Short answer: Place high-quality viral filters between the breathing circuit and the patient’s airway and between the expiratory limb and the machine. The use of these filters is essential to prevent contamination of the machine. (See previous article for details on which filtration devices to use). Note: Even with filters, breathing circuits should be discarded after every patient.
The anesthesia machine needs to be protected from contamination by a potentially infected patient for two reasons. First, if pathogens can enter the internal parts of the machine, they could be passed on to a subsequent patient. Second, respiratory gases sampled for gas analysis can pass pathogens on to other patients or health care professionals after leaving the gas analyzer if improperly managed.
The good news is that the same precautions can be applied to all patients. The strategy is the same regardless of the patient’s risk of infection. A high-quality filter placed between the breathing circuit and the patient’s airway will protect the machine from contamination and also filter gas sampled for analysis. Heat and moisture exchange filters (HMEFs) are a good choice because they preserve airway humidity and are designed so that sampled gas is filtered before it enters the gas analyzer. It is possible to use a filter at the airway that is not also an HMEF. If a filter only is used, lower fresh gas flows (1-2 L/min or less) are desirable during maintenance of anesthesia to preserve humidity in the circuit.
It is also recommended to add an effective viral filter between the expiratory limb of the circle system and the machine. Not only is this second filter a reasonable backup to protect the machine from any particles that pass the primary filter, but it significantly amplifies the effectiveness of the first filter. Given the fact that the primary filter can become less effective if soiled, the backup filter is a good recommendation. Another filter between the machine and the inspiratory limb is added sometimes but is not necessary to protect the machine from the patient nor to protect the patient if the machine is kept clean. The main reason to add an inspiratory limb filter is to eliminate the chance of error by placing a single filtered limb on the inspiratory rather than expiratory port.