C h a p t e r 2 1
Control of Respiratory Function
537
Coughing requires the rapid inspiration of a large
volume of air (usually about 2.5 L), followed by rapid
closure of the glottis and forceful contraction of the
abdominal and expiratory muscles. As these muscles
contract, intrathoracic pressures are elevated to levels of
100 mm Hg or more. The rapid opening of the glottis at
this point leads to an explosive expulsion of air.
A number of conditions interfere with the cough
reflex and its protective function. The reflex is impaired
in persons whose abdominal or respiratory muscles are
weak. This problem can be caused by disease conditions
that lead to muscle weakness or paralysis, by prolonged
inactivity, or as an outcome of surgery involving these
muscles. Bed rest interferes with expansion of the chest
and limits the amount of air that can be taken into the
lungs in preparation for coughing, making the cough
weak and ineffective. Disease conditions that prevent
effective closure of the glottis and laryngeal muscles
interfere with production of the marked increase in
intrathoracic pressure that is needed for effective cough-
ing. For example, the presence of a nasogastric tube may
prevent closure of the upper airway structures and may
fatigue the receptors for the cough reflex that are located
in the area. The cough reflex also is impaired when there
is depressed function of the medullary centers in the
brain that integrate the cough reflex.
Although the cough reflex is a protective mechanism,
frequent and prolonged coughing can be exhausting
and painful and can have undesirable effects on the car-
diovascular and respiratory systems and on the elastic
tissues of the lungs. This is particularly true in young
children and elderly persons.
Dyspnea
Dyspnea
is the perceived shortness of breath or diffi-
culty breathing. It may occur at rest or with exertion, be
continuous or intermittent, or have a pattern of acute or
chronic occurrences. Dyspnea may occur in otherwise
healthy persons, as during exercise or exposure to low
ambient levels of oxygen. It is a common complaint of
persons with primary lung diseases such as pneumonia,
asthma, and emphysema; heart disease that is charac-
terized by pulmonary congestion; and neuromuscular
disorders such as myasthenia gravis and muscular dys-
trophy that affect the respiratory muscles.
The physiological mechanisms underlying the sensa-
tion of dyspnea remain elusive. Dyspnea is not a sin-
gle phenomenon. There are at least three varieties of
breathing difficulty—air hunger, labored breathing, and
chest tightness. The sensation of
air hunger
is thought
to be mediated by transmission of excessive chemore-
ceptor stimulation of the medullary respiratory center
to sensory centers in the forebrain.
Labored breath-
ing,
a sensation of working hard to breathe, is a com-
mon complaint of persons with weakened respiratory
muscles. It is thought to be mediated by excessive input
from stretch receptors in the chest muscles or chest wall.
The sensation of
chest tightness
, an early symptom of
an asthmatic attack, appears to be related to input from
lung receptors that monitor bronchial constriction. In
contrast, the dyspnea that accompanies pulmonary
congestion due to heart failure appears to be related to
input from lung receptors that monitor vascular disten-
tion (i.e., the previously described J receptors). More
than one mechanism may be responsible for the dyspnea
seen in a particular disease state. For example, severe
flow limitations in chronic pulmonary disease can pro-
duce stimuli that give rise to the sensation of increased
breathing difficulty, and the presence of hypoxia and/or
hypercapnia may produce the sensation of air hunger.
Like other subjective symptoms, such as fatigue and
pain, dyspnea is difficult to quantify because it relies on
a person’s perception of the problem. Like pain, dyspnea
is also a multidimensional sensation, involving the sen-
sation of both sensory intensity (i.e., work of breathing)
and unpleasantness (i.e., air hunger or chest tightness).
A commonly used method for assessing dyspnea is a ret-
rospective determination of the level of daily activity at
which dyspnea is experienced. The visual analog scale
may be used to assess breathing difficulty that occurs
with a given activity, such as walking a certain distance.
The visual analog scale consists of a line (often 10 cm
in length) with descriptors such as “easy to breathe” on
one end and “very difficult to breathe” on the other.
The treatment of dyspnea depends on the cause. For
example, persons with impaired respiratory function
may require oxygen therapy, and those with pulmonary
edema may require measures to improve heart function.
Methods to decrease anxiety, breathing retraining, and
energy conservation measures may be used to decrease
the subjective sensation of dyspnea.
SUMMARY CONCEPTS
■■
Pulmonary ventilation or the act of breathing
involves movement of the diaphragm,
intercostal muscles, and other respiratory
muscles. These muscles are controlled by
neurons of respiratory centers in the pons and
medulla with input from higher brain centers
and peripheral receptors.
■■
Control of breathing has both automatic and
voluntary components.The automatic regulation
of ventilation is controlled by two types of
receptors: chemoreceptors, which monitor
blood levels of carbon dioxide, oxygen, and pH;
and lung receptors, which monitor the status
of breathing in terms of airway resistance and
lung expansion. Voluntary respiratory control is
needed for integrating breathing and actions such
as speaking, blowing, and singing.These acts,
which are initiated by the motor and premotor
cortex, cause temporary suspension of automatic
breathing.
(continued )
