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.
Halogenated agents:
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.
The above diagram highlights how the scavenging system is incorporated into the anesthesia machine. It should be noted that when the anesthesia machine is in manual or bag ventilation mode, when the APL value "pop-off" valve is opened all the way to relieve "pressure" in the reservoir bag, the excess flow is directed into the scavenging system (which contains another reservoir bag [see figure above]). Modern day OR's have active scavenging systems. In other words, the excess flow from the anesthesia machine is hooked up to the hospital vacuum system (see figure above), and the excess gas is actively suctioned off. If there is any obstruction to this vacuum system or its tubing, you will quickly develop a build up of pressure in the reservoir bag (see figure above). This will create a back up of pressure leading to the ventilator piston (setting off an alarm as indicated above) or to the reservoir bag on the anesthesia machine. Opening the APL valve "pop-off valve", to relieve the reservoir bag will not help the problem at all since the "pop-off" valve simply leads directly to the scavenging system reservoir bag, the source of the high pressure. The scavenging interface on anesthesia machines come in two varieties. An open interface system has no valves between the interface and the outside vent. Therefore, this provides a safety mechanism in that there are no valves that can malfunction leading to pressure build up to the patient. Our anesthesia machine, The Datex Ohmeda Asys CS2, has a scavenging interface that is a closed system, and therefore, relies on a positive pressure and negative pressure relief valve. In our particular case, after removing the patient from the anesthesia ventilator, and opening the APL valve to vent excess pressure to no avail, it became evident that pressure was building up distal to the APL valve, which would be the scavenging system. The scavenging interface allowed pressure to build up until 10 cmH2O, when the positive pressure valve vented excess pressure, but after 15 seconds of continuous pressure greater than 10 cmH2O, the "PEEP high" alarm is triggered on the machine.
It turns out that the Asys CS2 also has an option for an open scavenging interface system. In this system, there would be no alarm because there would be no build up of pressure in the case of insufficienct extraction of WAGs. In these systems, they are designed so that extraction flow is greater than average waste gas flow, therefore, the hospital vacuum entrains OR air during anesthesia. There is a 2L reservoir bag to hold WAGs should extraction flow fall below waste anesthesia gas flow for any amount of time. Should this state continue until the reservoir bag is full, then WAGs would be vented to the OR proper.
In our situation, we needed to proceed without the ability to scavenge.We recognized the need to attempt to perfectly match the anesthesia gas flow to the patient's metabolic rate of oxygen (closed anesthesia system) or periodically vent excess pressure with disconnects of the patient from the anesthesia machine.
While modern day ORs all have active scavenging systems, there are passive systems as well. A passive system would have tubing allowing the passive flow of the WAGs from the anesthesia machine into the environment or other collecting system. All modern day ORs also must have high air exchange in order to reduce the concentration of anesthetic gases. Despite active scavenging, waste anesthesia gases (WAGs) may escape from the machine during mask ventilation, or any time the patient is disconnected from the ventilator. Obviously, high flows should be avoided to reduce excess WAGs into the ambient room air during cases when the patient can be disconnected from the anesthesia.
