Generic Anesthesia Machine
A diagram of a generic two-gas anesthesia machine is shown in
Figure 9-3
. The pressures
within the anesthesia machine can be divided into three circuits: a high-pressure,
an intermediate-pressure, and a low-pressure circuit (see Fig.
9-3
). The high-pressure circuit is confined
to the cylinders and the cylinders' primary pressure regulators. For oxygen, the
pressure range of the high-pressure circuit extends from a high of 2200 pounds per
square inch gauge (psig) to 45 psig, which is the regulated cylinder pressure. For
nitrous oxide in the high-pressure circuit, pressures range from a high of 750 psig
in the cylinder to a low of 45 psig. The intermediate-pressure
circuit begins at the regulated cylinder supply sources at 45 psig, includes
the pipeline sources at 50 to 55 psig, and extends to the flow control valves. Depending
on the manufacturer and specific machine design, second-stage pressure regulators
may be used to decrease the pipeline supply pressures to the flow control valves
to even lower pressures, such as 14 or 26 psig within the intermediate-pressure circuit.
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The low-pressure circuit extends from the flow control
valves to the common gas outlet. The low-pressure circuit includes the flow tubes,
the vaporizers, and a one-way check valve on most Datex-Ohmeda machines.[8]
Both oxygen and nitrous oxide have two supply sources: a pipeline
supply source and a cylinder supply source. The pipeline supply source is the primary
gas source for the anesthesia machine. The hospital piping system provides gases
to the machine at approximately 50 psig, which is the normal working pressure of
most machines.
Figure 9-3
Diagram of a generic two-gas anesthesia machine. (Modified
from Check-Out, a Guide for Preoperative Inspection of an Anesthesia Machine. Park
Ridge, IL, American Society of Anesthesiologists, 1987.)
The cylinder supply source serves as a backup if the pipeline fails. The oxygen
cylinder source is regulated from 2200 to approximately 45 psig, and the nitrous
oxide cylinder source is regulated from 745 to approximately 45 psig.[8]
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A safety device traditionally referred to as the fail-safe
valve is located downstream from the nitrous oxide supply source. It
serves as an interface between the oxygen and nitrous oxide supply sources. This
valve shuts off or proportionally decreases the supply of nitrous oxide (and other
gases) if the oxygen supply pressure decreases. Contemporary machines have an alarm
device to monitor the oxygen supply pressure. A high-priority alarm is actuated
when the oxygen supply pressure declines to a predetermined threshold, such as 30
psig.[8]
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Most Datex-Ohmeda machines have a second-stage oxygen regulator
located downstream from the oxygen supply source in the intermediate-pressure circuit.
It is adjusted to a precise pressure level, such as 14 psig.[9]
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This regulator supplies a constant pressure
to the oxygen flow control valve, regardless of fluctuating oxygen pipeline pressures.
For example, the flow from the oxygen flow control valve remains constant if the
oxygen supply pressure is greater than 14 psig.
The flow control valves represent an important landmark because
they separate the intermediate-pressure circuit from the low-pressure circuit. The
low-pressure circuit is the part of the machine that is downstream from the flow
control valves. The operator regulates flow entering the low-pressure circuit by
adjusting the flow control valves. The oxygen and nitrous oxide flow control valves
are linked mechanically or pneumatically by a proportioning system to help prevent
delivery of a hypoxic mixture. The flow travels through a common manifold and may
be directed to a calibrated vaporizer. Precise amounts of inhaled anesthetic can
be added, depending on the setting of the vaporizer control dial. The total fresh
gas flow then travels toward the common gas outlet.[8]
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Many Datex-Ohmeda anesthesia machines have a one-way check valve
between the vaporizers and the common gas outlet.[9]
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Its purpose is to prevent backflow into the
vaporizer during positive-pressure ventilation, thereby minimizing the effects of
downstream, intermittent pressure fluctuations on inhaled anesthetic concentration
(see "Intermittent Backpressure"). The presence or absence of this check valve profoundly
influences which preoperative leak test is indicated (see "Checking Anesthesia Machines").
The oxygen flush connection joins the mixed-gas pipeline between the oneway check
valve (when present) and the machine outlet. When the oxygen flush valve is activated,
the pipeline oxygen pressure has a "straight shot" to the common gas outlet.[8]
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