Download CASE 16

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CASE 16
A 24-year-old pregnant woman presented to the hospital in preterm labor and
subsequently delivered a premature infant at only 27 weeks gestation (normal
term pregnancy is 37-42 weeks). After the delivery, the infant cried, but it subsequently began to grunt and showed signs of hypoxia despite oxygen supplementation. The baby immediately was intubated by endotracheal tube and
given surfactant down the endotracheal tube. The baby’s hypoxia resolved, and
he was transferred to the neonatal intensive care unit for further stabilization.
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What is the role of surfactant in the lung?
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Where is the major site of airway resistance?
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How does stimulation of the parasympathetic system affect airway
resistance?
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CASE FILES: PHYSIOLOGY
ANSWERS TO CASE 16: MECHANICS OF BREATHING
Summary: A premature infant at 27 weeks gestation develops respiratory distress syndrome (RDS) that requires intubation and surfactant.
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Role of surfactant: Reduces surface tension.
Major site of airway resistance: Medium-sized bronchi.
Stimulation of parasympathetic system: Increased airway resistance
because of smooth muscle constriction in bronchi.
CLINICAL CORRELATION
The use of surfactant has decreased the mortality of infants with RDS dramatically. Infants who are born prematurely are at much greater risk of having this
disorder than are term infants. Surfactant production begins in utero and
increases throughout pregnancy. Infants beyond 36 completed weeks have a
low incidence of RDS. Infants born with RDS present with grunting, hypoxia,
atelectasis, retraction, and excess use of accessory muscles such as the intercostal muscles. During pregnancy, it is possible to perform an amniocentesis
to assess for lung maturity. Patients with pregestational diabetes or a history of
previous cesarean delivery who want a repeat cesarean often are delivered
before labor begins, and if there is any question about the gestational age, an
amniocentesis can be performed to assess lung maturity before delivery.
APPROACH TO MECHANICS OF BREATHING
Objectives
1.
2.
3.
Discuss the relative roles of the muscles involved in breathing.
Discuss the roles of compliance of the lung and compliance of the
chest wall in determining lung volumes and capacities.
Define surface tension and discuss its role in the mechanics of
breathing.
Definitions
Surfactant: A lipoprotein produced by type II alveolar cells.
Compliance: The ability of a structure to yield elastically when a force is
applied.
Interdependence: A condition in which the size and shape of a structure is
dependent upon the size and shape of adjacent structures.
CLINICAL CASES
135
DISCUSSION
When all the muscles involved in respiration are relaxed, the volume of air in
the lungs, called the functional residual capacity (FRC), is determined by
the interplay between the elastic recoil of the lungs inward and the elastic
recoil of the chest wall outward. This interplay results in an intrapleural pressure that is subatmospheric. Inspiration is accomplished by expanding the
volume of the chest, making intrapleural pressure even more subatmospheric. This drop in pressure outside the alveoli results in the flow of air from
the atmosphere into the alveoli, thus expanding them. During respiration at
rest, chest expansion is accomplished largely by means of contraction of the
diaphragm. Expiration is accomplished by simply relaxing the diaphragm,
allowing the elastic recoil of the lung to return intrapleural pressure to its
previous resting value, thus providing the force to move air out of the alveoli
so that they return to their resting size. Contraction of the external intercostal
muscles to stabilize and/or move the ribs upward and outward to expand the
chest also occurs during inspiration. Deeper inspirations involve greater contraction of the diaphragm and external intercostals as well as the scalene and
the sternomastoids. During increased breathing, expiration is aided by contraction of the abdominal muscles (rectus abdominis, internal and external
obliques, and transversus abdominis) and the internal intercostals.
The lung volumes, capacities, and flows (eg, forced expired volume during
the first second of expiration [FEV1]) discussed in Case 15 depend not only on
the muscles of respiration but also on the compliance of the lungs and the
compliance of the chest wall. Compliance is defined by the equation
Compliance = Dvolume/Dpressure
Lung compliance basically is a measure of the stiffness of the lung and is
inversely related to lung elastic recoil: The lower the compliance, the greater
the elastic recoil and the greater the compliance, the lower the elastic recoil. In
part, lung compliance depends on lung architecture. In a healthy lung, there is
little connective tissue in the terminal airways and alveoli. Thus, the connective tissue contributes little to compliance. (However, fibrous tissue is
increased in several disease states and can decrease compliance significantly.)
Compliance in the normal lung comes primarily from two sources: surface
tension and alveolar interdependence.
Alveolar membranes present a surface where air and fluid interface. The
fluid lining this interface exerts a surface tension that tends to collapse the
alveoli, thus decreasing compliance. If this fluid were just a saline solution,
surface tension would be quite high; however, type II alveolar cells secrete a
phospholipid-protein solution that acts as a surfactant to lower surface tension. If this solution is missing or is not of the proper composition, as in this
case, surface tension and elastic recoil will be high. This decreased compliance
will make the work of breathing greater, and accessory muscles of respira-
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CASE FILES: PHYSIOLOGY
tion will be used for normal breathing. Surfactant is unique and differs from
a simple detergent in that the magnitude to which it can lower surface tension is inversely related to surface area. Alveoli communicate with one
another and vary in size. If surface tension were the same in all alveoli, small
alveoli would tend to collapse and larger ones would tend to expand. (This is
the case because pressure is directly related to surface tension and indirectly
related to radius, as stated in the Laplace law.) This does not happen because
surfactant is more compressed in the smaller alveoli, thus lowering surface
tension to a greater degree.
