Safety Devices for Oxygen Supply Pressure Failure
Oxygen and nitrous oxide supply sources existed as independent
entities in older models of anesthesia machines, and they were not pneumatically
or mechanically interfaced. Abrupt or insidious oxygen pressure failure could lead
to the delivery of a hypoxic mixture. The 2000 ASTM F1850-00 standard states, "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."[7]
Contemporary anesthesia machines have a number of safety devices that act together
in a cascade manner to minimize the risk of hypoxia as oxygen pressure decreases.
Several of these devices are described in the following sections.
Pneumatic and Electronic Alarm Devices
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. The 2000 ASTM F1850-00 standard mandates that medium priority
alarm shall be activated within 5 seconds when the oxygen pressure decreases below
a manufacturer-specified threshold pressure.[7]
Electronic alarm devices are used to meet this guideline.
Fail-Safe Valves
A fail-safe valve is present in the gas line supplying each of
the flow meters except that for oxygen. Controlled by oxygen supply pressure, the
valve shuts off or proportionally decreases the supply pressure of all other gases
(e.g., nitrous oxide, air, carbon dioxide, helium, nitrogen) as the oxygen supply
pressure decreases. Unfortunately, the misnomer "fail-safe" has led to the misconception
that the device prevents administration of a hypoxic mixture. This is not the case.
Machines that are not equipped with a flow proportioning system (see "Proportioning
Systems") can deliver a hypoxic mixture under normal working conditions. The oxygen
flow control valve can be closed intentionally or accidentally. Normal oxygen pressure
keeps other gas lines open so that a hypoxic mixture can result.[8]
[9]
Datex-Ohmeda machines are equipped with a fail-safe valve known
as the pressure sensor's shut-off valve ( Fig.
9-4
). The valve operates on a threshold principle, and it is open or closed.
Oxygen supply pressure opens the valve, and the valve's return spring closes the
valve. Figure 9-4
shows
a nitrous oxide pressure sensor's shut-off valve with a threshold pressure of 20
psig.[11]
[12]
[14]
[15]
[16]
In Figure
9-4A
, an oxygen supply pressure greater than 20 psig is exerted on the
mobile diaphragm. This pressure moves the piston and pin upward, and the valve opens.
Nitrous oxide flows freely to the nitrous oxide flow control valve. In Figure
9-4B
, the oxygen supply pressure is less than 20 psig, and the force of
the valve's return spring completely closes the valve.[9]
Nitroux oxide flow stops at the closed fail-safe valve, and it does not advance
to the nitrous oxide flow control valve.
Dräger Medical uses a fail-safe valve known as the oxygen
failure protection device (OFPD) to interface the oxygen pressure with
that of other gases, such as nitrous oxide or other inert gases.[19]
[20]
[21]
It differs
from Datex-Ohmeda's oxygen pressure sensor's shut-off valve because the OFPD is based
on a proportioning principle rather than a threshold principle. The pressure of
all gases controlled by the OFPD decreases proportionally with the oxygen pressure.
The OFPD consists of a seat-nozzle assembly
Figure 9-4
Shut-off valve of the pressure sensor. A,
The valve is open because the oxygen supply pressure is greater than the threshold
value of 20 psig. B, The valve is closed because
of inadequate oxygen pressure. (Adapted from Bowie E, Huffman LM: The Anesthesia
Machine: Essentials for Understanding. Madison, WI, Ohmeda, a Division of BOC Health
Care, 1985.)
connected to a spring-loaded piston ( Fig.
9-5
). The oxygen supply pressure (see Fig.
9-5, left panel
) is 50 psig. This pressure pushes the piston upward, forcing
the nozzle away from the valve seat. Nitrous oxide and other gases advance toward
the flow control valve at 50 psig. The oxygen pressure in the right panel is 0 psig.
The spring is expanded and forces the nozzle against the seat, preventing flow through
the device. The center panel shows an intermediate oxygen pressure of 25 psig.
The force of the spring partially closes the valve. The nitrous oxide pressure delivered
to the flow control valve is 25 psig. There is a continuum of intermediate configurations
between the extremes (0 to 50 psig) of oxygen supply pressure. These intermediate
valve configurations are responsible for the proportional nature of the OFPD.[19]
It is important to understand the differences of these fail-safe devices; the Datex-Ohmeda
pressure sensor's shut-off valve is an all-or-nothing threshold device, whereas the
Dräger OFPD is a variable-flow proportioning system.
Second-Stage Pressure Regulator for Oxygen
Most contemporary Datex-Ohmeda workstations have a second-stage
pressure regulator for oxygen that is set at a specific value between 12 and 19 psig.
[11]
[12]
[13]
[14]
[15]
[16]
The oxygen flow meter's output is constant when the oxygen supply pressure exceeds
the threshold value. The Datex-Ohmeda pressure sensor's shut-off valves are set
at a higher threshold value (20 to 30 psig). This ensures that oxygen is the last
gas flow to decrease if oxygen pressure fails.
