Detecting airways obstruction from the tidal flow profile.

The two papers in this issue from the Osler Chest Unit at Oxford University [1, 2] exploit the fact that the breathing pattern in humans is affected in characteristic ways when the pulmonary airways are obstructed. MORRIS et al. [1] examined the latter part of the tidal expiratory flow pattern in patients with varying degrees of airflow obstruction and found that the expiratory time constant correlated positively with standard measures of airflow obstruction. WILLIAMS et al. [2] also showed that a shape index related to the rate of decay of post-maximal expira-tory flow correlated strongly with airflow obstruction, as determined by conventional methods. These studies suggest that a simple, noninvasive procedure may be used to derive an important physiological parameter that is not normally so easily obtained. Assessing airflow obstruction directly means measuring airway or lung resistance, which requires subjecting the subject to a technique that is either cumbersome or invasive. For example, airway resistance (Raw) can be assessed by body plethysmography [3], but this requires a significant degree of subject cooperation and the ability to deal with claustrophobia. Lung resistance can be determined from flow at the mouth and the pressure drop across the lung [4], but this requires the placement of an oesophageal balloon, which is not a trivial matter and is out of the question for routine screening. Respiratory system impedance can be determined by the forced oscillation method [5], in which perturbations in flow are applied to the mouth by an oscillator. Although promising, this method is attended by numerous technical difficulties and has yet to be put to widespread use. Direct visualization of constricted airways is also possible, in principle, but at, the expense of the X-ray dose resulting from a computed tomographic scan [6]. The results of MORRIS et al. [1] and WILLIAMS et al. [2] are, therefore, interesting from a clinical perspective because they suggest that the severity of airflow obstruction, a parameter of key concern to the respiratory specialist, may be derived easily and noninvasively, namely from airflow measurements at the mouth. The physiological basis of this approach is not difficult to appreciate. A spontaneously breathing individual indulges in a process of feedback control, whereby the respiratory centres in the brain direct the respiratory musculature to overcome the mechanical impedance of the lungs and thorax in such a fashion as to achieve the necessary alveolar ventilation. If the respiratory impedance increases, as …