The word ASTHMA is of Greek origin, meaning "breathless" or "to breathe with open mouth", and was originally applied to breathlessness of any cause. An accurate and comprehensive definition has proved far more elusive than expected, but from a clinician's viewpoint the currently most acceptable is that asthma is a disease characterised by wide variations over short periods of time in resistance to flow in intrapulmonary airways, The definition implies objective measurement and 15-20% variation

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... s a significant level. This vague definition avoids the problem of dividing patients absolutely into categories of asthma and chronic bronchitis and then debating whether patients with chronic bronchitis have reversible airways disease. Even if symptoms suggest bronchitis, the finding of significant variation in resistance to airflow means that asthma co-exists and patients should be regarded as having "chronic bronchitis with asthmatic features." Difficulties with regard to definition make accurate assessment of the prevalence of the disease impossible and there are wide variations in the results of surveys in general practice. However, this is a common disease with a cumulative prevalence of up to about 5% in the United Kingdom.