Influence of knowledge of peak flow on self assessment of asthma: studies with a coded peak flow meter.

A portable peak flow meter based on a turbine transducer that can display results in code has been developed. Its performance compares well with the Wright peak flow meter. Records of subjective self assessment of asthma on a visual analogue scale and of peak flow (PEF) were compared in 12 subjects with asthma. PEF measurements were made with a coded meter for two weeks and an uncoded meter for two weeks in random order. The correlation between visual analogue scale score and PEF was invariably stronger when PEF was known. Changes in perception of asthma were measured by comparing the slopes and relative positions of the regressions of visual analogue on PEF. When PEF was uncoded awareness of asthma was significantly increased in five patients, predominantly those whose perception was poorest while they were using the coded meter, and decreased in only one patient. In two patients the results were unsuitable for this type of analysis. Knowledge of PEF therefore may influence subjective self assessment in patients with bronchial asthma. For objective studies of symptoms of asthma, PEF readings should be unknown to the patient. Perception of asthma may, however, be improved in patients with poor ability to detect changes in bronchial calibre by uncoded measurement of peak flow at home.