Anesthesiology - Biomedical Engineering Desk Reference - page 416

between a person and the anesthesia machine is required.
Breathing circuits convert the machine's steady gas
output to a flow and pressure cycle that is consonant with
the human breathing cycle.
In keeping with the Ideal Gas Law, PV ¼ nRT, to
maintain a constant baseline pressure, one must also
maintain a constant baseline volume. If there is excessive
(or a growing) volume in a closed system, pressure will
decrease. Respiratory baseline pressure is measured at
end-tidal expiration when there is no longer significant
flow. This measurement is known as ''positive end-tidal
expiratory pressure'' (PEEP). A second ventilation in-
dicator is ''peak inspiratory pressure'' (PIP), the maxi-
mum pressure attained during the inspiratory cycle.
When a patient is paralyzed to the point where they no
longer spontaneously ventilate, positive-pressure venti-
lation is used. This involves delivering pressurized tidal
volumes of gas to the patient's lungs for oxygen uptake
supporting their required metabolism. Under normal
circumstances (no leaks), an anesthesia machine's gas
output has two places to go: Patient uptake or its scav-
enging system.
A primary component of breathing circuits is the
tubing and its configuration to the patient. Some
breathing circuits rely on high fresh gas flow rates to
prevent rebreathing of respiratory gases while others in-
volve absorbent to neutralize expired carbon dioxide. A
thorough discussion of all the breathing circuit (open,
semi-open, semi-closed, and closed) options is too great
for this text. The two most common circuits used in the
Unites States, the Bain Circuit and Circle System, are
described below.

Carbon dioxide absorbers

Recirculating breathing circuit gases require the patient's
expired carbon dioxide to be removed, to prevent
hypercarbia. This is accomplished by means of a CO 2
absorbent contained in a housing, called an ''absorber.''
The breathing circuit absorber must allow the use of both
automated (machine ventilator) and manual (breathing
circuit bag) ventilation. Most present-day absorbers are
stand-alone subassemblies that tie into related functions
of the machine. They are modular and can be exchanged
easily if needed. They are the union between patient and
machine and are where respiratory gases mix with fresh
gas flow from the machine. Delivered oxygen concentra-
tion is measured in the absorber's inspiratory limb that is
most proximal to the patient and in a relatively safe lo-
cation where it is unlikely to be damaged or disconnected.
One advantage of modern absorbers is that they measure
PIP after the inspiratory check valve. Older style ab-
sorbers require the use of a second gauge when a PEEP
valve is used for clinical reasons (discussed below). Gages
on older designs only display interior absorber pressure.
When the inspiratory check valve closes, they do not
display true airway pressure and do not reflect PEEP.
In an effort to address some of the limitations of the
relatively simplistic time-cycled, volume-controlled ven-
tilators that are typically found in anesthesia, manufac-
turers are introducing a new generation of absorbers that
are more fully integrated into the machine. This design
approach has advantages and disadvantages. New designs
can be sterilized. While not required in the United States,
parts of the European Community require that breathing
circuit components be sterilized between patients.
One significant point to consider is that absorbers re-
quire daily service and therefore have greater potential
for problems such as leaks. Although they are designed
with this in mind, personnel who are not technically
trained might partially disassemble the absorber. The
system is subject to repeated changes of disposable
breathing circuit tubing, spills, daily cleaning, and ab-
sorbent

Bain circuit

A Bain Circuit is a semi-open breathing circuit that does
not recirculate respiratory gases and relies on high fresh
gas flow rates to prevent rebreathing. It is most often
used for neonatal and thoracic applications. Its greatest
advantages are that it creates little dead space, it is
lightweight, and gases can be scavenged easily. The most
significant drawbacks to this circuit are that it requires
high fresh gas flow (delivering cold dry gases to the pa-
tient) and can be cost-prohibitive with costly volatile
agents.

changes, and is otherwise exposed to

a demanding environment.

Absorbent

Various commercially available absorbents are used to
absorb carbon dioxide. For practical purposes, the CE
and BMET must be aware and concerned with the absor-
bent's systematic application and its use. By-products of
the chemical reactions between the absorbent and CO 2
are beneficial to the patient and to the proper function of
the absorber. However, one reaction might prove to be
a hindrance in excess. Removal of carbon dioxide is
clearly beneficial and a primary function of the absorbent.
In the process, a chemical reaction produces a color

Circle system

By far, the most common is the circle system, where
respiratory gases move through a housing incorporating
unidirectional valves to recirculate gases, minimizing
waste and maximizing warmth and humidity in the cir-
cuit. The system's greatest drawback is its level of
complexity. However, nearly all anesthesia machines are
designed to operate with a circle system.