Low flow anesthesia (usually considered to be 1 L/m of fresh gas flow as elaborated by Foldes in 1952) and minimal flow (@ 0.5L/m) was defined by virtue in 1972. These techniques can reduce WAGs dramatically. There are the additional advantages of cost savings on the volatile anesthetics, reduction of the drying affects of high fresh gas flows on mucous membranes and avoidance of cooling the patient, as well as the reduction of WAGs in the atmosphere. Open circuit anesthesia (FGF exceeds minute ventilation (V
m), can be compared to closed circuit anesthesia where FGFs is set as low as the metabolic rate of the patient, and would, in theory, maximize the above benefits noted. There are three main contraindications to using minimal or closed circuit anesthesia. These include patients acutely intoxicated with alcohol, patients in decompensated diabetic metabolic acidosis, and patients suffering from acute carbon monoxide poisoning. The rationale is simply that the patients would be rebreathing the alcohol, acetones, or carbon monoxide respectively. Desflurane is capable of being used in low flow anesthesia with little controversy. Sevoflurane has been shown to produce compound A at low flows which caused acute tubular necrosis in rats at concentrations greater than 250 ppm. At FGFs of 1 L/min compound A is usually fond to be around 15 ppm in clinical anesthesia but the amount of compound A produced is a function of the concentration of sevoflurane delivered, the duration of exposure to the absorbent, lower FGFs, the temperature and desiccated absorbents. Since the reaction that eliminates CO2 is an exothermic reaction, as the amount of CO2 absorbed increases temperature increases linearly producing greater degradation of sevoflurane to compound A. Therefore, in spontaneously breathing patients with significant hypercarbia, the risk of elevated compound A is elevated. Howeve, recent evidence suggests that compound A is not as clinically relevant with soda lime absorbers vs baralyme [2]. Furthermore, the studies raising concerns initially, were invalidated due to the marked difference between rat and human renal biochemistry. Nevertheless, The FDA issued a lower limit FGF rate when using sevoflurane at 2 L/m when using sevoflurane for greater than 2 MACs hours. Typically, to be safe, clinicians set FGFs to at least 2 L/m when using sevoflurane. However, this doubles the cost and production of WAGs when compared to a setting of 1 L/m. I have created a quick calculator to help a clinician determine how long the anesthetic may continue with sevoflurane at low flow (1 L/min) before exceeding the FDA recommendation of 2 MAC hours (see calculator
here). It should be noted that the studies the FDA relied upon to publish a warning label on sevoflurane were problematic and it is not clear at all that there is significant real clinical risk from compound A to humans at low FGFs.
In fact, The Brigham and Women’s Hospital Anesthesiology Clinical Practice Committee approved the use of sevoflurane at any fresh gas flow when CO2 absorbents are used that limit the added strong base. The justification was that these products permit safe, effective and planet-friendly use of low-flow or closed-circuit delivery of anesthetic agents. Similarly, the University of California, San Francisco implemented a fresh gas flow alert within the electronic medical record that notifies providers if their flows exceed 1 liter per minute and does not require a minimum fresh gas flow for sevoflurane.
Regulatory bodies:
OSHA does not mandate any particular scavenging system for WAGs. However, I did find this reference related to Joint commission where it states that they mandate scavenging be used when WAGs could be a problem (i.e. they are used for anesthesia). [1] In 1977, the national institute of safety and health (NIOSH) made a recommendation that workers should not be exposed to a greater than eight hour time weighted average of 2 ppm of halogenated agents. The limit is only 0.5 ppm if nitrous oxide is being used. The limit for nitrous oxide is eight hours time weighted average of 25 ppm. Obviously, the ability to make these calculations on the fly and track it is not done in the real world and would be expensive and difficult requiring the workers wear a badge of some sort that could record the exposure and then maintain that reading. To me, the message is that there is really little tolerance for any exposure and a scavenging system is mandatory for inhalation anesthesia.
The scavenging system on modern anesthesia machines actively emits anesthetics into the atmosphere. Therefore, there is a plausible concern of the effects of these hydrofluorocarbons in the atmosphere. Indeed, anesthetic gases are recognized as green house gases (GHGs). The US health care sector is responsible for about 10% of US GHG emissions. If the US healthcare sector were a country, it would rank 13th in the world for GHG emissions, ahead of the entire United Kingdom. Kaiser Permanente, one of the nation’s largest non-governmental health care systems, has a robust sustainability program. Anesthetic gases account for 3% of the organization’s greenhouse gas emissions – over half of which was from desflurane [3]. In 2014, WAGs stood at the equivalent of 3 million tons of carbon dioxide. Greenhouse Gases (GHGs) differ in their abilities to trap heat. The effect of this heat trapping over a 100-year period is described using a scale called the global warming potential over 100 years (GWP100). This allows a direct comparison a variety of different gases using CO2 as a baseline with a value of 1.
