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Ventilator
Definition
A ventilator is an automatic mechanical device
designed to provide all or part of the work the body must
produce to move gas into and out of the lungs. The act of
moving air into and out of the lungs is called breathing,
or, more formally, ventilation.
Background
on Ventilation
During breathing, a volume of air is
inhaled through the airways (mouth and/or nose, pharynx, larynx,
trachea, and bronchial tree) into millions of tiny gas exchange
sacs (the alveoli) deep within the lungs. There it mixes with
the carbon dioxide-rich gas coming from the blood. It is then
exhaled back through the same airways to the atmosphere. Normally
this cyclic pattern repeats at a breathing rate, or frequency,
of about 12 breaths a minute (breaths/min) when we are at
rest (a higher resting rate for infants and children). The
breathing rate increases when we exercise or become excited.
Gas exchange is the function of the
lungs that is required to supply oxygen to the blood for distribution
to the cells of the body, and to remove carbon dioxide from
the blood that the blood has collected from the cells of the
body. Gas exchange in the lungs occurs only in the smallest
airways and the alveoli. It does not take place in the airways
(conducting airways) that carry the gas from the atmosphere
to these terminal regions. The size (volume) of these conducting
airways is called the anatomical "dead space" because
it does not participate directly in gas exchange between the
gas space in the lungs and the blood. Gas is carried through
the conducting airways by a process called "convection".
Gas is exchanged between the pulmonary gas space and the blood
by a process called "diffusion".
One of the major factors determining
whether breathing is producing enough gas exchange to keep
a person alive is the 'ventilation' the breathing is producing.
Ventilation is expressed as the volume of gas entering, or
leaving, the lungs in a given amount of time. It can be calculated
by multiplying the volume of gas, either inhaled or exhaled
during a breath (called the tidal volume), times the breathing
rate (e.g., 0.5 Liters x 12 breaths/min = 6 L/min).
Therefore, if we were to develop a
machine to help a person breathe, or to take over his or her
breathing altogether, it would have to be able to produce
a tidal volume and a breathing rate which, when multiplied
together, produce enough ventilation, but not too much ventilation,
to supply the gas exchange needs of the body. During normal
breathing the body selects a combination of a tidal volume
that is large enough to clear the dead space and add fresh
gas to the alveoli, and a breathing rate that assures the
correct amount of ventilation is produced. However, as it
turns out, it is possible, using specialized equipment, to
keep a person alive with breathing rates that range from zero
(steady flow into and out of the lungs) up to frequencies
in the 100's of breaths per minute. Over this frequency range,
convection and diffusion take part to a greater or lesser
extent in distributing the inhaled gas within the lungs. As
the frequency rates are increased, the tidal volumes that
produce the required ventilation get smaller and smaller.
We will consider two classes of ventilators
here: those that produce breathing patterns that mimic the
way we normally breathe (i.e., at rates our bodies produce
during our usual living activities: 12 - 25 breaths/min for
children and adults; 30 - 40 breaths/min for infants) - these
are called conventional ventilators; and those that
produce breathing patterns at frequencies much higher than
we would or could voluntarily produce for breathing - called
high frequency ventilators.
There are two sets of forces that can
cause the lungs and chest wall to expand: the forces produced
when the muscles of respiration (diaphragm, inspiratory intercostal,
and accessory muscles) contract, and the force produced by
the difference between the pressure at the airway opening
(mouth and nose) and the pressure on the outer surface of
the chest wall. Normally, the respiratory muscles do the work
needed to expand the chest wall, decreasing the pressure on
the outside of the lungs so that they expand, which in turn
enlarges the air space within the lungs, and draws air into
the lungs. The difference between the pressure at the airway
opening and the pressure on the chest wall surface usually
does not play a role in this activity because, both of these
locations being exposed to the same pressure (atmospheric),
this difference is zero. However, when the respiratory muscles
are unable to do the work required for ventilation, either
or both of these two pressures can be manipulated to produce
breathing movements.
It is not difficult to visualize that,
if the pressure at the mouth and nose of an individual were
increased while the pressure surrounding the rest of the person's
body remained at atmospheric, the person's chest would expand
as air is literally forced into the lungs. Likewise, if the
pressure on the person's body surface were lowered as the
pressure at the person's open mouth and nose remained at atmospheric,
then again the pressure at the mouth would be greater than
that on the body surface and air would be forced into the
lungs. Thus, we have two approaches that can be used to mechanically
ventilate the lungs: apply positive pressure (relative to
atmospheric) to the airway opening - devices that do this
are called positive pressure ventilators; or, apply
negative pressure (relative to atmospheric) to the body surface
(at least the rib cage and abdomen) - such devices are called
negative pressure ventilators.
Mechanical
Ventilators
The simplest mechanical device we
could devise to assist a person's breathing would be a hand-driven,
syringe-type pump that is fitted to the person's mouth and
nose using a mask. A variation of this is the self-inflating,
elastic breathing bag. Both of these require one-way valve
arrangements to cause air to flow from the device into the
lungs when the device is compressed, and out from the lungs
to the atmosphere as the device is expanded. Also, it can
be appreciated that such arrangements are not automatic, requiring
an operator to supply the energy to push the gas into the
lungs through the mouth and nose.
Automating the ventilator so that continual
operator intervention is not needed for safe, desired operation
requires 1) a stable attachment (interface) of the device
to the patient, 2) a source of energy to drive the device,
3) a control system to make it perform appropriately, and
4) a means of monitoring the performance of the device and
the condition of the patient.
