EDITORIAL
USEFULNESS AND LIMITATIONS OF PEF
In recent years there has been a notable advance in the treatment of asthma as a result of the availability of highly selective ß-agonist bronchodilators and effective anti-inflammatory medications, such as corticoids and chromones, which are administered by inhalation, the most effective route of administration. This has led to the simplification of asthma treatment as a result of the management of only a few medications, so asthmatic patients are being cared for more and more often by general practitioners or pediatricians without specialist supervision (1). This trend has been favored by the development of a simple method for the control of respiratory function, the Peak Expiratory Flow Meter (PEFM), which is available to non-specialist physicians. From this development, it has been just a short step to recommending self-control of the disease by the patient, or by the patient''s familily in the case of children, and this has been proposed by international consensus (2). Patient control of the disease is accompanied by a degree of risk because the patient usually lacks the necessary knowledge and awareness of the problems that asthma can cause, and may underestimate the importance of symptoms, or perhaps more frequently, overestimate them, which leads to the unnecessary use of medications which are not free of side effects.
In 1959 Wright described the first portable PEFM which was sufficiently robust to guarantee reliable readings. However, for years Peak Expiratory Flow (PEF) measurement was not well accepted. Acceptance of PEFM came in the 80s, coinciding with the popularity of the medications mentioned and the development of inhalation systems that could be easily managed by patients. Since then, many devices for peak flow measurement have been manufactured. However, in order to bring their cost within reach for patients, quality has sometimes been less than optimal.
The most generalized use of portable PEFM is for control by the patient of his or her daily clinical condition. Peak flow should be determined, at least, when the patient wakes up, before going to bed at night, and before and after inhaling a ß-mimetic if this agent if used regularly. This regimen allow the patient to confirm the onset of bronchospasm and gauge its intensity by comparing PEF readings with the usual values. Likewise, physicians can evaluate broncholability by checking the variability of PEF values throughout the day. Another use for this device is in occupational asthma, as a tool for investigating the influence of the patient''s working environment on the appearance of respiratory problems.
In theory, PEFM use could provide valuable data if it were not for a few drawbacks. Above all, a reliable PEFM is necessary. The construction and durability of some PEFM suggest a questionable quality. Electronic devices and the primitive Wright device are expensive and usually not available to patients, who may only have to use them for a short time. Since not all simple and inexpensive models are of good quality, the prescribing physician should check this. On the other hand, normal PEF values must be determined because they may differ with PEFM model, sex, race, and stage of growth, which is important in pediatric patients (3). Baseline PEF values do not always coincide with estimated normal values for the patient''s age, size, etc., so it is recommended that measurements be made for several days in "baseline conditions" in order to obtain a "normal" value for the individual patient. The percentage reduction in PEF informs the patient of the degree of obstruction present and of the need for bronchodilators. However, dependence on PEF measurements is not advisable for all asthmatics, only for those with moderate, severe, or unstable asthma (4). Various studies have shown that a simple diary of symptoms kept by the patient yields results that are comparable to those obtained with PEFM, that is, that the patient is familiar with his or her clinical condition and does not need any measurement, although measurements are more objective (5, 6). For daily PEF measurements, it is recommended that the readings obtained the first 4 or 5 days be ignored, until the patient becomes adept with the technique. The initial readings will be lower or more irregular than later readings. However, after two or three weeks, patients, particularly pediatric patients, tend to lose interest and often forget to take readings or are not conscientious about making them (3, 7).
In order to evaluate the degree of instability, which is the same as the broncholability or bronchial hyperreactivity, at least two daily readings should be made, although a third reading at mid-day is advisable. These measurements can be used to calculate an index that will simplify the interpretation of results. However, different indexes have been proposed and no consensus has been reached. Since this determination is difficult and time-consuming and must be done over a period of several days, it demands a high level of cooperation from the patient. The same information about broncholability can be obtained by inhalation of a bronchoconstrictor, such as methacholine or histamine, in tests carried out in a functional exploration laboratory.
As a technique for studying occupational asthma, PEF measurements present certain difficulties. On the one hand, there are technical problems because repeated measurements must be made in the working environment at certaine times, as well as outside of work at the same times. Moreover, the reliability or results depends on the patient, who may be interested in demonstrating that the occupational environment is harmful (3, 8).
Finally and most importantly, PEF offers only partial information about the status of respiratory capacity (9, 10). It is roughly equivalent to FEV1 because both parameters indicate the state of the central airways, but the two parameters are different. However, the most important abnormality in the asthmatic''s respiratory function is located in the peripheral airways. Bronchiolar permeability is better indicated by the middle part of the forced expiratory flow curve (FEF25-75) (10-12). Therefore, PEF determinations alone may lead to errors, because this test can indicate normal airway permeability when, in fact, bronchiolar (small airways) obstruction is present. The evolution of asthma toward chronicity is conditioned by this, which should not be overlooked in children and, especially, adolescents (13).
To conclude this discussion, PEF measurements have limited value and the respiratory function of asthmatic patients should be studied by flow-volume curves at least twice a year in a specialized laboratory, as recommended by international consensus opinion (2, 4). The advantages inherent to patients being able to control their own respiratory function using this simple method, combined with possibility of using medication as needed ("on demand"), and the supervision of the system by a general practitioner is that it overlooks the fact that the evolution of asthma is conditioned by on parameters that are not evident in either clinical manifestations or PEF readings. Asthma could be better controlled if simple devices for the measurement of FEF25-75 became available, as has been noted by various authors (4, 11).
F. Muñoz-López
REFERENCES
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