Obstructive Lung Disease

Hutchinson correctly predicted that premature morbidity and mortality could be identified by a reduced vital capacity, a term he coined because he recognised that spirometric abnormalities accurately predict the capacity to live.

In obstructive airways disease the hollow lumen of the airways fails to maintain adequate patency during expiration as the pressure outside of the airway is much greater than the inside. This leads to a reduced velocity of expired air.

Airflow limitation is best measured by spirometry and this is the most widely used and reproducible test of lung function.

The pathogenesis of obstructive lung disease ultimately results in reduces airflow through the airways (“obstruction”). An obstructive spirometry pattern is a result of these pathophysiological changes:

diminished elasticity of the lung parenchyma increases lung compliance – this means the lungs are easy to inflate but they empty slowly. This is typically found in subjects with emphysema

increased tone of the airway smooth muscle causes the airway lumen to narrow – this raises airway resistance and impairs airflow, particularly during expiration. This is typically evident in subjects with asthma

mucous in the airways will raise airway resistance and impair airflow. This is typically seen in patients with chronic bronchitis, bronchiectasis and cystic fibrosis, for example

The volume-time trace of a subject with obstructive airways disease typically shows a forced expiratory time (FET) greater than that expected in a normal patient. This is a consequence of relatively normal volumes being expired through narrowed airways – it simply takes longer to get out.

In fact, in patients with severe airways disease, the ‘true FVC’ (which is a plateau of the volume-time trace) may not be achievable within the recommended expiration time of 15 seconds, or beyond.

This may lead to an underestimation of the FVC, and care must be taken, particularly when the FVC value falls bellow the predicted lower limit of normal. In such circumstances, an abnormally low FVC in an obstructive subject may not necessarily indicate a superimposed restrictive disease. Therefore if the measured VC is greater than the FVC it is important to quote the VC for an accurate measure of volume.

A normal or elevated TLC may exist. The pathogenesis of this observation is increased air trapping within the lungs, which may increase the RV. An elevated RV will effectively result in an equal and opposite reduction in the FVC as airways disease progresses.

The TLC of course, cannot be measured during spirometry.

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