
the anesthesia delivery system has evolved
from a simple pneumatic device to a
complex multisystem workstation

For anesthesia care providers understanding
of its operation is essential.

Caplan and coworkers found that although
claims related to the medical gas delivery
system were rare, when they did occur, they
were usually severe and death or
permanent brain injury frequently resulted.
we examine the anesthesia workstation
piece by piece.
 We will describe the normal operation,
function, and integration of major
anesthesia workstation subsystems.
 we illustrate potential problems and
hazards associated with the various
components of the anesthesia delivery
system
 We overview appropriate preoperative
checks that may help detect and
prevent such problems.


Standards for Anesthesia Machines and
Workstations
› 1979: American National Standards Institute
(ANSI) Z79.8-1979[2]
› 1988: American Society for Testing and
Materials (ASTM) F1161-88[3]
› 1994: ASTM F1161-94[4] (reapproved in 1994
and discontinued in 2000)
› 2000: ASTM F1850-00[5]

Newly manufactured workstations must
have monitors that measure the following
parameters: continuous breathing system
pressure, exhaled tidal volume, ventilatory
carbon dioxide concentration, anesthetic
vapor concentration, FiO2, oxygen supply
pressure, SpO2, BP, and EKG.

The anesthesia workstation must have a
prioritized alarm system that groups the
alarms into three categories: high, medium,
and low priority
A complete anesthesia apparatus
checkout procedure must be performed
each day before the anesthesia
workstation is first used.
 An abbreviated version should be
performed before each subsequent case.
 The user must always refer to the original
equipment manufacturer's operator's
manual for special procedures or
precautions related to particular
workstations.


Every anesthesia care provider must be
aware that the ultimate responsibility for
proper machine pre-use safety checks
rests on the provider using the machine
to deliver anesthetic care.

The three most important preoperative
checks are:
› (1) calibration of the oxygen analyzer
› (2) the low-pressure circuit leak test
› (3) the circle system tests

It is the only machine safety device that
evaluates the integrity of the lowpressure circuit in an ongoing fashion

The only machine monitor that detects
problems downstream from the flow
control valves is the oxygen analyzer

sensing element must be exposed to
room air for calibration to 21%
checks the integrity of the anesthesia machine from the
flow control valves to the common gas outlet
 Leaks in the low-pressure circuit can cause hypoxia and/or
patient awareness
 Loose filler caps on vaporizers are a common source of
leaks
 Several different methods have been used:

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›
›
›
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the oxygen flush test,
the common gas outlet occlusion test,
the traditional positive-pressure leak test,
the North American Dräger positive-pressure leak test,
the Ohmeda 8000 internal positive-pressure leak test,
the Ohmeda negative-pressure leak test,
the 1993 FDA universal negative-pressure leak test,
and others.
Inappropriate use of
the oxygen flush
valve to check the
low-pressure circuit

Left, A negative-pressure leak-testing device is attached directly
to the machine outlet. Squeezing the bulb creates a vacuum in
the low-pressure circuit and opens the check valve.

Right, When a leak is present in the low-pressure circuit, room air
is entrained through the leak and the suction bulb inflates

if the bulb re inflates in less than 10
seconds, a leak is present somewhere in
the low-pressure circuit.

The “universal” negative-pressure leak
test is the most sensitive of all
contemporary leak tests because it is not
dependent on volume

It can detect leaks as small as 30
mL/min

The leak test is performed by closing the
pop-off valve, occluding the Y-piece, and
pressurizing the circuit to 30 cm H2O with
the oxygen flush valve. The value on the
pressure gauge will not decline if the circle
system is leak free

The flow test checks the integrity of the
unidirectional valves: The operator should
be able to inhale but not exhale through
the inspiratory limb and able to exhale but
not inhale through the expiratory limb.

self-diagnostic tests varies from one
model and manufacturer to another

to detect internal vaporizer leaks on this
type of system, the “leak test” portion of
the self-diagnostic must be repeated
separately with each individual vaporizer
turned to the “on” position
Diagram of a generic two-gas anesthesia machine
The pipeline supply: primary gas
source
 must supply: correct gases at
appropriate pressure
 In a survey: 31% reported
difficulties with pipeline systems (The

most common problem was inadequate oxygen
pressure)

The most devastating reported
hazard: accidental crossing of
oxygen and nitrous oxide
pipelines, (which has led to many deaths)

In the event that a pipeline crossover is
ever suspected immediately:
› First, turned on the backup oxygen cylinder
› Second, disconnect pipeline supply

The pipeline inlet fittings are gas-specific
Diameter Index Safety System (DISS)

Medical gases attached to the
anesthesia machine via the hanger yoke
assembly.

Each hanger yoke is equipped with the
Pin Index Safety System (PISS).

The PISS is a safeguard introduced to
eliminate cylinder interchanging
Pin Index Safety System (PISS)
Pin-Indexed Yoke Assemblies
and
Cylinder Valve Connections

A check valve is located downstream
from each cylinder and serves several
functions.
› First, it minimizes transfer of gas from a cylinder
at high pressure to one with lower pressure.
› Second, it allows an empty cylinder to be
exchanged for a full one while gas flow
continues from the other cylinder into the
machine
› Third, it minimizes leakage from an open
cylinder to the atmosphere if one cylinder is
absent.



Each cylinder supply source has a pressurereducing valve known as the cylinder
pressure regulator
The oxygen cylinder pressure regulator
reduces the oxygen cylinder pressure from
a high of 2200 psig to approximately 45
psig.
The nitrous oxide cylinder pressure regulator
receives pressure of up to 745 psig and
reduces it to approximately 45 psig.

The gas supply cylinder valves should be turned off when
not in use, except during the preoperative machine
checkout period.

the volume of gas remaining in the cylinder is proportional
to cylinder pressure.

use of a pneumatically driven mechanical ventilator will
dramatically increase oxygen utilization rates,

Hand ventilation at low fresh gas flow rates may consume
less than 5% of the amount of oxygen consumed by
intermediate flow meter settings coupled with the use of
pneumatically powered mechanical ventilation

Nitrous oxide cylinder ??

abrupt or insidious oxygen pressure failure
had the potential to lead to the delivery of
a hypoxic mixture.

The 2000 ASTM F1850-00 standard states
that “The anesthesia gas supply device shall
be designed so that whenever oxygen
supply pressure is reduced to below the
manufacturer specified minimum, the
delivered oxygen concentration shall not
decrease below 19% at the common gas
outlet.”

A fail-safe valve is present
in the gas line supplying
each of the flow meters
except oxygen. Controlled
by oxygen supply pressure

the valve shuts off or
proportionally decreases
the supply pressure of all
other gases as the oxygen
supply pressure decreases.
An oxygen failure protection device that responds
proportionally

Many older anesthesia
machines have a
pneumatic alarm device
that sounds a warning when
the oxygen supply pressure
decreases to a
predetermined threshold
value, such as 30 psig

Electronic alarm devices are
now used to meet this
guideline.

Most contemporary Datex-Ohmeda workstations have
a second-stage oxygen pressure regulator set at a
specific value ranging from 12 to 19 psig

precisely controls
and measures
gas flow to the
common gas
outlet:
› Traditional glass
flow meter
› Electronic flow
sensors
 numerical
 graphic
 combination