Biomedical Engineering Reference
In-Depth Information
fluid dynamics. A fluid is anything (liquid, gas, or both)
that takes the shape of its container. Materials have
three phases: gas, liquid, and solid. Vapor is the gaseous
phase of a liquid at room temperature and atmospheric
pressure. Pressure, volume, and temperature are related.
A gas is fully saturated when it contains the maximum
amount of vapor possible without precipitating out to
a liquid. If two or more gases are mixed in one container,
the total pressure exerted is made up of the sum of
partial pressures created by each individual gas.
Operating principles
Side-stream
All of the mixed gas flows through the inside of a side-
stream vaporizer. The main stream of this gas (when
flowing through a vaporizer it is also known as the ''carrier
gas'') has a fraction diverted to the wick/sump assembly.
The amount of flow of this side-stream diverted to the
wick assembly is controlled by the setting on the vapor-
izer output dial. The higher the output setting, the
greater the amount diverted. This side-stream flow
becomes fully saturated with anesthetic vapor in the
wick assembly and then returns to the mainstream con-
trolling agent concentration output in the fresh gas flow.
Two physical principles cool this style of vaporizer,
requiring temperature compensation. Forced convection
(flow of gas) and latent heat of vaporization (energy re-
quired to vaporize a liquid) cool the vaporizer while in
use. Cooling an anesthetic agent reduces its partial
pressure, which in turn lowers the vaporizer's output. To
compensate for this cooling, vaporizers incorporate
a bimetallic temperature-sensitive diverter that adjusts
the amount of flow fed to the side-stream from the main
carrier gas. As temperature drops, the side-stream of gas
increases to maintain a constant output over varying
temperature during normal operation. Side-stream va-
porizers, which are relatively simple in design, are the
most commonly used and are available for all of the
currently used agents, with the exception of Suprane.
Partial pressure
An understanding of partial pressure is required, to
support anesthesia technology effectively. Partial pres-
sures are an absolute measurement defining the total
number of molecules where percentages are relative to
total gas mixture. Partial pressure is used and applied
in a number of technologies within anesthesia (e.g.,
vaporization, ventilation, and respiratory gas monitor-
ing). One example to help clarify the term is dry (versus
moist) air. Air is made of 21% oxygen. The remainder is
mostly nitrogen and trace gases (negligible partial pres-
sure for this example). Atmospheric pressure is ap-
proximately 760 mmHg. On a dry, warm day (98
For
37
C) of 0% relative humidity, oxygen partial pressure
is 21% of 760 (0.21
760), or 159.6 mmHg. The
remaining pressure must be from nitrogen (760-159.6),
or 600.4 mmHg. Then, on the following day, the tem-
perature and atmospheric pressure are the same as the
day before, but the relative humidity is 100% (the air is
fully saturated with water vapor and cannot contain any
more without rain-out). At 37
C, fully saturated water-
vapor pressure is 47 mmHg. On the second day, atmo-
spheric pressure contains three gases (nitrogen, oxygen,
and water vapor). Oxygen concentration remains con-
stant at 21%, but its partial pressure is a function of the
three gases [0.21
(760-47)] or 149.7 mmHg. The
partial pressure of nitrogen is 0.79
(760-47) mmHg,
or 563.3 mmHg.
Because volatile inhalation agents have varying partial
pressures that affect concentration output, modern
vaporizers are made agent-specific. The most common
agents currently in use are forane, ultane and suprane
(manufacturer trade names are isoflurane, sevoflurane
and desflurane, respectively). Vaporizers are available as
either funnel- or key-filled. Funnel-filled vaporizers can
be more convenient and reduce the possibility of vapor
lock when filling, but they are susceptible to being filled
with the wrong agent. Key-filled vaporizers are more
cumbersome and require use of a filler, but they virtually
eliminate the possibility of cross-contamination because
the agent is not poured directly into the vaporizer as in
the case of funnel-filled units.
Mainstream injection
Suprane is a volatile agent with high partial pressure that
makes it difficult to deliver with a traditional side-stream
vaporizer. It can be delivered only using a mainstream
injection vaporizer. Suprane vaporizers are heated and
pressurized to control more precisely the environment in
which is it stored when in use. Mainstream vaporizers
control output by injecting measured amounts of agent
directly into the main stream of carrier gas.
Future machines might incorporate new designs that
are more closely related to the mainstream vaporizers
for other agents. Vaporizers are also used on heart-lung
machines to control the anesthetic agent delivered
through the oxygenator during cardiopulmonary bypass
surgery.
Breathing circuits
Breathing circuits serve as the interface between patient
and machine. Because people breathe in volumetric flow
and pressure cycles and anesthesia machines deliver
a unidirectional stream of gas (at a specific oxygen con-
centration and controlled anesthetic agent), an interface
