving a flow output, such as variable orifice meters, a calibration check is beyond the capability of all but the most sophisticated laboratories, since a pump system is needed that can deliver calibrated flows [14]. Human calibration can be used where, for instance, lung function laboratory staff who are free of lung disease regularly check their own PEF on a meter. This can soon detect an unexpected trend in the output of the device. If the device records volume as well as flow, then discharging a 3-L syringe several times through the device using a range of flows can give an indication if there is a significant error in the device. Resistance to flow PEF meters offer a complex impedance to flow, which will be referred to here as a resistance. When compared with variableorifice meters, some devices, such as pneumotachographs or ultrasonic devices, offer very little resistance. This can be easily appreciated when blowing through one such device and then through a variable-orifice meter. Standards for PEF meters state what is the acceptable limit for resistance [8, 13]. Turbine devices offer greater resistance at higher flows, while for variable orifice meters it is the reverse [15]. A highresistance meter has been found to reduce the achieved PEF by an average of 8% compared with PEF measured with a low-resistance device, but the PEF readings were also less variable [16], which may be an advantage in some settings. Reliability The variable-orifice meters such as the original mini-wright meter have working parts that have been found to work correctly for up to 15 years of continuous patient use without loss of accuracy [17]. However, if a PEF meter is mistreated then it can fail to work properly even when new. Thus instruction to the user about how to use and care for the meter is a key part of the clinician s role in suggesting the patient uses THE BUYERS GUIDE TO RESPIRATORY CARE PRODUCTS 11

9 01 THE EU PEAK FLOW METER a PEF meter to help manage their asthma. This is equally true for healthcare professionals in accident and emergency depts or in primary care who use PEF meters. Cleaning of PEF devices can cause more harm than benefit and so the manufacturer s instructions must be followed. Disassembling a device can lead to damage to important key components, such as the spring in the mini-wright meter and other variable-orifice meters, which then render all subsequent readings invalid. Many devices are affected by excess heat and so leaving a device close to a heater may distort its component parts, rendering it useless. Environmental conditions Most PEF meters are used at home, at work or in a hospital setting. The ambient conditions in these settings do not usually present a problem. If accurate PEF recordings are needed at altitude, then turbine devices or pneumotachographs would be the best choice, as their readings are not affected by the changes in gas density at altitude [14]. Clinical settings Accident and emergency depts and outpatient depts In this setting, the instrument needs to be readily available and suitable for multi-patient use. Devices through which the patient cannot inhale can be used for different patients without the necessity for ensuring cleaning of the insides of the instrument itself. One-way mouthpieces can also be used to facilitate the use of one instrument for difference patients. Ideally this environment also requires rugged, robust and calibration-free instruments. In-patient wards Ward patients with asthma will be recording their PEF for several days while they recover and can individually use a single instrument for this time. A number of patients will take this instrument home on discharge to facilitate the monitoring of their condition. The environment is otherwise as demanding as the accident and emergency dept. Cleaning of such meters should be undertaken, especially for high-risk patients such as those with cystic fibrosis and tuberculosis. Suggestions have been made about how this can be done for simple mechanical variable orifice meters [18]. The manufacturer s instructions must be followed to ensure the PEF readings remain accurate after cleaning. Home, school, work These settings require the PEF meter to be used by the patient without supervision. Thus the instrument has to be reliable, robust and calibration free. Most devices for patient use fulfil these requirements. However, the results from unsupervised recordings can be less than accurately registered for a number of reasons [19, 20]. Electronic data logging meters may then be necessary to obtain faithful results. Another consideration is that if the readings are to be taken in open social environments, the instruments should not be awkward to use. For children the device may need to look cool to be acceptable and so increase the likelihood it will be used appropriately. Primary care Increasingly, it is hoped that satisfactory lung function measurements will be made in the primary care setting rather than in a hospital laboratory. There are tensions between the technical requirements for using instruments for diagnosis, which require that their calibration and results are accurate, and the simplicity of cheaper calibration-free instruments. In primary care other lung function indices, such as forced expiratory volume in one second (FEV1) may 12 THE BUYERS GUIDE TO RESPIRATORY CARE PRODUCTS

