Gregory G. King

Perception of bronchodilation in subjects with asthma and smokers with airflow limitation

Perception of the efficacy of bronchodilators in relieving airflow obstruction is a likely determinant of compliance with treatment in patients prescribed these drugs on an ‘as needed’ basis. This study aimed to determine whether bronchodilator‐induced improvements in lung function are associated with improvements in breathing difficulty in subjects with asthma or smokers with airflow limitation. Twenty smokers with airflow limitation and 16 subjects with previously physician‐diagnosed asthma received salbutamol (200 μg) and ipratropium bromide (80 μg). Spirometry and lung volumes were measured before and 40 min after bronchodilator. Subjects recorded changes in ‘difficult breathing’ on a visual analogue scale (VAS). After bronchodilator, forced expiratory volume in 1 s (FEV1) increased by 23.0 ± 6.4% of baseline (mean ± 95%CI) in smokers, and by 25.2 ± 8.5% in the asthmatics, while VAS improved by 31 ± 23% in smokers and 45 ± 25% in asthmatics. However, these changes were not significantly correlated in either smokers (r = – 0.04) or asthmatics (r = 0.15). In the asthmatic subjects, good perceivers (> 25% improvement in VAS) had greater improvements in lung volumes, as percentage predicted, than did poor perceivers. In the smokers, changes in lung function did not differ significantly between good and poor perceivers. Improvement in FEV1, as percentage predicted, was significantly correlated with improvement in VAS in good perceivers (asthma: r = 0.78, P < 0.01; smokers: r = 0.68, P < 0.05), but not in poor perceivers. Asthmatic subjects had good perception of improvements in lung function. However, in smokers with airflow limitation there is little correlation between improvement in lung function and sensation of breathing difficulty. In these subjects symptoms appear to be an unreliable guide for ‘as needed’ use of bronchodilators.

Distribution of Air: Ventilation Distribution and Heterogeneity

As air is breathed in, it follows a complex pathway of branching airways on its way to the alveolar spaces of the lung. Two basic regions of ventilation are recognized, one that participates in gas exchange, the alveolar ventilation, and one that serves as the conduit for air to reach the gas exchange region, the dead space ventilation. Ventilation is not evenly distributed throughout the lung. First, because of the weight of the lung, the more dependent regions of the lung are more compliant and therefore receive more ventilation than less dependent regions. Secondly, topographic ventilation heterogeneity also exists because of the varying time constants (resistance x compliance) scattered throughout the lung, resulting in fast and slow filling and emptying of different regions. This unevenness of ventilation can be detected by a variety of methods, but inert gas washout techniques, including single and multiple-breath nitrogen washout, are most common. This chapter will explore in more detail the physiology of ventilation heterogeneity, how it is measured, and what its implications may be for clinical medicine.