- Patient Interface. Positive
Pressure Ventilators: The ventilator delivers gas
to the patient through a set of flexible tubes called a
patient circuit. Depending on the design of the ventilator,
this circuit can have one or two tubes. The circuit connects
the ventilator to either an endotracheal or tracheostomy
tube that extends into the patient's throat (causing this
arrangement to be called invasive ventilation), or
a mask covering the mouth and nose or just the nose (referred
to as noninvasive ventilation). Each of these connections
to the patient may have a balloon cuff associated with it
to provide a seal - either inside the trachea for the tracheal
tubes or around the mouth and nose for the masks. Negative
Pressure Ventilators: The patient is placed
inside a chamber with his or her head extending outside
the chamber. The chamber may encase the entire body except
for the head (e.g., iron lung), or it may enclose just the
rib cage and abdomen (cuirass). It is sealed to the body
where the body extends outside the chamber. Although it
is not generally necessary, the patient may have an endotracheal
or tracheostomy tube in place.
- Power Sources. Positive
Pressure Ventilators are typically powered by electricity
or compressed gas. Electricity is used to run compressors
of various types. These provide compressed air both for
motive power as well as air for breathing. More commonly,
however, the power to expand the lungs is supplied by compressed
gas from tanks, or from wall outlets in the hospital. The
ventilator is generally connected to separate sources of
compressed air and compressed oxygen. This permits the delivery
of a range of oxygen concentrations to support the needs
of sick patients. Because compressed gas has all moisture
removed, the gas delivered to the patient must be warmed
and humidified in order to avoid drying out the lung tissue.
A humidifier placed in the patient circuit does this. A
humidifier is especially needed when an endotracheal or
tracheostomy tube is used since these cover or bypass, respectively,
the warm, moist tissues inside of the nose and mouth and
prevent the natural heating and humidification of the inspired
gas. Negative Pressure Ventilators are usually
powered by electricity used to run a vacuum pump that periodically
evacuates the chamber to produce the required negative pressure.
Humidification is not needed if an endotracheal tube is
not used. Oxygen enriched inspired air can be provided as
needed via a breathing mask.
- Control System. A control
system assures that the breathing pattern produced by the
ventilator is the one intended by the patient's caregiver.
This requires the setting of control parameters such as
the size of the breath, how fast and how often it is brought
in and let out, and how much effort, if any, the patient
must exert to signal the ventilator to start a breath. If
the patient can control the timing and size of the breath,
it is called a spontaneous breath. Otherwise, it
is called a mandatory breath. A particular pattern
of spontaneous and mandatory breaths is referred to as a
mode of ventilation. Numerous modes, with a variety
of names, have been developed to make ventilators produce
breathing patterns that coordinate the machine's activity
with the needs of the patient.
- Monitors. Most ventilators
have at least a pressure monitor (measuring airway pressure
for positive pressure ventilators, or chamber pressure for
negative pressure ventilators) to gauge the size of the
breath and whether or not the patient is properly connected
to the ventilator. Many positive pressure ventilators have
sophisticated pressure, volume and flow sensors that produce
signals both to control the ventilator's output (via feedback
in the ventilator's control system) and to provide displays
(with alarms) of how the ventilator and patient are interacting.
Clinicians use such displays to follow the patient's condition
and to adjust the ventilator settings.
Conventional
Ventilators
The vast majority of ventilators used in
the world provide conventional ventilation. This employs breathing
patterns that approximate those produced by a normal spontaneously
breathing person. Tidal volumes are large enough to clear
the anatomical dead space during inspiration and the breathing
rates are in the range of normal rates. Gas transport in the
airways is dominated by convective flow and mixing in the
alveoli occurs by molecular diffusion. This class of ventilator
is used in the ICU, for patient transport, for home care and
in the operating room. It is used on patients of all ages
from neonate to adult..
High
Frequency Ventilators
It has been known for several decades
that it is possible to adequately ventilate the lungs with
tidal volumes smaller than the anatomic dead space using breathing
frequencies much higher than those at which a person normally
breathes. This is actually a common occurrence of which we
may not be fully aware. Dogs do not sweat. They regulate their
temperature when they are hot by panting as you probably know.
When a dog pants he takes very shallow, very fast, quickly
repeated breaths. The size of these panting breaths is much
smaller that the animal's anatomical dead space, especially
in dogs with long necks. Yet, the dog feels no worse for this
type of breathing (at least all the dogs interviewed for this
article).
Devices have been developed to produce
high frequency, low amplitude breaths. These are generally
used on patients with respiratory distress syndrome (lungs
will not expand properly). These are most often neonates whose
lungs have not fully developed, but can also be older patients
whose lungs have been injured. High frequency ventilators
are also used on patients that have lungs that leak air. The
very low tidal volumes produced put less stress on fragile
lungs that may not be able to withstand the stretch required
for a normal tidal volume.
There are two main types of high frequency
ventilator: high frequency jet ventilators (HFJV) and
high frequency oscillatory ventilators (HFOV). The
HFJV directs a high frequency pulsed jet of gas into the trachea
from a thin tube within an endotracheal or tracheostomy tube.
This pulsed flow entrains air from inside the tube and directs
it toward the bronchi. The HFOV uses a piston arrangement
that moves back and forth rapidly to oscillate (vibrate lengthwise)
the gas in the patient's breathing circuit and airways. Both
of these techniques cause air to reach the alveoli and carbon
dioxide to leave the lungs by enhancing mixing and diffusion
in the airways. Convection plays a minor role in gas transport
with these ventilators while various forms of enhanced diffusion
predominate.
Although high frequency devices that
drive the pressure on the chest wall have been developed,
most high frequency ventilators in use today are applied to
the airway opening.
In future articles, the authors will
explore topics such as how ventilators work, the controls and
monitors that can be available on a ventilator, interpretation
of graphical displays of ventilatory variables, as well as various
clinical aspects of ventilator use.
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