10 THE EU PEAK FLOW METER 01 also be required from the instrument in order to facilitate making a diagnosis in chronic obstructive pulmonary disease or asthma. When measurements are made at a single interaction that then lead to a diagnosis being made, there is a necessity for the measurements to be undertaken with the highest technical standards being achieved [13]. Laboratory In a pulmonary function laboratory, PEF is often recorded incidentally at the same time as other spirometric measures. Evidence suggests that it is essential that the device must be recording flow as the primary measure in order to get an accurate representation of PEF. Differentiation of volume measurements with respect to time to obtain flow accentuates any noise on the signal and complex smoothing algorithms have been recommended [7, 21] to obtain an estimate of PEF from volume measuring equipment. This practice is not recommended since the frequency response characteristics to record PEF [12] are beyond that which volume measuring equipment can readily achieve. Table 1 includes a nonexhaustive list of manufacturers and their devices with some essential characteristics that can help determine the best choice for the patient or setting. Table 2 outlines the various clinical settings for recording PEF and the likely characteristics for choice of device. Conclusion PEF meters are useful in a wide variety of settings. From its origins in small, cheap, handheld PEF meters the recording of PEF has moved on to now include equally small and useable devices which are more sophisticated, with additional features such as recording FEV1 and data logging for subsequent download into a computer. It is remarkable and a credit to engineering that for extremely low cost there are very accurate and reliable devices available. With the information contained in this article, it should be possible to choose the best device for the required clinical application. REFERENCES 1. Higgins ITT. Respiratory symptoms, bronchitis and ventilatory capacity in a random sample of agricultural population. BMJ 1957; 2: Wright BM, McKerrow CB. Maximum forced expiratory flow rate as a measure of ventilatory capacity with a description of a new portable instrument measuring it. BMJ 1959; 2: The BTS/SIGN British Guideline on the management of asthma. Thorax 2003; 58: Supplement 1. Updated Nov 2005 from: /asthmaupdatenov05.pdf. Date last accessed: June 14, Miller MR, Dickinson SA, Hitchings DJ. The accuracy of portable peak flow meters. Thorax 1992; 47: Gardner RM, Crapo RO, Jackson BR, Jensen RL. Evaluation of accuracy and reproducibility of peak flow meters at 1400 m. Chest 1992; 101: Miles JF, Tunnicliffe W, Cayton RM, Ayres JG, Miller MR. Potential effects of correction of inaccuracies of the mini- Wright peak expiratory flow meter on the use of an asthma self-management plan. Thorax 1995; 51: American Thoracic Society. Standardization of spirometry update. Am J Respir Crit Care Med 1995; 152: European Standard pren Peak Flow Meters. London, British Standards Institute, Fry DL, Hyatt RE, McCall B, Mallos AJ. Evaluation of three types of respiratory flowmeters. J Appl Physiol 1957; 10: Miller MR, Sigsgaard T. Prevention of thermal and condensation errors in pneumotachographic recordings of the maximal forced expiratory manoeuvre. Eur Respir J 1994; 7: Miller MR, Pincock AC. Linearity and temperature control of the Fleisch pneumotachograph. J Appl Physiol 1986; 60: Miller MR, Atkins PR, Pedersen OF. Inadequate peak expiratory flow meter characteristics detected by a computerised explosive decompression device. Thorax 2003; 58: Miller MR, Hankinson J, Brusasco V, et al. Standardisation of spirometry. Eur Respir J 2005; 26: Miller MR, Jones B, Xu Y, Pedersen OF, Quanjer PH. Peak expiratory flow profiles delivered by pump systems: limitations due to wave action. Am J Respir Crit Care Med 2000; 161: Pedersen OF, Miller MR, Sigsgaard T, Tidley M, Harding RM. Portable peak flow meters: physical characteristics, influence of temperature, altitude, and humidity. Eur Respir J 1994; 7: Pedersen OF, Rasmussen TR, Omland Ø, Sigsgaard T, Quanjer PH, Miller MR. Eur Respir J 1996; 9: Miles JF, Bright P, Ayres JG, Cayton RM, Miller MR. The performance of Mini Wright peak flow meters after prolonged use. Respir Med 1995; 89: Bell J. Peak flow meter manufacturer s comment single patient use? Prim Care Respir J 2002; 11: Verschelden P, Cartier A, L Archevêque J, Trudeau C, Malo JL. Compliance with and accuracy of daily selfassessment of peak expiratory flows (PEF) in asthmatic subjects over a three month period. Eur Respir J 1996; 9: Malo JL, Trudeau C, Ghezzo H, L Archevêque J, Cartier A. Do subjects investigated for occupational asthma through serial peak expiratory flow measurements falsify their results? J Allergy Clin Immunol 1995; 96: American Thoracic Society. Standardization of spirometry: 1987 update. Am Rev Respir Dis 1987; 136: THE BUYERS GUIDE TO RESPIRATORY CARE PRODUCTS 13

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