Adding peak flow measurement to the assessment of asthma control introduced a whole new spectrum of objective assessment to what had been a very subjective exercise. Peak expiratory flow (PEF) is a cheap and simple test and shows a strong correlation with other measures of airflow obstruction. PEF is quick to record, and serial measures also provide an indication of the variability of airflow. It is tempting to extrapolate all the above to COPD. However, the physiology and uses of the measurements are very different in the two conditions.
PEF measures the maximum expiratory flow a patient can achieve over a fraction of a second. The level of PEF is related to the airway calibre in asthma, and there is a reasonable correlation of falling PEF with increasing symptoms and vice versa. Asthma is very variable, and the PEF may vary by 200L/min between periods of wellness and periods of illness. This may be from 50% of predicted to normal. This is greatly in excess of the variability of the measurement (single measurements can vary by ± 60L/min), and moreover the effect of variability of an individual reading is reduced by making serial readings over a day or week. The serial PEF chart is a measure of the variability of the airways and of the average levels of function being achieved. It provides a useful method for monitoring average levels of lung function and for documenting the improvement that should follow a successful change in asthma therapy.
In COPD, the situation is quite different. Symptomatic COPD patients have much less variability. The range of possible PEF variability is reduced, such that most COPD patients will not exceed the variation that might be expected from the measurement itself. If the airflow limitation is essentially fixed, it is not helpful to use change in lung function as a primary outcome variable, either when assessing treatment or as a marker of short-term decline. The prime reasons for a measurement in COPD are to make the diagnosis of airflow obstruction and to assess the severity of the abnormality. This requires a single-visit measure that is robust and repeatable.
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