Mandatory minimal flow of oxygen (200 ml/min)
Flowmeters on many anesthesia machines (including the Ohmeda Modulus
II anesthesia machine modeled here) have a mandatory minimum oxygen
flow rate of 200 ml/min when the machine is turned on.
If an oxygen supply is available, the bobbins of
the O2 flowmeter will not completely return to zero in the simulation.
Without an O2 supply, the bobbins return to zero.
Hypoxic safe-guard : the Link-25 system
The hypoxic safe-guard links the nitrous oxide (N2O) flow control valve
to the oxygen (O2) flow control valve. The Link-25 system from Ohmeda
connects the flowmeter control valves in such a way that the ratio of
O2 to N2O flow can never be less than 1:3. In other words, it prevents
the administration of a fresh gas flow with an FiO2 of less than 0.25.
N2O flow is automatically lowered when O2 flow is decreased too much
by the user, but the N2O flow is not changed when O2 flow is increased.
O2 flow is automatically increased if the N2O flow is increased too
much, but is unaffected when N2O is decreased. A few examples:
- If O2 flow is decreased to its minimum (200 ml/min), N2O flow will
also automatically be decreased to 600 ml/min if it was higher.
- If N2O flow is increased to maximum (6 l/min), O2 flow will also automatically
be increased to 2 L/min if it was lower.
The hypoxic safe-guard does not detect whether the correct gas is actually
flowing through the flowmeters (i.e., whether the gas flowing in the
O2 flowmeter is really O2) and it should be noted that only N2O and
O2 flows are interlinked.
No precise flowrates can be set on the simulation
but the flowmeters can be turned on or off. The behavior of the Link-25
system is modeled in the simulation by discrete movements of the bobbins.
Function of the common gas outlet check valve
When ventilation is controlled or assisted, positive pressure from the
breathing system could potentially be transmitted back to the vaporizers
and flowmeters. Use of the oxygen flush valve could also potentially
create the same problem. Backflow of gas to the vaporizer could cause
changes in the delivered inhalational anesthetic concentration. In addition,
it could also cause inaccurate flowmeter readings. The addition of the
common gas outlet check valve however, minimizes backflow of gas to
the vaporizers and flowmeters. Not all types of anesthesia machines
have a common gas outlet check valve.
The Ohmeda Modulus machine does have a common gas
outlet check valve which is shown in the simulation.
Contribution of fresh gas flow to the tidal
Fresh gas continues to flow from the flowmeters during inspiration.
Depending on the magnitude of the fresh gas flow (FGF), it can contribute
significantly to the delivered tidal volume. For example, if the respiratory
rate of mechanical ventilation is 10 breaths/minute with an I:E ratio
of 1:2, then the inspiration time (Ti) will last 2 seconds. If the fresh
gas flow is 6000 ml/min or 100 ml/second then 200 ml will be added to
each tidal volume breath. The tidal volume is augmented by the fresh
gas flow by the product of FGF times Ti.
Upon increasing the fresh gas flow during mechanical
ventilation, the lungs can be seen to inflate more in the simulation.