Scavenging refers to the method of extracting waste gases from the breathing system and venting them to an area where they will not be directly inhaled by staff or other patients.
Scavenging systems can be classified as open or closed.
Open refers to the basic system of extracting the gas from its point of entry into the theatre
Closed systems are more common and can be further subdivided into active and passive
No conservation of volatile agent is possible with either the active or the passive systems; conservation must occur within the anaesthetic breathing system itself, by the use of a circle system and low-flow anaesthesia.
The active and the passive both pass any waste gas to the atmosphere, polluting it to the same extent.
In the passive scavenging system an exhaust port collects the waste gases from the expiratory valve of the breathing system or from the ventilator and the gases pass through the transfer system (which consists of 30 mm low-resistance tubing) to the outside of the building, preferably above roof level.
If the theatre air is not recirculated, the waste gases can be piped to the exit port of the theatre ventilation system.
In the passive system, the gases are pushed to the atmosphere solely by the expiratory power of the patient
If the pathway to the atmosphere involves a vertical passage of gas, then the patient must overcome the atmospheric pressure required to push the gas over this distance; may be several floors of hospital! Means significant forces has to be overcome.
The use of gases with higher density, like nitrous oxide, adverse atmospheric conditions like high winds etc, will further increase the forces required to expel waste gases; this can even affect the cardiopulmonary status of the patient.
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