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Abstract
A lung with decreased compliance is stiff and requires more work to inflate.2 Normally, 30 to 40 per cent of the work of breathing is devoted to overcoming viscous resistance (most of which is due to resistance to airflow), and 60 to 70 per cent is devoted to overcoming elastic resistance.2 3 4 5 6 With obstructive diseases -- for example, obstructive emphysema -- the normal situation is reversed, and 60 to 70 per cent of the work of breathing is spent in overcoming viscous resistance.2, 6 The rate and depth of respiration are adjusted to minimize the work performed against elastic and viscous resistances.2 3 4 5 6 Adjustment is accomplished through the inflation reflex, whose receptors are probably located in the smooth muscle of the trachea and bronchioles; afferent impulses travel in the vagus nerve.7 Although patients with asthma have lungs with reduced compliance, the increased airway resistance is much more important in determining breathing patterns. [...]patients breathe at higher lung volumes, since expansion of the lungs gives support to the airways, increasing airway diameter and reducing airflow resistance.8 They breathe slowly because the increased resistance produces a long time constant for expiration.8 Most important, they breathe slowly and deeply, thereby reducing turbulent airflow and, hence, viscous resistance.2, 7, 8 Such a breathing pattern minimizes the larger viscous resistance at the expense of elastic resistance -- a compromise that reduces the overall work of ventilation in the asthmatic patient. On the contrary and in spite of what some textbooks claim, respiratory frequency is increased during the asthmatic attack.1, 2 The mechanism for the increase in respiratory frequency may be vagally mediated, but this is not yet clearly established in human beings.3 There are many factors that increase the work of breathing in asthma in addition to the increase in airway resistance and the fall in dynamic lung compliance.