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Last Updated 8/18/03
The Scavenging System

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.