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10. A 42-year-old woman complains to her physician of
muscle fatigue. A series of tests confirm a diagnosis of
myasthenia gravis.Which of the following is most likely
decreased in this woman with inspiratory muscle weakness?
(A) Functional residual capacity
(B) Lung compliance
© Lung elastic recoil
(D) Residual volume
(E) Vital capacity
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The correct answer is E. Deterioration of pulmonary
function is a serious complication of myasthenia gravis.
Inspiratory muscle weakness decreases the maximum
amount of air that can be inhaled and exhaled, thereby
decreasing the vital capacity of the lungs. Recall that the
vital capacity is the difference between the total lung
capacity and the residual volume, i.e., the difference in
volume between maximum inspiration and maximum
expiration. The patient™s inability to take a deep breath
is also reflected in a reduced total lung capacity, inspiratory
capacity, and forced expiratory volume.
The functional residual capacity (choice A) is the volume
of air in the lungs at the end of a normal expiration.
It represents a balance between the tendency of the
lungs to collapse and the tendency of the chest wall to
expand. The functional residual capacity should not be
affected very much by muscle paralysis because it is
measured in the absence of muscle contraction.
The pulmonary compliance (choice B) and elastic
recoil of the lungs (choice C) are both functions of the
viscoelastic properties of the lungs themselves and are
thus independent of the skeletal muscle system.