GWP100 for several anesthetics
CO2 ......................1
Methane..................30
SEVO......................130
N2O ........................298
ISO ......................510
DES ......................2540
SEVO is much lower than DES in part because it remains in the atmosphere for 1.1 years vs. 14 years for Desflurane. Nitrous remains in the atmosphere for about 150 years making its GWP100 fairly high.
To use another analogy to better put in perspective the GHG effect of anesthetics vs CO2 it is helpful to compare a typical anesthetic to driving a car some distance. As an example, a two hour anesthetic with desflurane with fresh gas flow (FGF) of 1 L/min using a 1 MAC setting would be the equivalent of driving a car 378 miles (from Los Angeles to Phoenix). The same anesthetic with sevoflurane but at double the FGF would be the equivalent of driving about 16 miles. see chart below:
| Table. One hour of anesthetic is like driving a car [how many?] miles.a | | 0.5 L/min | — | 4 | 93 | 29 | | 1.0 L/min | 4 | 7 | 189 | 57 | | 2.0 L/min | 8 | 15 | 378 | 112 | | 5.0 L/min | 19 | 38 | 939 | 282 | | 10.0 L/min | 38 | 74 | 1,876 | 564 | a Assumes EPA 2012 fuel efficiency average of 23.9 miles per gallon. b Because N2O cannot be delivered at 100%, the more typical percentage of 60% is used. In combination, 0.6 MAC-hour of N2O would be added to 0.4 MAC-hour of a volatile. EPA, Environmental Protection Agency; MAC, minimal alveolar concentration; N2O, nitrous oxide |
|
After the choice of anesthetic, the FGF used is the next most important determinant of the carbon footprint of a typical anesthetic. Any anesthetic with FGF beyond the metabolic needs and system requirements will be vented into the atmosphere as described earlier via the scavenging system. Thus, if you double the FGF of sevoflurane from 1 l/m to 2 l/m, the output of CO2 equivalent GHG is doubled for sevoflurane and for desflurane. Adding N2O to the anesthetic, but keeping the FGFs equivalent, will increase the CO2 equivalent emissions by 20 times. Therefore, the ability to decrease the concentration of the halogenated agent because of the N2O does not come close to compensating. Furthermore, N2O is known to deplete the ozone layer in the atmosphere.
However, to put this all into perspective, an equivalent of 6% of global carbon dioxide emissions result from nitrous oxide, where 1% of these are medicinal. Furthermore, as a waste anesthetic gas, N2O contributes roughly 0.1% of the whole GHG. In addition, N2O contributes by far, the largest amount to GHG when compared to all waste anesthetic gases as the consumption values of N2O far exceed those of other anesthetic gases.
From a GHG perspective the perfect inhalation anesthetic would be Xenon which acts via inhibition of calcium pumps in cell membranes which may increase intraneuronal calcium concentrations altering excitability. Xenon also inhibits NMDA receptors, as well as nicotinic acetylcholine receptors. Other beneficial characteristics of Xenon include:
- 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.
- Neuroprotective
- Lacks cardiovascular depression
So why doesn't Xenon replace all current inhalation anesthetics? It costs $10.00/Liter, far more than current inhalation agents.
In summary, when performing anesthesia outside of the regular OR, the anesthesia provider will be forced to determine if WAGs are scavenged as it is likely this component has been forgotten. WAGs have a real, although small, negative health effect over time on exposed personnel. Regulatory bodies demand that WAGs be eliminated in the anesthetizing location as best as possible which requires scavenging. Anesthesia technique, utilizing low FGFs, etc can reduced exposure of the OR worker as well as mitigate to some degree contaminants into the environment.
2. Kharasch ED, Powers KM, Artru AA. Comparison of Amsorb, Sodalime, Baralyme degradation of volatile anesthetics and formation of carbon monoxide and compound in swine in vivo. Anesthesiology. 2002;96:173–82
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