Function of the Scavenging Bag and Overinflation
With an active waste gas scavenging system, a constant vacuum (suction)
is applied to the scavenging system and outflow from the scavenging
system to the vacuum line is continuous. However, discharge of gas into
the scavenging system from the breathing circuit is usually intermittent
because more gas is vented during mechanical expiration. The scavenging
bag provides a reservoir or buffer that accommodates surges of gas with
peak inflow rates exceeding the continuous outflow to the vacuum line.
The scavenging bag allows the flow rate to be set just above the average
outflow over time from the anesthesia breathing circuit, rather than
at the peak outflow rate of gases. During mechanical inspiration, the
inflated scavenging bag will then be slowly suctioned.
This behavior of the scavenging bag is modeled in the simulation. The bag can be seen to inflate and deflate depending upon the phase of ventilation and the ventilation mode (manual or mechanical).
Function of the Positive and Negative Pressure
Relief Valves
The scavenging system has negative and positive pressure relief valves
to prevent excessively high or low pressures from developing in the
scavenging manifold and being transmitted to the patient via the breathing
circuit. The positive pressure relief valve will lift up and open at
approximately 5 cm H2O. This happens when the the vacuum is too weak
or if the vacuum adjustment valve is completely closed. A slight hissing
or rattling sound can be heard (indicating the venting of exhaust gases
into the room) during the expiratory phase of mechanical ventilation
or when the manual bag is squeezed. In addition, the scavenging bag
will be fully inflated. To prevent negative pressure from being transmitted
to the breathing circuit, the negative pressure relief valve will actuate
at approximately -0.25 cm H2O and allow room air to enter the scavenging
system to "break" the vacuum. The scavenging bag will be collapsed
flat.
The positive and negative pressure relief valves have been modeled in the simulation and their function can be observed by disconnecting the waste gas evacuation wall outlet, by adjusting the vacuum adjustment valve, or by adjusting the fresh gas flow.
Optimal Setting for the Vacuum Adjustment Valve
The vacuum adjustment valve allows the user to control the outflow generated
by the vacuum. The optimal setting of the vacuum adjustment valve depends
on the amount of excess gas that is vented to the scavenging system
and as such depends on the fresh gas flow settings. The waste gas evacuation
bag should neither be fully inflated or fully deflated for prolonged
periods of time.
In the simulation the size of the scavenging bag is affected by the position of the vacuum adjustment valve and the flow of excess gas.