The other day I arrived to our main hospital to relieve one of my partners who was working with several CRNAs. One of the CRNAs had just arrived in the cath lab where we had been asked to anesthetize a patient for a cardiac catheterization procedure. We induced general anesthesia with an ETT. Shortly thereafter I was called to the room for an alarm with warning indicator of " PEEP high. Blockage?,". I quickly determined that there was high pressure in the 3 Liter scavenging system bag and this was a result of not having the vacuum line connected to a central vacuum. I thought it odd that the anesthesia machine had been placed in the cath lab without having been connected to the central vacuum for scavenging of waste anesthesia gases (WAGs). But it turns out there are no connections for vacuum in this particular room.
In the typical OR, the anesthesia machine has a scavenging system set up so that all WAGs are emitted into the atmosphere and do not contaminate the OR. Below is a figure with the basic outline of how this system would be set up.
Because all modern ORs are built the same most anesthesia providers take it for granted that WAGs will be scavenged.
However, at times we are asked to provide anesthesia services outside of the OR setting. In this case, the anesthesia tech for the hospital placed an anesthesia machine in a site that had no medical gas column therefore, no site for suction for the anesthesia suction hook up. Therefore, the hose leading from the scavenging system meant for active suction of anesthetic gases from the machine to the environment was placed on the ground. The frequency of the build up of pressure could be decreased by using low flows (less than 1 L/m). However, this did not eliminate pressure build up. Therefore, periodically we were forced to disconnect the circuit from the anesthesia machine as a manual vent of a pressure build up in the anesthesia. The rationale for this will be discussed in more detail below.
The rationale for scavenging of WAGs comes from a number of studies on small concentrations of anesthetics on animals (predominantly rats) and humans.
Nitrous Oxide has been shown to increase fetal death in rats and also to result in an increased rate of rib and vertebral defects. In humans, dental assistants exposed to nitrous oxide in a setting without a scavenging system had a 59% decrease in fertility vs unexposed females. Dental assistants working with nitrous oxide and using a scavenging system had no decrease in fertility rates. The exposure limit resulting in fertility problems was at least 5 hours per week of work with nitrous oxide. Likewise, similar findings were noted for the occurrence of spontaneous abortion in exposed workers (at least 3 hours of exposure per week) vs un exposed female dental assistants (working with a scavenging system). The relative risk of spontaneous abortion was 2.6. In humans, there is no clear evidence for an increase rate of congenital abnormalities in females exposed to nitrous oxide.
Dental assistants exposed to 8 hours per week of halogenated agents had an elevated rate of spontaneous abortion (19.1 per 100 pregnancies) vs dental assistants who worked with a scavenging system (8.1 per 100 pregnancies). The wives a dentists with heavy exposure to halogenated agents also experienced spontaneous abortion at higher rates than non exposed females (10.2 vs 6.7 events per 100 pregnancies). A subsequent study found that spontaneous abortion was increased in exposed females and spouses of exposed males to halogenated anesthetics. In this study, they are were able to determine that congenital abnormalities were also increased in female workers exposed as well as to spouses of male exposed workers.
Unfortunately, due to the difficulties in conducting an RCT that is prospective, all of the data we have currently related to the harmful effects of anesthetics are retrospective in nature and therefore, not robust. However, there are multiple different studies in both animals and humans plus the biologic plausibility of danger related to long term chronic exposure to anesthetics that lend to the credibility of the current evidence available to us.
In the above studies, it is important to note that the groups who suffered an increase in spontaneous abortion from anesthetic agents (presumably) were working in an environment where waste anesthetic gases were not scavenged, whereas, they were often times compared to workers who had access to scavenging of waste anesthetic gases. Modern day anesthesia machines all come equipped with a scavenging system to dispose of all waste anesthetic gases to the environment. It should be noted that ALL air flow (oxygen, air or nitrous) that is not consumed and metabolized by the patient will make its way into the scavenging system. Therefore, much of the problem with waste anesthetic gases can be mitigated by low flow anesthesia. The scavenging system is made up of several parts with complicated names that are not terribly helpful in understanding their function.
- non-flammable and non-explosive
- Rapid onset/offset (partition co-efficient of 0.12 (lowest of all anesthetics)
- Zero metabolism, low toxicity, and devoid of teratogenicity.
- Produces high regional blood flow in brain, liver, kidney and intestine.
- Lacks cardiovascular depression