Although alveoli often are depicted as a bunch of grapes, they actually are
connected to one another and to small airways to form a meshwork. In this
arrangement, if one alveolus tended to collapse, it would tend to expand adjacent alveoli, and that expansion of adjacent alveoli would oppose its collapse.
This is referred to as interdependence. In diseased areas of lung in which the
alveolar structure is lost, as is seen in emphysema, interdependence is
reduced and lung compliance is increased.
Changes in compliance alter the elastic recoil of the lung and are reflected
in the FRC. With decreased compliance as is seen in fibrosis and in lack of
surfactant, FRC is reduced. With increased compliance as is seen in emphysema, FRC is increased.
COMPREHENSION QUESTIONS
[16.1]
A 24-year-old medical student awakens to get ready for classes.
When he arises from a lying down to a standing up position, changes
occur to his lungs. Which of the following describes the alterations
that occur in the alveoli at the top of his lungs compared with those at
the bottom of his lungs?
A.
B.
C.
D.
E.
[16.2]
Exhibit greater compliance
Have a lower ventilation-perfusion ratio
Have larger radii
Receive a greater percentage of the pulmonary blood flow
Receive a greater percentage of the tidal volume
A premature newborn infant is noted to have a deficiency of pulmonary surfactant. Which of the following muscles will the infant
likely require to use to accomplish adequate respiration?
A. Diaphragm, internal intercostals, scalene, sternomastoids
B. Diaphragm, internal intercostals, sternomastoids, rectus abdominis
C. Diaphragm, external intercostals, internal intercostals, rectus
abdominis
D. Diaphragm, external intercostals, scalene, rectus abdominis
E. Diaphragm, external intercostals, scalene, sternomastoids
CLINICAL CASES
[16.3]
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A pulmonary physiologist notes that the lung characteristics of individuals who lack surfactant and those with pulmonary fibrosis are
similar. Which of the following occurs in patients who lack surfactant
but not in patients with pulmonary fibrosis?
A.
B.
C.
D.
E.
Collapse of small alveoli and expansion of large alveoli
Decreased lung compliance
Decreased total lung capacity
Increased work of breathing
Intrapleural pressures more subatmospheric than normal
Answers
[16.1]
C. Under the influence of gravity, the weight of the lung stretches
structures at the top of the lungs and compresses those at the bottom.
Thus, alveoli are larger at the top of the lung. Because they are
stretched, these alveoli are less compliant and have smaller changes
in volume with changes in intrapleural pressure upon inspiration.
Thus, they are less well ventilated. Because pulmonary artery pressures are relatively low, blood flow to these alveoli also is low.
However, ventilation is not as low as blood flow; thus,
ventilation/perfusion (V/Q) is high.
[16.2]
E. As a result of the lack of normal surfactant, surface tension in the
lung is higher than normal. This results in a tendency for alveoli to
collapse and a decrease in lung compliance. Thus, greater than normal muscular effort is needed to expand the lung during inspiration.
Contraction of the internal intercostals and the rectus abdominis
occurs with a forced expiration.
[16.3]
A. The lack of surfactant not only results in an overall decrease in
compliance as does fibrosis, it also results in unstable alveoli.
Without surfactant, surface tension will be nearly the same in alveoli
of all sizes. Thus, pressures will be greater in smaller alveoli, as predicted by the Laplace law. This leads to the collapse of small alveoli
into larger alveoli. The decreased compliance in both conditions
leads to decreased total lung capacity, increased work of breathing,
and intrapleural pressures that are more negative than normal.
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CASE FILES: PHYSIOLOGY
PHYSIOLOGY PEARLS
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During quiet respiration, inhalation is accomplished mainly by contraction of the diaphragm. Exhalation is passive because of elastic recoil of the lungs.
Compliance of the lung is due in large part to the surface tension of
fluid lining the alveoli.
Stability of alveolar size is maintained by the presence of surfactant
in the fluid lining the alveoli and by the interdependence among
adjacent alveoli and small airways.
In the absence of surfactant, the work of breathing is markedly
increased.
REFERENCES
Cloutier MM, Thrall RS. Respiratory system. In: Levy MN, Koeppen BM, Stanton
BA, eds. Berne & Levy, Principles of Physiology. 4th ed. Philadelphia, PA:
Mosby; 2006:361-426.
Powell FL. Mechanics of breathing. In: Johnson LR, ed. Essential Medical
Physiology. 3rd ed. San Diego, PA: Elsevier Academic Press; 2003:277-288.