Oxygen Flush Valve
The oxygen flush valve allows direct communication between the
oxygen high-pressure circuit and the low-pressure circuit (see Fig.
9-3
). Flow from the oxygen flush valve enters the low-pressure circuit
downstream from the vaporizers and downstream from the Datex-Ohmeda machine outlet
check valve. The spring-loaded oxygen flush valve stays closed until the operator
opens it by depressing the oxygen flush button. Actuation of the valve delivers
100% oxygen at a rate of 35 to 75 L/min to the breathing circuit.[9]
The oxygen flush valve can provide a high-pressure oxygen source
suitable for jet ventilation when the anesthesia machine is equipped with a one-way
check valve positioned between the vaporizers and the oxygen flush valve and when
a positive-pressure relief valve exists downstream of the vaporizers; this pressure
relief valve must be upstream of the outlet check valve. Because the Ohmeda Modulus
II has such a one-way check valve and its positive-pressure relief valve is upstream
from the check valve, the entire oxygen flow of 35 to 75 L/min is delivered to the
common gas outlet at a high pressure of 50 psig. However, the Ohmeda Modulus II
Plus and some Ohmeda Excel machines are not capable of functioning as an appropriate
oxygen source for jet ventilation. The Ohmeda Modulus II Plus, which does not have
the check valve, provides only 7 psig at the common gas outlet because some oxygen
flow travels retrograde through an internal relief valve located upstream from the
oxygen flush valve. The Ohmeda Excel 210, which does have a one-way check valve,
also has a positive-pressure relief valve downstream from the check valve and is
therefore unsuitable for jet ventilation.[16]
The
older North American Dräger Narkomed 2A, which does not have the check valve,
provides an intermediate pressure of 18 psig to the common gas outlet because some
oxygen flow travels retrograde through a pop-off valve located in the vaporizers.
[48]
The oxygen flush valve is associated with several hazards. A
defective or damaged valve can stick in the fully open position, resulting in barotrauma.
[49]
A valve sticking in a partially open position
can result in awareness by the patient because the oxygen flow from the incompetent
valve dilutes the inhaled anesthetic.[50]
Improper
use of normally functioning oxygen flush valves also can result in problems. Overzealous
intraoperative oxygen flushing can dilute inhaled anesthetics. Oxygen flushing during
the inspiratory phase of positive-pressure ventilation can produce barotrauma in
patients if the anesthesia
machine does not incorporate fresh gas decoupling or an appropriately adjusted inspiratory
pressure limiter. Anesthesia systems (i.e., Dräger Narkomed 6000, Julian, and
Fabius GS and Datascope Anestar) with fresh gas decoupling are inherently safer from
the standpoint of minimizing the chance of producing barotrauma from inappropriate
use of an oxygen flush valve (see "Fresh Gas Decoupling"). With traditional anesthesia
breathing circuits, excess volume cannot be vented during the inspiratory phase of
mechanical ventilation because the ventilator's relief valve is closed and the adjustable
pressure-limiting (APL) valve is "out of circuit" or closed.[51]
One caveat is illustrated by the Datex-Ohmeda S/5 ADU and Aestiva. These circle
systems use an integrated, adjustable pressure limiter. If this device is properly
adjusted, it functions like the APL valve to limit the maximum airway pressure to
a safe level, thereby reducing the possibility of barotrauma.[34]
[35]
[52]
If a
machine
is equipped with a freestanding vaporizer downstream from the common gas outlet,
oxygen flushing can deliver large quantities of inhaled anesthetic to the patient.
Inappropriate preoperative use of the oxygen flush to evaluate the low-pressure
circuit for leaks can be misleading, particularly on Datex-Ohmeda machines with a
one-way check valve at the common outlet.[53]
Because
backpressure from the breathing circuit closes the one-way check valve air-tight,
major leaks in the low-pressure-circuit can go undetected with this leak test (see
"Checking Anesthesia Machines").
