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The role
of the respiratory system in producing dyspnea may be viewed from
the perspective of the "controller," the "ventilatory
pump," and the "gas exchanger." Derangements in
any of these may lead to dyspnea and may involve one or more of
the basic physiological mechanisms described previously.
The controller is the
neural respiratory center in the medulla and is the "central
command" for breathing. Heightened output from the brain
is the primary manifestation of dyspnea in which the controller
may be playing a role. Abnormalities in gas exchange along with
stimulation of pulmonary receptors, as evidenced by the hyperventilation
seen in patients with asthma and pulmonary embolism, may contribute
to heightened activity in the controller. Hormones, such as progesterone
which also stimulate the respiratory centers in the medulla, can
also play a role. The dyspnea commonly seen in the first trimester
of pregnancy is likely due to the effect of progesterone on the
controller.
The ventilatory pump
consists of the chest wall, muscles of ventilation, airways, and
pleura. All components are necessary to move air from the outside
world to the alveoli and back out again. Patients with a non-compliant
wall, increased airway resistance, weakened muscles, or thickened
pleura may all be said to be suffering from problems with the
ventilatory pump.
The gas exchanger is
comprised of the alveoli and the pulmonary capillaries. The interface
between these structures is the site for uptake of oxygen into
the blood and elimination of carbon dioxide. Derangements of the
gas exchanger, such as pneumonia, lead to hypoxia and/or hypercapnia
with resulting dyspnea.
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