Background information respiration | Pathophysiology | Lung capacities

Effects of airway obstruction

In the case of severe airway obstruction it is obvious that respiratory reserve volumes are very small. Flow during normal expiration is nearly the same as during forced expiration. In fact each normal tidal breath is a forced expiration and involves much work of breathing. During exercise patients can increase the inspiratory, but not the expiratory flow leading to hyperinflation, i.e. an increase in end-expiratory lung volume probably on top of the hyperinflation already present when at rest.

Hyperinflation in the case of severe airway obstruction results in marked flattening of the diaphragm. The flatter the diaphragm, the poorer it functions as an inspiratory muscle. In extreme cases paradoxical inspiratory movements of the lower ribs occur indicating that the diaphragm has turned into an expiratory muscle by causing inward motion of the thoracic cage instead of outward motion.

Both airway hyperresponsiveness and airway obstruction contribute to the airflow limitations observed in asthmatics. Most accepted methods to determine the level of limitation are measurement of forced expiratory volume in 1 second (FEV 1) and its accompanying forced vital capacity (FVC) and measurement of peak expiratory flow (PEF).

Daily variability in PEF (or FEV1) is larger in asthmatics than in healthy subjects, and is used as an index of the activity of the disease process. Variability is expressed as the ratio of the difference between the highest and lowest PEF divided by the average of all measurements of one day (morning, noon and night value). Variations within and between days in PEF as well as FEV1 of 20% or more are highly characteristic for asthmatics. The underlying variation in airway diameter again reflects increased airway hyperresponsiveness. An improved or lower level of PEF, of its variability, or both, signifies improvement or deterioration of asthma. In non-asthmatics variability is < 20% in adults, < 31% in children.

In most patients airflow obstruction is completely reversible after treatment with inhaled glucocorticosteroids. Still in several asthmatics residual airway obstruction is observed, which probably represents remodelled airway structures.

Literature:

O'Byrne P. GINA Executive Commitee. Global strategy for asthma management and prevention. 2004. National Institutes of Health. Publication No 02-3659

Respiration profile depending on resistance and bronchial diameter

Description
In this interactive tool sliders can be moved to choose a certain resistance value or bronchial diameter respectively. The tidal breathing profile shown in the window will change according to the chosen value. Additionally, clicking on the buttons "normal" or "obstructive" will display the corresponding breathing profile.

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