Biomedical Engineering Reference
In-Depth Information
Holes in cylinders work in conjunction with pins in the
yokes. For anesthesia-supply gases (oxygen, nitrous
oxide, and air), one pin location is fixed, and the other
varies depending on gas type. One inherent weakness in
this system is that if the wrong pin pulls out or breaks, it
could lead to a misconnection.
Oxygen pressure detecting system
and distribution
To detect the presence of oxygen pressure, there is
a pressure-controlled, normally off, valve called the ''fail
safe,'' which is intended to protect the patient from
a hypoxic mixture. Its function is to shut off secondary
and tertiary gases in case of oxygen loss. The name is
a misnomer because the device does fail. It is ineffective
when other gases are delivered in the oxygen pipeline or
when flow-control valves or hypoxic mixture interlock
systems are out of adjustment. Machines also have
a pressure-sensing alarm that indicates when supply
pressure has fallen below a threshold to inform the user
to take evasive action (e.g., to turn on cylinder supply).
Oxygen-pipeline supply is plumbed to at least five
locations at full pressure within the machine. The
oxygen-flush valve is always active and capable of de-
livering 50-65 lpm. It incorporates a safety-pressure
relief at about 120 mmHg for catastrophic failures that
far exceed normal breathing-circuit pressures. The flush
valve is dangerous to the patient in the hands of an un-
informed user. Activating the flush for 1 second will in-
crease the tidal volume delivered by 1 liter, which
is easily enough to cause barotrauma to a susceptible
patient. The fail-safe and pressure-sensing alarms, as
described above, require line pressure to operate cor-
rectly. Flowmeters combined with the flow-control
valves feed the oxygen supply to the patient via vapor-
izer(s) and breathing circuit. They are the interfaces
between the high- and low-pressure sides of the machine.
Ventilators need a working gas. The United States has
standardized on oxygen, but other parts of the world
could use air. Auxiliary flow meter and other power
outlets are fed from accessory connections.
The anesthesia machine has two limits for the mini-
mum oxygen-flow rate. One is fixed and sets at the
absolute minimum deliverable, while the other varies in
proportion to the nitrous oxide flow-rate setting. Most
machines are designed for a minimum of 200 ml, but
some go as low as 50 ml. This is intended to reduce the
possibility of a hypoxic mixture because there is always
the minimum patient-uptake requirement supplied. An
average adult will metabolize about 2-300 ml/min of
oxygen.
Nitrous oxide flow control is linked directly or in-
directly to the flow of oxygen, also to prevent delivery of
Figure 4.4-2 Anesthesia machine.
mmHg and about 70 cmH
2
O. In the same units of
measure, an anesthesia machine needs to safely reduce
and control gases fed at 3500 cmH
2
O and to supply them
to the breathing circuit normally operating at about
35 cmH
2
O. In other words, the supply pressures are 100
times that of the breathing circuit. Machines are
constructed with a high-pressure side and a low-pressure
side. The high-pressure side is primarily the supply,
and the low-pressure side is any part operating near
breathing-circuit pressure.
There are a number of items with designed in-
compatibility in anesthesia machines. Pipeline gas-supply
connections are available in a few different configura-
tions. The diameter indexing safety system (DISS) is a
common configuration. The thread and inner diameters
vary to prevent accidental connections to a wrong
supply. Virtually all machines use E cylinders as backup
supplies. Backup oxygen is vital for patient safety and
must always be available. Cylinders use the pin index
safety system (PISS) to prevent wrong connections.
