Instead of the current option of heating it, engineers designing the
desflurane vaporiser also had the option of cooling it to a controlled temperature
below normal room temperatures. However, cooling is technologically more
difficult to achieve in such a small vaporiser. Also, if the cooling
mechanism failed, the desflurane might boil and possibly expose the patient to a sudden high
concentration of vapor.
TANK (SUMP) SHUT OFF VALVE
There are a lot of safety features packed into the vaporiser. There is a
electronically controlled tank (sump) shut off valve.
This valve can completely stop the flow of desflurane out of the vaporising
chamber. For an example, it is shut closed by the computer when:
The dial is at a setting of zero %
It detects faults in the system
It detects, using a tilt sensor, that the vaporiser is being tilted (e.g. to
over 15 degrees from vertical)
There is no power
MULTIPLE SAFETY SYSTEMS
There are multiple parallel systems for safety. For an example there are
two separate "differential pressure transducers" working independently. If
the readings between them don't agree, the computer senses that something is
wrong and shuts down the tank shut off valve. It also alarms for you to take action.
There are multiple heaters and multiple heat sensors. There is a back
up system to shut power to the heaters if there is over heating which can
otherwsie cause excessive vaporiser pressure.
WARNING SYSTEM
The
vaporiser has a panel of lights and an alarm to communicate with you.
As
the vaporiser is powered up, all the lights briefly light up to show you that
non of the bulbs are burnt. After the vaporiser is warm enough and
hasn't detected any faults, the green operational light comes on to say that it
is ready to serve you.
BATTERY
The vaporiser does have a small battery. However, when there is a power
failure, this battery is completely inadequate to heat and operate the
vaporiser. The role of the battery is only to supply power to the indication
systems and alarms (to inform you that there is power failure!). Hopefully
your hospital is equipped with a back up electrical generator.
EFFECT OF NITROUS OXIDE
The
use of nitrous oxide in the fresh gas flow can lower the delivered concentration
of desflurane to less than that set on the dial. Refer to the diagram
below when reading the explanation of this effect.
The
flow across the fixed restriction [10] is laminar (this type of flow is
discussed elsewhere). Laminar flow is affected by the viscosity of the
gas. Lower the viscosity, easier is the flow.
Nitrous oxide has a lower viscosity than oxygen and thus passes the
fixed resistance [10] easier than oxygen does. Therefore, for a given flow
in pipe [2], nitrous oxide raises the pressure in pipe [2] less than what
oxygen would.
The
vaporiser however has been calibrated by the manufacturers to respond to pressure changes caused by
various flows of 100% oxygen. The vaporiser thus assumes that whatever pressure changes
that are sensed by the differential transducer [11] are due to the flow of
oxygen and this causes it to under deliver anaesthetic concentration.
Hopefully, the diagrams below will help you to understand this problem better.
Below, we are sending a fresh gas flow of 5 liters of oxygen through the
vaporiser. The flow of oxygen meets the fixed resistance and the pressure
rises by a certain amount. This causes the diaphragm of the differential
pressure transducer to move and the system responds by giving a certain flow of
desflurane.
Now
let us change the mixture of fresh gas flow that we give. We give 5 liters of
a 50% oxygen and 50 % nitrous oxide mixture ( 2.5 L + 2.5 L ) instead of 5
liters of 100% oxygen. Note that
we have not changed the total fresh gas flow, which remains at 5 liters.
The diagram below shows that, because the nitrous oxide can more easily pass via
the resistance (because it is less viscous) the pressure rise is not as great as
before. The diaphragm of the pressure transducer therefore moves less and
the system assumes that the fresh gas flow must have dropped. So it
reduces the flow of desflurane. But in reality , the fresh gas flow is
still 5 liters and for the same flow as before, the amount of desflurane
injected is now less. This is how nitrous oxide reduces the concentration of
delivered desflurane to below what is set on the dial.
