Circle system with flows of 2 L/min
Circle system with flows just sufficient to replace uptake of oxygen
34.3 Non-rebreathing Systems
The term insufflation refers to a breathing system that is not in contact with the patient but instead blows oxygen and the anesthesia gases across a patient’s face. There are 2 common methods where insufflation is used in modern practice. The first is in pediatric patients who will not allow the placement of an intravenous line or the application of an oxygen face mask to their face. In this case the oxygen mask is held near the patient’s face and when they breath in, they inhale the oxygen and the anesthesia gases that are supplied by the mask. The other example of insufflation is during cataract surgery and procedures performed typically under conscious sedation or monitored anesthesia care (MAC) when the patient’s face is fully covered with the surgical drapes. Frequently oxygen tubing is then placed under the drapes to blow oxygen at high flows near the patient’s face to minimize the rebreathing of carbon dioxide. The disadvantage of insufflation is positive pressure ventilation cannot be done and the concentration of oxygen delivered is variable even at high flow rates.
34.3.2 Open-Drop Anesthesia Systems
In modern medicine, open-drop anesthesia systems are no longer in use, but since they were one of the earliest methods for providing anesthesia, they deserve a brief description. These were the primary methods for administering ether or chloroform anesthesia to patients. They consisted of a face mask with a gauze-covered opening where a highly volatile anesthetic was then dripped onto the gauze. When the patient inhaled, the flow of air through the gauze caused the liquid volatile anesthetic to become a gas. The depth of anesthesia was controlled by the amount of volatile anesthetic liquid applied to the gauze. The major disadvantage of this system is the patient had to be breathing spontaneously and there was no way to monitor the concentration of volatile anesthetic the patient was inspiring.
34.3.3 Mapleson Circuits
The development of the Mapleson circuits attempted to overcome the major shortcomings of the insufflation and open-drop systems. Compared to previous breathing systems, the Mapleson circuits had the following advantages:
Ability to deliver a reliable concentration of oxygen and volatile anesthetics
Provide positive pressure ventilation
Ability to scavenge waste gases
Portable and relatively simple design
Since there are no unidirectional valves or CO2 absorbers, the CO2 is eliminated by fresh gas flows that flush the CO2 into the atmosphere or scavenger via the pop-off valve. The Mapleson circuits are classified based on the location of the following parts as seen in ◘ Fig. 34.1:
Fresh gas inlet – Supplies oxygen, air, and volatile anesthetics to the system
Reservoir bag – Acts as a reservoir for the gases and a method for creating positive pressure
Pop-off valve/Adjustable pressure relief valve – Allows the release of excess gases from the system. Closing of the valve allows increasing levels of pressure to be achieved
a-f Mapleson system. FGF fresh gas flow, P patient
Although the Mapleson circuits all have the same components, the comparison of the circuits and the amount of CO2 rebreathing that occurs can be complex. Rebreathing that occurs for each circuit is affected by the position of the pop-off valve in relationship to the fresh gas input and the reservoir tubing and bag. Each type of Mapleson system has different levels of fresh gas flows required to prevent rebreathing. In addition, each class of circuit responds differently to spontaneous versus controlled ventilation in relation to the fresh gas flows required to prevent rebreathing.
Mapleson A (see ◘ Fig. 34.1) is considered the most efficient circuit for spontaneous ventilation but is not recommended for controlled ventilation due to the need for unpredictably high fresh gas flows required to prevent rebreathing. By changing the position of the pop-off valve and the fresh gas inlet the system becomes a Mapleson D (see ◘ Fig. 34.1), which is significantly more efficient for controlled ventilation. This is because the fresh gas flushes the exhaled gases away from the patient and toward the pop-off valve. The Bain circuit is a modified Mapleson D system that incorporates the fresh gas inlet tubing inside the larger corrugated breathing tube. This decreases the size of the system and allows warming of the inspired gases by the exhaled gases. A major disadvantage of Bain circuit is if the fresh gas inlet tubing is disconnected or kinked, this will lead to significant rebreathing of exhaled gases.
Efficiency of Mapleson Circuits
Spontaneous Ventilation A > DFE > CB
Controlled Ventilation DFE > BC > A
(Refer to ◘ Fig. 34.1 for the different Mapleson circuits by letter.)
While the Mapleson systems were a significant improvement from previous breathing systems, the major disadvantage remained to be poor conservation of heat and humidity. In addition, the minimum flows required to prevent rebreathing are typically 5–10 L/min for an adult.
