Lecture 8
Factors affecting Pulmonary Ventilation
By
Dr. Khaled Khalil
By the end of this session, the student should be able to:
o Outline the various factors affecting airway resistance and
correlate it to changes in pulmonary ventilation.
o Describe
the
metabolism
of
surfactant,
discuss
its
significance and relate its deficiency to clinical conditions.
o Define compliance of the lung and chest wall, illustrate and
discuss the compliance curve and describe the effect of
surfactant on it.
o Discuss work of breathing and relate it to clinical conditions.
Surfactant
- When the water forms a surface with air, the water molecules on
the surface (in contact of air) are strongly attracted to each other
trying to reduce the surface area of contact. This is called "surface
tension".
- In the lungs, a thin water film lines
the alveoli. Since, it is in contact with
air, it tends to contract to force air out
of alveoli and cause the lungs to
collapse. Fortunately, this does not
occur due to presence of surfactant.
- Surfactant is a surface active agent that decreases surface
tension of the fluid lining the alveoli.
- Surfactant is a complex mixture of phospholipids, proteins and
ions.
 The
most
important
phospholipid
is
dipalmitoyl
phosphatidyl-choline (DPPC) which is surface active
agent.
 Surfactant proteins as: SP-A, SP-B and SP-C.
 Calcium ions.
Functions of surfactant:
1. It decreases the surface tension of the fluid lining the alveoli
through:
a) It scatters among the fluid molecule decreasing the
attraction between them.
b) It also spreads over the fluid preventing air-fluid interface.
* This decreased surface tension:
 Helps lung expansion during inspiration.
 Protects against pulmonary edema as it decreases the filtration
forces for the fluid from pulmonary capillaries into alveoli.
 Prevents lung collapse during expiration.
 Decreases the work of breathing needed.
2. Surfactant activates alveolar macrophages.
3. It has a bacteriolytic effect, making the bacteria susceptible to
alveolar macrophages.
4. The phospholipids of surfactant may act directly on T-
lymphocytes cell membrane to prevent excess immune
response.
5. It assists cilliary movement in upper respiratory tract (so, helps
to remove any particles and mucus away from air passage).
- Surfactant decreases in the following conditions:
1. Respiratory distress syndrome (RDS):
- In fetal life, surfactant is not produced in sufficient amounts until
about the eighth month. Thus, premature babies lacking sufficient
surfactant are born with collapse alveoli.
- Because of the high surface tension, plasma fluid leaks into the
alveoli
producing
pulmonary
edema.
Due
to
“glistening
membrane” appearance, the condition is also called "hyaline
membrane disease".
- RDS babies can be saved by mechanical ventilators that keep
them alive until their lung can manufacture sufficient surfactant.
Otherwise, bovine or synthetic surfactant can be administrated.
- When premature delivery (before 8 months) is life-saving for the
mother, elective caesarean section may result in baby with
collapsed lungs. It is better in this condition to inject the mother
with cortisol hormone which is essential for surfactant formation.
2. Cigarette smoking decreases surfactant. It leads to destruction
of type II alveolar cells.
3. After cardiac surgery during which the pulmonary circulation
is interrupted, alveolar epithelium fails to secrete surfactant.
4. Long-term inhalation of 100% O2.
Pulmonary Compliance
The compliance is the ratio between the change in
volume
to
the
change
in
distending
(transmural pressure). i.e., C = ∆V / ∆P.
pressure
- Compliance of the lung can be defined as the extent to which
the lung expands for each unit increase in its transmural
pressure.
- For the lungs, transmural pressure = intrapulmonary pressure –
intrapleural pressure. It is called "transpulmonary pressure".
- The normal compliance of the lungs in the average adult human
is approximately 200 ml/cm of water pressure. That is every
time, the transpulmonary pressure increase by one centimeter of
water, the lungs expand 200 milliliters.
Dynamic Lung Compliance Curve
- From the curve, one can conclude:
 The compliance of the lung is more during expiration
(BZA) than during inspiration (AXB). This is known as
"hysteresis" (i.e., forward path is different from the
reverse path). Why????????
 The compliance of the lung is determined by the diagonal
line (BYA) connecting the two ends of the curve (average
lung compliance)
Factors affecting compliance:
1. Factors in the lung:
- It is affected by elastic forces of the lung (1/3 of compliance) and surface tension
of the fluid lining the alveoli (2/3 of the compliance).
- Lung compliance is decreased in pulmonary congestion, pulmonary fibrosis and
pulmonary edema whereas it is increased in emphysema.
2. Factors in the chest wall:
- It is affected by the elastic properties of the thorax caused by elasticity of the
muscles, tendons and connective tissues in the chest wall.
- Chest wall compliance is decreased in case of:
* Deformities
of spine.
*Skeletal muscle disease e.g. poliomyelitis.
*Arthritis of vertebra.
* Obesity.
Work of breathing
- During normal quiet respiration, work is done only during
inspiration (during which respiratory muscles contract), whereas
expiration is entirely a passive process.
- At rest, work of breathing is about 1-2% of the total body energy
expenditure. Even during heavy exercise, work is only about 3-
5% of the total energy expenditure of the body.
Work of breathing can be divided into:
1- Compliance work
(elastic work)
2- Tissue resistance work
3- Airway resistance work
65% of total work
7% of total work
28% of total work
- It is the work required to - It is the work required to - It is the work required to
expand the lung against its overcome the resistance of overcome airway resistance
elastic force.
non-elastic
tissue
of
the during
the
- This can be calculated as lungs and thoracic cage.
movement of air into the
follow:
lung.
Compliance work = volume
- The major factor, affecting
x pressure needed to inflate
airway
this volume
diameter
ways.
resistance
of
The
the
is
the
passage
larger
the
diameter of the airway, the
less work done.
Conditions associated with increased work of breathing:
1- Increased compliance work:
- Decreased elasticity (fibrosis).
- Lung congestion (heart failure).
2- Increased tissue resistance work:
- Diseases affecting thoracic cage e.g. kyphoscoliosis.
- Muscle disease.
3- Increased airway resistance work:
- Obstruction of air way.
- Chronic bronchitis.
- Asthma.
Thank you