CO2 is usually removed by an absorbent but at flows > 5 L/min can also be removed by washout. All anesthesia machines have failsafe valves which shut off or decrease gas flows when the O2 pressure drops below 30 PSI, although this system will not prevent the delivery of 100% N2O (thus an oxygen analyzer is necessary).
Note that the oxygen flowmeter is last in the sequence of meters, reducing the likelihood that a leak will produce deceptively normal values for inspired oxygen. However, a leak is still possible, thus inspired O2 is measured and there is a second mechanism by which O2 can be delivered – the oxygen flush valve (which bypasses the flowmeter and, consequently, permits the delivery of high pressures to the lungs, possibly causing barotrauma).
Note that desflurane has a vapor pressure of 664 mm Hg at 20C, thus it is heated to 23-25C and backpressure of 1500 mm Hg is applied, lowering volatility but making it more predictable. All modern vaporizers are of the “variable bypass” design whereby some gas bypasses the liquid agent, and the rest passes through a baffle system over the agent. A temperature compensating valve ensures that output is linear between 20 and 35C.
In the US Gas tank colors do not follow the ISO standard and are not regulated by law . The definitive description of contents are therefore on the USP Label.
Standard US colors are:
A full oxygen tank contains 625L at 2000 PSI (half tank = 312L at 1000 PSI). N2O, by contrast, is in gas/liquid form, thus the tank must be weighed (it is normally 750 PSI as long as liquid is present). When N2O pressure drops below 750 PSI, less than 400 L remains.
Most popular system in the US. Because of its circular nature, relies on an absorbent to remove CO2. Its circular design also allows for significant conservation of airway moisture and heat, as well as decreased pollution/waste of anesthetic gases. Disadvantages include increased resistance to breathing (because of unidirectional valves as well as the CO2 absorbent), bulkiness, and complexity/potential for failure. Also, rebreathing of exhaled gases can slow uptake of gases during induction, as fresh gas flow is diluted by exhaled gas. Can be classified as semiopen, semiclosed, or closed depending on the fresh gas inflow.
Mapleson F (Jackson-Rees) System
T-piece arrangement with a reservoir bag and adjustable overflow bag on the distal end of the reservoir bag. It can be adapted to scavenging systems as needed. popular in pediatric patients because it has minimal dead space and offers minimal resistance (no moving parts except the overflow valve). Simple, inexpensive, can be used with a facemask or ETT, is lightweight and easily repositioned, thus it is often used in transport as well. Disadvantages include 1) need for high fresh gas inflow to prevent rebreathing 2) possibility of barotrauma if the overflow valve is occluded 3) lack of humidification.
Coaxial Mapleson D (Bain) System
As opposed to Mapleson F, the overflow valve is on the proximal portion of the reservoir bag, and the fresh gas flow inlet travels coaxially, i.e., in a separate tube that passes through the corrugated tubing. This allows warming of the fresh gas inflow. Other advantages include conservation of moisture as a result of partial rebreathing and ease of scavenging waste gases through the overflow valve. This system is used most often when access to the patient is limited (ex. ENT surgery). Disadvantages include unrecognized disconnection or kinking of the inner gas tube.
Total rebreathing of all gases, APL valve is completely closed. A circle system is considered closed when fresh gas inflow satisfies the patient’s metabolic requirements (150-250 mL/min). Advantages of closed systems are 1) maximal humidification and warming 2) less pollution and 3) economy. Disadvantages include 1) inability to rapidly change delivered concentration of gases 2) relative unpredictability of oxygen concentration [alleviated by an inspired O2 sensor]and 3) unknown and possibly excessive concentration of gases [secondary to unknown tissue uptake – can be partially offset by FGF > 3L/min for 15 mins before instituting a closed system].
Ventilators, Moisture, and Heat
Oxygen drives the bellows in case of leak (will increase O2, rather than air). Bellow rise during exhalation, this way they will not rise if there is a leak in the breathing system or a disconnection.
The upper respiratory tract (mostly the nose) is the principal heat and moisture exchanger of inspired gases. Anesthetic gases, however, are waterless (to prevent corrosion), thus leading to mucosal dehydration and impaired ciliary function, impaired surfactant function, inspissation, atelectasis, and an increase in the A-a gradient. CO2 absorbers produce water and help ameliorate this loss.
Heat loss is even more important than water loss. HME (heat/moisture exchangers) contain porous hydrophobic (or hygroscopic) membranes that trap humidified gases and return them to the patient on inhalation. They are inexpensive and easy to use, however they do add resistance to breathing, are not as effective as heated water vaporizers/humidifiers, can become clogged, and increase dead space (which is particularly problematic in pediatric patients). Heated water vapors provide superior temperature control but are associated with the risk of thermal injury, nosocomial infection, and malfunction, in addition to also increasing the work of breathing.
Amsorb is made of water, Ca(OH)2, and CaCl2. CO2 with water produces carbonic acid, which reacts with Ca(OH)2 to form Ca(HCO3)2, carbonate, water, and heat. Unlike sodalime, Amsorb does not contain NaOH or KOH. Soda lime is made of water, Ca(OH)2, NaOH, and KOH. Reaction of CO2 with water produces carbonic acids, which then react with hydroxides to form bicarbonates, water, and heat. These canisters should be warm – if not, they may not be functioning. Note that “mesh size” refers to the number of openings per linear inch in a sieve through which granular particles can pass (normal sizes are 4-8 mesh). Channeling can be minimized by shaking the canister before use. Note that soda lime can be desiccated by reverse flow (bottom up, ex. over weekends, requires ~ 48 hrs), which can lead to carboxyhemoglobin formation.
Degradation of Inhaled Anesthetic
Amsorb does not degrade inhaled anesthetics, but soda lime can degrade sevo, des, and isoflurane into CO. Soda lime can also degrade sevoflurane into compound A, which at high doses is a nephrotoxin in rats [Kharasch ED et al. Anesth Analg 93: 1511, 2001]; FREE Full-text at Anesthesia & Analgesia.