There are two reasons to protect the anesthesia machine from contamination by a potentially infected patient. First, if pathogens are allowed to 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 healthcare workers after leaving the gas analyzer if not managed properly.
The good news is that the same precautions can be applied to all patients. The strategy is not different depending upon 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 gases sampled for analysis. Heat and moisture exchange filters (HMEFs) are a good choice since they preserve airway humidity, and are designed so that sampled gas is filtered before it enters the gas analyzer (note that HMEs without filters provide no protection to the anesthesia machine or gas analyzer). It is possible to use a filter only at the airway that is not also a heat and moisture exchanger, but strategies like low flow anesthesia should be employed to preserve humidity. If a filter only is used at the airway, lower fresh gas flows (1-2 L/min or less) are desirable during maintenance of anesthesia to preserve humidity in the circuit.
While a high quality filter at the airway will protect both the sampled gas and the anesthesia machine, there are reasons to consider adding a second filter at the end of the expiratory limb. The airway filter can become soiled and lose effectiveness and in small patients, airway filters are designed to minimize dead space and as a result are generally less effective. Not only is the 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.
We're using a circuit with viral filters on both sides of the circuit as well as between the circuit itself and airway (3 total filters). Initially we had to use our supply of viral filters plus the circuit, but now we buy the whole thing pre-packaged from the manufacturer.
1, we usually put COVID patients as last case of that OR
2, we try to avoid GA whenever possible due to aerosolization risk
3, we place 2x viral filters on, i'm told it reduces the cross contamination risk to zero