34.3.4 The Circle System
The circle system was designed to overcome some of the limitations of the Mapleson circuits. Through the rebreathing of exhaled gases humidity and heat are maintained. Since the exhaled gases are rebreathed, this leads to a conservation of volatile anesthetics, oxygen, and air. These advantages have resulted in the circle system being the ventilation system used in all modern anesthesia machines. While the oxygen, nitrogen, and the volatile anesthetics are rebreathed, CO2 absorbers chemically neutralize the CO2 so there is no rebreathing of CO2. However, to achieve these results the complexity of the breathing system was substantially increased. As shown in ◘ Fig. 34.2 the necessary components of a circle system include:
Fresh gas inlet
Adjustable pressure relief valve (“pop-off valve”)
Carbon Dioxide Absorber
To prevent rebreathing of carbon dioxide, a carbon dioxide absorber is an essential part of any circle system. The absorber is able to convert carbon dioxide gas to calcium carbonate. In addition, the reaction leads to the production of heat and water:
Soda lime is the most commonly used agent to absorb carbon dioxide. It is capable of absorbing 23 L of CO2 per 100 g of soda lime. It is important to note that for the reaction to occur water is necessary to begin the reaction, but as the reaction continues each molecule of CO2 leads to the net production of 1 molecule of H2O. Since the carbon dioxide absorber requires water to start the reaction, they are packaged by the manufacturer with 14–19% water content.
As the absorber is exhausted, hydrogen ions accumulate. To monitor the pH an indicator dye (ethyl violet) is added to the absorber granules, which causes the white absorber granules to turn purple as they are exhausted. If an exhausted carbon dioxide absorber is not used for a time, the granules may return to the original white color. However, the absorber is still exhausted and cannot absorb additional carbon dioxide. All modern anesthesia machines monitor the inspired and expired carbon dioxide. With a properly functioning carbon dioxide absorber, inspired carbon dioxide should be zero and the absorber should be changed once the inspired carbon dioxide is present. If it is not possible to replace the carbon dioxide absorber, high flows (>5 L/min) can effectively wash out the carbon dioxide from the system.
The absorber granules are capable of absorbing and releasing volatile anesthetic gases. This is clinically relevant when the absorber canisters are changed during a surgical case. When this occurs, the new absorber canister will absorb the volatile anesthetic in the anesthesia machine and the depth of anesthesia will decrease temporarily. The other clinically significant scenario is in patients at risk of malignant hyperthermia. In these patients if an anesthesia machine is used that had previously been used with volatile anesthetics, then the patient may be exposed to volatile anesthetics. Since this could trigger an episode of malignant hyperthermia, a new carbon dioxide absorber should be used.
In a circle system, unidirectional valves are essential to maintain the flow of gases in the correct direction. This is important to prevent rebreathing of the expired gases with carbon dioxide present. There are 2 valves in the system. During inspiration the inspiratory valve opens and the expiratory valve closes. During expiration the expiratory valve opens and the inspiratory valve closes. The most common malfunction of the unidirectional valves is incomplete closure—usually due to a warped valve. If this occurs, then rebreathing of carbon dioxide is possible and hypercapnia can result.
Fresh Gas Inlet
One of the advantages of the circle system is the rebreathing of oxygen and volatile anesthetic gases. However, due to the uptake of oxygen and volatile gases by the patient there must be continuous addition of oxygen and volatile gases to the circle system. This occurs via the fresh gas inlet. When adding gases to the system, it is important to remember that gases from the fresh gas inlet will be diluted by the gases already in the circuit. For example, a typical circle system in a modern anesthesia machine will have a volume of 7 L. If the fresh gas inlet is at low flows (<1 L/min), there can be a significant difference between the concentration of volatile gases coming from the vaporizer via the fresh gas inlet and the concentration delivered to the patient. This is most apparent during induction and emergence from anesthesia. To compensate for the dilution effect, a high flow rate (>5 L/min) can be used.
Adjustable Pressure Relief Valve (“Pop-Off Valve”)
As previously mentioned, higher fresh gas flows can be used to speed changes in the concentration of volatile anesthetics in the circuit. The adjustable pressure relief valve will vent off the excess gases from the circle system to maintain a set volume. The adjustable pressure relief valve is also used to adjust the pressure in the system during positive pressure ventilation. During spontaneous ventilation, the adjustable pressure relief valve should be fully open to minimize the pressure to which the patient is exposed.