CHEST DRAINAGE AS A THERAPEUTIC INTERVENTION
Normal Anatomy and Physiology
CHEST DRAINAGE AS A
THERAPEUTIC INTERVENTION


The clinical need for chest drainage arises anytime
the negative pressure in the pleural cavity is disrupted
by the presence of air and/or fluid resulting in
pulmonary compromise. The purpose of a chest
drainage unit is to evacuate the air and/or fluid from
the chest cavity to help re-establish normal
intrathoracic pressure. This facilitates the re-expansion
of the lung to restore normal breathing dynamics.
The need also arises following heart surgery to
prevent the accumulation of fluid around the heart.

Patients with continual air or fluid leaks have a chest
tube, also called a thoracic catheter, inserted. The
distal end, which will be inside the patient’s chest,
has a number of drainage holes. The last eyelet can
be detected on a chest x-ray as intermittent breaks in
the radiopaque line. Once the chest tube has been
properly positioned and secured, the x-ray should be
checked to ensure that all drainage holes are inside
the chest wall.

The location of the chest tube depends on what is
being drained. Free air in the pleural space rises, so
the tube is placed above the second intercostal space
at the mid-clavicular line. Pleural fluid gravitates to
the most dependent point, so the tube is placed at
the 4th to 5th intercostal space along the mid-axillary
line (figure 1). Mediastinal tubes placed to drain the
pericardium after open-heart surgery are positioned
directly under the sternum (figure 2). Once the chest
tube is in place, it is connected to a chest drainage
unit.

NORMAL ANATOMY AND PHYSIOLOGY

Before we discuss the chest drainage unit in detail, it
is important to briefly review normal anatomy and
physiology of the thorax with emphasis on the
physiology of respiration. This will help us understand
what can go wrong in the structure and function of
the chest and how these problems can be treated.

CHEST WALL

The chest wall is made up of bones and muscles. The
bones, primarily ribs, sternum and vertebrae, form a
protective cage for the internal structures of the
thorax. The main muscles of the chest wall, the
external and internal intercostals, extend from one rib
to the rib below (figure 3). The external intercostals
enlarge the thoracic cavity by drawing the ribs
together and elevating the rib cage, while the internal
intercostals decrease the dimensions of the
thoracic cavity.

MEDIASTINUM

Within this musculoskeletal cage of the thorax are
three subdivisions. The two lateral subdivisions hold
the lungs. Between the lungs is the mediastinum,
which contains the heart, the great vessels, parts of
the trachea and esophagus, and other structures
(figure 4).

LUNGS

The lungs consist of airways (trachea and bronchi)
that divide into smaller and smaller branches until
they reach the air sacs, called alveoli. The airways
conduct air down to the alveoli where gas exchange
takes place (figure 5).

The lung itself is covered with a membrane called the
visceral (or pulmonary) pleura. The visceral pleura is
adjacent to the lining of the thoracic cavity which is
called the parietal pleura. Between the two
membranes is a thin, serous fluid which acts as a
lubricant – reducing friction as the two membranes
slide across one another when the lungs expand and
contract with respiration. The surface tension of the
pleural fluid also couples the visceral and parietal
pleura to one another, thus preventing the lungs
from collapsing. Since the potential exists for a space
between the two membranes, this area is called the
pleural cavity or pleural space (figure 6).

RESPIRATION

Respiration is a passive, involuntary activity. Air moves
in and out of the thorax due to pressure changes.
When the diaphragm, the major muscle of
respiration, is stimulated, it contracts and moves
downward. At the same time, the external
intercostals move the rib cage up and out. The chest
wall and parietal pleura move out, pulling the visceral
pleura and the lung with it. As the volume within the
thoracic cavity increases, the pressure within the lung
decreases. Intrapulmonary pressure is now lower than
atmospheric pressure; thus air flows into the lung —
inhalation (figure 7a).

When the diaphragm returns to its normal, relaxed
state, the intercostal muscles also relax and the chest
wall moves in. The lungs, with natural elastic recoil,
pull inward as well and air flows out of the lungs —
exhalation (figure 7b). The lungs should never
completely collapse for there is always a small
amount of air, called residual volume, in them.

Under normal conditions, there is always negative
pressure in the pleural cavity. This negative pressure
between the two pleurae maintains partial lung
expansion by keeping the lung pulled up against the
chest wall. The degree of negativity, however,
changes during respiration. During inhalation, the
pressure is approximately –8 cm H2O; during
exhalation, approximately –4 cm H2O. If a patient
takes a deeper breath, the intrapleural pressure will be
more negative. Under normal conditions, the
mechanical attachment of the pleurae, plus the
residual volume, keep the lungs from collapsing.
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