Effects of equipment and technique on peak flow measurements

Background Different lung function equipment and different respiratory manoeuvres may produce different Peak Expiratory Flow (PEF) results. Although the PEF is the most common lung function test, there have been few studies of these effects and no previous study has evaluated both factors in a single group of patients. Methods We studied 36 subjects (PEF range 80–570 l/min). All patients recorded PEF measurements using a short rapid expiration following maximal inspiration (PEF technique) or a forced maximal expiration to residual volume (FVC technique). Measurements were made using a Wright's peak flow meter, a turbine spirometer and a Fleisch pneumotachograph spirometer. Results The mean PEF was 8.7% higher when the PEF technique was used (compared with FVC technique, p < 0.0001). The mean PEF recorded with the turbine spirometer was 5.5% lower than the Wright meter reading. The Fleisch spirometer result was 19.5% lower than the Wright reading. However, adjustment of the Wrights measurements from the traditional Wright's scale to the new EU Peak Flow scale produced results that were only 7.2% higher than the Fleisch pneumotachograph measurements. Conclusion Peak flow measurements are affected by the instruction given and by the device and Peak Flow scale used. Patient management decisions should not be based on PEF measurement made on different instruments.


Background
It is customary for spirometers to print out Peak Expiratory Flow (PEF) measurements as well as measurements of Forced Expiratory Volume in one second (FEV1) and Forced Vital Capacity (FVC). It is not known if these PEF measurements correspond to those made on Wright's or Mini-Wrights meters. Previous studies have found that different spirometers and different Peak Flow Meters can record PEF differently with error rates of up to 26% in laboratory calibration tests [1]. Furthermore, some spirometers use a traditional Wright scale to record PEF whilst others use a scale which corresponds more closely to the new European scale which is closer to the true PEF measured in laboratory studies [2]. The Wright Peak Flow Meter was developed to measure Peak Expiratory Flow [3]. This requires the subject to exhale as quickly as possible into a recording device following maximum inspiration. Maximal expiratory flow lasts for only a fraction of a second and occurs very early in expiration. It is not necessary for the subject to continue exhaling to residual volume. For most subjects, a short but forceful blow will be sufficient to register the maximal expiratory flow ("PEF technique").
The FVC measurement requires a blow that starts from maximal inspiration and proceeds to residual volume. Although PEF measurements and FEV1/FVC measurements both require a rapid exhalation, the instructions given to the subject are different and it is possible that the two techniques are not interchangeable.
Wensley and colleagues found that the PEF of children was 3% higher when measured using a PEF technique compared with a FVC technique on a turbine spirometer [4]. They concluded that this small difference was not clinically significant but they did not compare the measurement made on a Peak Flow meter. In our chest clinic, technicians record the FEV1/FVC of all patients using a Fleisch pneumotachograph-based spirometer and the Peak Flow is measured by the same technician using a Wright's Peak Flow Meter using "PEF technique". We noticed that the peak flow measurement from the two devices differed by up to 20%.
To explore these issues further, we studied the effect of the instruction given to adult subjects (PEF technique or FVC technique) when the peak expiratory flow was measured. We further investigated the effect of different devices using a Wright's Peak Flow Meter and two types of spirometer (turbine spirometer and Fleisch pneumotachograph). This allowed comparison of the instrument effect and the instruction effect using a group of subjects with a wide range of peak flow values.

Methods
We invited 38 sequential patients attending a hospital chest clinic to participate in a study comparing two different Peak Flow techniques on three devices. Informed consent was obtained from all patients willing to participate. Exclusion criterion was inability or unwillingness to take part in the study. Two patients declined and 36 agreed to take part in the study; clinical details are given in Each patient undertook 12 expiratory manoeuvres. This consisted of duplicate measurements of peak expiratory flow using PEF technique and FVC technique with each of the above three devices. The instruction given was standardized. For "PEF technique", the subject was instructed to inhale as deeply as possible and to blow out as fast as possible into the mouthpiece. The best of two technically satisfactory blows into each instrument (as judged by the investigator) was recorded. For "FVC technique", the subject was instructed to inhale as deeply as possible and to blow forcefully into the mouthpiece as fast as possible until their lungs were empty. The individuals were randomly allocated to one of six groups. Each group consisted of 6 participants and had a pre determined order of technique and device to be used.
The letters A -E represent the different devices and techniques used as follows.    figure 1). Removal Three devices using "peak flow technique" Comparing the same technique on each of three devices, the turbine spirometer gave a PEF which averaged 5.5% lower than that recorded by the Wright's meter and the Fleisch pneumotachograph device gave a reading that was, on average 19.5% lower than the Wright's meter reading ( figure 2). However, the pneumotachograph measurement was much more closely matched to the "corrected Wright's" value on the new EU PEF scale ( figure  3). This effect appeared to be symmetrical across a wide range of peak flow values from below 100 l/min to above 500 l/min. However, the converted PEF (EU scale) was closer to the Wright's scale than to the Fleisch pneumotachograph reading for 12 patients with Wright's PEF above 400 L/min.

Discussion
This is not the first study to compare the results of different devices for the measurement of PEF but it is the first study to look at the combined effect of device and instruction given in a group of adult patients with a wide range of lung diseases and a wide range of PEF values. The present study confirms previous reports that the instruction given, the device chosen and the scale used on the device can all affect the PEF measurement [1,2,4,[6][7][8][9][10].
The instruction given (PEF technique or FVC technique) had a small effect for most patients (5.3% lower with FVC technique) but there was a major effect of >50 l/min for three of 36 patients which was reproducible on 3 different devices. The overall difference in PEF (8.7%) was greater than the 3% difference reported in a previous study of children with normal peak flows [4]. This may be explained by adult patients having a better understanding of the subtle difference in the instructions given. The dif-"Peak Flow technique" with Wright meter and Fleisch pneumotachograph spirometer and conversion to new EU scale ference in PEF attributable to the instruction given was smallest for the low resistance Fleisch device.
The different lung function devices gave different values for PEF even when the same technique was used with each device. This difference (up to 19%) was in line with a previous laboratory study which showed "error" rates up to 26% in some devices [1]. The difference between turbine and Wright meter was small but the difference between Wright Meter and Fleisch meter could be sufficiently large to lead to a change in a patient's treatment if the two readings were to be used inter-changeably. Most of the differences can be explained by the use of different scales to measure PEF. The Wright's meter that we used and the turbine spirometer give a PEF reading on the "old" Wright's scale that has been used since 1959 but the Fleisch pneumotachograph gives a more accurate PEF measurement [2,3]. Converting the Wright's PEF measurements to the new EU scale abolished most of the difference between the Wright meter and the Fleisch pneumotachograph with virtually identical measurements in the range of 80-350 L/min.

Conclusion
In laboratory studies, the Wright's meter has been shown to over-estimate the PEF in the mid-range compared with pre-determined machine-generated airflows [2,6,8]. The scales of the Wright meters and Mini-Wright meters were changed to the new EU scale in September 2004 and the results of this experiment indicate that measurements from a Wright meter using the new EU scale are likely to be very similar to those from a Fleisch pneumotachograph but only if the same technique is used on both devices. It is also important to note that patients and doctors should not compare readings made on different Wright meters (new scale or old scale) when deciding on whether to change a patient's treatment. The key point is that a patient's serial PEF should be measured on a single type of device using a consistent technique and measurements made on different machines should not be used to monitor a patient's progress.