Jun 21, 2013 - An electronic peak flow meter with optional coded display ... The minus sign and decimal point are incorporated in the coding. Figures 3(a) and ...
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An electronic peak flowmeter with optional coded display
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 1984 Clin. Phys. Physiol. Meas. 5 201 (http://iopscience.iop.org/0143-0815/5/3/004) View the table of contents for this issue, or go to the journal homepage for more
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Clin.
Physiol. Meas., 1984, Vol. 5, No. 3, 201-206. Printed in Great Britain
An electronic peak flow meter with optional coded display R B Richardson?, C M B HiggsS, D A Lea$, G T R Lewis and G Laszlo Work conducted at Respiratory Department, Bristol Royal Infirmary, Bristol, England* iDepartment of Medical Physics, Frenchay Hospital, Bristol $ Senior Medical Registrar, Royal United Hospital, Bath 8 Department of Electrical Engineering, Polytechnic of Wales, Pontypridd, South GIamorgan Received 16 April 1984 Abstract. A portable electronic spirometer giving peak expiratory flow rate has been developed and used successfully in studies of asthma. A turbine flow transducer produces a pulsed output whose frequency is proportional to flow. The output was coded to keep the results ‘blind‘to the patient. The code is suitable for use on other electronic instruments employing a similar display. It has been successfully used to investigatepatients’ perception of their asthma.
1. Introduction
In the investigation of asthma, it is customary to give patients a peak flow meter and a diary card to record their peak expiratory flow rates (PEFR),together with some index of symptoms. However, there are situations when it is desirable that the PEFR value should remain unknown to the patient. For example, documentary evidence of changes in PEFR in relation to occupational exposure or in the home may, in theory at least, be falsified by the patient. Making the PEFR unknown to the patient should increase reliability of PEFR records in cases of litigation. It has also been shown that knowledge of PEFR can influence subjective self-assessment of asthma (Higgs and Laszlo 1982). This is important in studies of patients’ perception of asthma using the relationship between subjective assessment and objective measurement. Portable peak flow meters (PFM)of a mechanical nature, such as the Wright and mini-Wright meters, are available. Rotating vane transducers measuring forced expiratory volume in one second and forced vital capacity have been described (Chowienczyk and Lawson 1982). We have developed a portable electronic peak flow meter employing a rotating vane transducer whose display can be coded electronically. We compared this instrument with a Wright PFM and have successfully used it in studies of asthma. 2. Specification
(i) Small and robust enough for portable use. (ii) Accuracy and range to be comparable to standard peak flow devices such as the Wight PFM. (iii) Option of a coded display which is easy to read, but very difficult to decode by the patient. (iv) Capable of one month’s, four times daily self-monitoring, without the need for new batteries or recalibration. ‘Correspondence to Mr R B Richardson at Frenchay Hospital.
0143-08~5/84/03~2~i +06$02.25 @ 1984 The Institute of Physics
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Figure 1. T h e Codaflow.
3. The coded peak flow meter The coded peak flow meter (Codaflow) as shown in figure 1, consists of a rotating vane transducer developed by D A Lea from a design by Dr C L Smith (Smith 1976). Air flow through the turbine rotates a low-inertia vane, which sets up an electrical impulse as it passes a metal plate. The pu,lses are preamplified within the transducer handle and relayed to the electronics housed in the instrument case.
Figure 2. Block diagram of the electronics.
The electronics are shown in block diagram form in figure 2. The instrument is powered by eight HW batteries to give a * 6 V supply. To conserve power CMOS components were used throughout. The pulses obtained from the preamplifier are shaped to a square wave and amplified by use of operational amplifiers. The output from a single chip frequency to analogue voltage (F/V) converter was analysed by a peak detector and hold circuit. A digital panel meter integrated circuit (DPM) incorporating an analogue to digital converter, together with decoder/drivers allows direct interfacing with a seven segment, three and a half digit liquid crystal display (LCD). The display c a n be coded. This is obtained by changing or omitting interconnections between the DPM and the LCD segments. Two segments, in some cases, are linked and driven by one driver. The minus sign and decimal point are incorporated in the coding. Figures 3 ( a ) a n d 3 ( b ) represent the hundreds part of the code. The coded
An electronic peak flow meter with optional coded display lo) Normal
203
connections
8 910
7
2 3 4 5
digit
II L
'
1 1
2 3 4 5
6
7
1
8 9 101
Figure 3. The connections between the drivers and the liquid crystal display in its normal ( a ) and coded ( b ) forms.
form of the display for the hundreds digit is shown in table 1. The tens employ a different variant of the changes in the interconnections. The complete code is represented by a variety of alphanumeric symbols, e.g., 1, 2, 3, 5 , 6, 8, 9, A, E, U, ., -. Then the position of the ones, tens and hundreds can be interchanged. On the prototype instrument only two and a half digits were used for display, providing 100 coded numerals, but three and a half digits are available providing 1000 coded numerals. We pre-set the range of each instrument so that the top of the scale represented a steady flow of 720 1 min-' (this is adjustable). We have used only one variant of the code, but no user (total use is now > 1500 user-days) has yet managed to break the code. f i e only information a user needs is the value for zero, i.e., -1U.8. There are three switches on the instrument case: on/off, reset (returns display to coded 'zero'), and blOw/read. During the peak flow manoeuvre the blow/read switch is at 'blow' and the display reads zero (- 1U.8), and after the manoeuvre the switch is turned to 'read' and the peak flow rate is displayed and held.
DISPLAY
0
CODED DISPLPiY
-1U.
1 1
2
-
3.
3 3.
4 A
5
E .
6 ~
E.
7 1.
8
-
8.
