Case Studies 8 & 9

Case Study #8—Asthma: Obstructive Lung Disease

The airways are inflamed in asthma and this leads to swelling of the airway lining. Apart from this, release of mediators and increase in mucus productions are other side effects of asthma. Consequently, the airway diameter decreases significantly (Respiratory Physiology: Lung Volumes & Capacities, n.d.). Airway resistance has an inverse correlation to airway radius or diameter. When there is a decrease in airway diameter, there is an increase in resistance to airflow. This is known as Poiseuille's law:

R refers to the resistance of airways

? refers to viscosity of the inspired air

L refers to the length of airways

r refers to the radius of airways

In a similar manner, airflow is inversely proportional to airway resistance and this can be explained by the following equation:

Q refers to airflow (L/min)

?P refers to the difference in pressure (mm Hg or cm H2O)

R refers to the resistance of airways (cm H2O/L/sec)

The asthma that Ralph is suffering from is linked to an increase in airway resistance. This increased resistance is the main reason why Ralph's expiratory functions have been compromised (Respiratory Physiology: Lung Volumes & Capacities, n.d.). Consequently, where all the air should have been expired from his lungs, a significant portion of it remains behind in the lungs due to increased resistance of airways. This not only increased his residual volume but also raises his FRC (Functional Residual Capacity), which is the lungs' volume in between breathing (Respiratory Physiology: Lung Volumes & Capacities, n.d.). Since there was an increase in Ralph's FRC, his normal tidal breathing occured at higher lung volumes.

The effort put into breathing is measured by the pressure changes needed for moving air into the lungs and out (Respiratory Physiology: Lung Volumes & Capacities, n.d.). When a person is suffering from asthma, there are two major reasons why the effort of the lungs increases.

Asthmatic patients have a higher FRC and therefore naturally breathe at higher lung volumes. When inhaling, it is necessary for an asthmatic patient to lower intrathoracic pressure to a much greater degree than that of a healthy person so that sufficient air can be brought into the lungs. As a result, inspiration requires much more effort.

Since resistance of airways is increased during expiration, there is a need to create greater pressure in order to force air out of the lungs (Respiratory Physiology: Lung Volumes & Capacities, n.d.). This increased expiratory work demands the help of accessory muscles.

FVC (Forced Vital capacity) refers to the volume that the lungs are able to force out after complete inspiration. On the other hand, FEV1 refers to the volume that is expired from the lungs within the first second of the forced expiration. FEV1/FVC refers to the fraction of FVC that is expired from the lungs in the first second (Respiratory Physiology: Lung Volumes & Capacities, n.d.). In a healthy person, FEV1/FVC is normally measured at around 0.8. Since airway resistance is increased in asthmatic patients, this affects FVC, FEV1, and FEV1/FVC. For example, during an asthma attack, FVC ...