9 8'
R B Richardson et a1
204 4. Performance
There are controversies and problems with peak flow calibration (Oldham e l a/ 1979 Fisher and Shaw 1980, Morrill et al 1981, Wright 1981). Calibration with stead; flow-rates is not the same as peak flow. We used a Cape ventilator to provide reproducible peak flows up to 475 1 min-', and compared the Codaflow and a Wright PFM against a pneumotachograph (figure 4). This showed the Codaflow to be linear to within 1% over the range 40-475 1 min-I. From this we calculated a calibration for Codaflow to Wright's peak flow meter PEFR. 500 1
1
0
160
100
200 3CO 400 Pneumotachograph ( 1 min-'1
'0
500
Figure 4. Comparison of Codaflow( +) and Wright PFM(0) in measuring artificially produced peak flows, using a Fleisch pneumotachograph as a standard.
Clinical evaluation was achieved by comparing the PEFR in over 100 outpatients with a variety of respiratory and non-respiratory disorders using a Codaflow and a Wright PFM. The patients produced five peak flows measured in the order AABBA with A and B randomly allocated to different types of meter. The maximum reading, for A and B, was taken as a satisfactory PEF reading if the difference between each patient's maximum and minimum A or B responses was less than 20%. If the difference was greater than 20% the results were rejected. The correlation coefficient comparing the two PFMS was 0.991, with a mean percentage difference of 6.4% explainable by variable patient performance. Transcription errors were not seen. If they occufled they would result in a non-existent coded value, which did not occur, or a value which was radically different and would be rejected by the above protocol. Eight instruments have now been tested. The maximum variation between instruments is 2%, for steady state and artificial peak flow calibration (over ranges and 40-475 1 min-' respectively). All instruments have been tested before and after prolonged use without change in calibration. The batteries provide power for severa1 months. The instruments have proved to be very reliable in extensive use in various asthma studies. 5. Discussion
d w e have developed a portable, reliable and accurate peak flow meter that can be code electronically. We compared and calibrated it against the Wright PFM because the
A n electronic peak flow meter with optional coded display
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latter and its derivative (the Minimeter) are in common use. The Codaflow has apparentlygreater linearity, at least in the range 40-475 1 min-’. f i e peak flow rate has been defined by Wright and McKerrow (1959) as the highest flow rate sustained by a subject for at least 10 ms. To achieve this the holding capacitor in the peak detector circuit has a critical value. If this capacitor has too low a value, readings at low flow rates are inaccurate; too high and the value displayed is more stable at the cost Of increasing the response time with an erroneous lowering of the peak value displayed. (The capacitor was chosen by trial and error to obtain a satisfactory result.) The reference voltages to both the DPM and the F/V converter were critical to the gain stability so precision voltage references and a battery low indicator were added. We have already used the Codaflow to show that knowledge of PEFR alters the relationship between subjective and objective measurement in asthma (Higgs and Laszlo 1982). In studies of asthma investigating extrinsic factors, occupational or otherwise, coding might be particularly useful where the patient could consciously or subconsciously alter the PEFR records. In conclusion, we describe a portable electronic peak flow meter based on a turbine flow transducer which can be coded to provide PEFR recording at home and work which is unavailable to the patient. The code is suitable for use on other instruments having a similar form of display. Acknowledgments The authors thank the Medical Physics Department, Frenchay Hospital for help with the development of the instrument and Phoenix Abboflex, Merthyr Tydfil for providing the transducer.
Rhme Dibimitre Clectronique 6 codage optionneI de I’affichage pour la mesure des dtbits maximum respiratoires.
Un spiromtrtre tlectronique portable donnant la valeur maximale du dtbit expiratoire a 6tt dtveloppC et utilis6 avec succks dans I’ttude de l’asthme. Un transducteur B turbine produit un signal pulS6 dont la friquence est proportionnelle au &bit. L‘infomation en sortie est codte afin que les rtsultats ne soient Pas accessibles au patient. Le code est utilisable par d’autres instruments Clectroniques employant un afichage similaire. Ce dispositif a t t t utilist avec succbs pour Ctudier la rtaction des patients vis-&-visde l e u &me.
Zusammenf assung elektronisches Peak-DurchfluflmeBgeratmit einstellbarer kodierter h z e i g e . Entwickelt wurde ein tragbares elektronisches Spirometer, das die Atmungsdurchfluflrate anzeigt und erfolgreich bei Asthmauntersuchungen angewendet wurde. Ein Turbinen-Flupwandler eneugt gepulste *usgangssignale deren Frequenz proportional zum Durchflufl ist. Das Ausgangssignal war kodiert, um die Ergebnissefur den Patienten nicht zuganglich zu machen. Der Kode ist geeignet, um auch bei anderen eiektronischenGeraten mit einer ahnlichen h z e i g e Venvendung zu finden und wurde erfolgreich venvendet, um untersuchen wie Patienten ihr Asthma empfinden.
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Fisher J and Shaw A 1980 Calibration of some Wright peak flow meters. Br. J. Anaesth. 52 4614 Higgs C M B and Laszlo G 1982 Coded peak flow and the perception of asthma Thorax 37 780 Morrill C G, Dickey D W, Weiser P C, Kinsman R A, Chai H and Spectors S L 1981 Calibration and stability of standard and mini-Wright peak flow meters Ann. Allergy. 46 70-3 Oldham H G, Bevan M M and McDermott M 1979 Comparison Of the new miniature Wright peak flow meter with the standard Wright peak flow meter Thorax 34 807-9 Smith C L 1976 Rotating vane flow measurement techniques PhD thesis Polytechnic of Wales, Pontypfidd Wright B M 1981 Calibration of peak flow meters Br. J. Anaesth. 53 777 Wright B M and McKerrow C B 1959 Maximum forced expiratory flow rate as a measure of ventilatory capacity Br. Med. 1.2 1041-7
