Although Mapleson circuits overcome some of the disadvantages of the insufflation and draw-over systems, the high fresh gas flows required to prevent rebreathing of CO2 result in waste of anesthetic agent, pollution of the operating room environment, and loss of patient heat and humidity (Table 3-3). In an attempt to avoid these problems, the circle system adds more components to the breathing system.
The components of a circle system include: (1) a CO2 absorber containing CO2 absorbent; (2) a fresh gas inlet; (3) an inspiratory unidirectional valve and inspiratory breathing tube; (4) a Y-connector; (5) an expiratory unidirectional valve and expiratory breathing tube; (6) an APL valve; and (7) a reservoir (Figure 3-8).
Components of the Circle System
Carbon Dioxide Absorber and the Absorbent
Rebreathing alveolar gas conserves heat and humidity. However, the CO2 in exhaled gas must be eliminated to prevent hypercapnia. CO2 chemically combines with water to form carbonic acid. CO2 absorbents (eg, soda lime or calcium hydroxide lime) contain hydroxide salts that are capable of neutralizing carbonic acid (Table 3-4). Reaction end products include heat (the heat of neutralization), water, and calcium carbonate. Soda lime is the more common absorbent and is capable of absorbing up to 23 L of CO2 per 100 g of absorbent. It consists primarily of calcium hydroxide (80%), along with sodium hydroxide, water, and a small amount of potassium hydroxide. Its reactions are as follows:
Table 3-4 Comparison of Soda Lime and Barium Hydroxide Lime. ||Download (.pdf)
Table 3-4 Comparison of Soda Lime and Barium Hydroxide Lime.
|Soda Lime||Barium Hydroxide Lime|
|Method of hardness||Silica added||Water of crystallization|
- Calcium hydroxide
- Sodium hydroxide
- Potassium hydroxide
- Barium hydroxide
- Calcium hydroxide
|Usual indicator dye||Ethyl violet||Ethyl violet|
|Absorptive capacity (liters of CO2/100 g granules)||14-23||9-18|
Note that the water and sodium hydroxide initially required are regenerated. Another absorbent, barium hydroxide lime, is no longer used due to the possible increased hazard of fire in the breathing system.
A pH indicator dye (eg, ethyl violet) changes color from white to purple as a consequence of increasing hydrogen ion concentration and absorbent exhaustion (Table 3-5). Absorbent should be replaced when 50% to 70% has changed color. Although exhausted granules may revert to their original color if rested, no significant recovery of absorptive capacity occurs. Granule size is a compromise between the higher absorptive surface area of small granules and the lower resistance to gas flow of larger granules. The granules commonly used as CO2 absorbent are between 4 and 8 mesh; the number of mesh corresponds to the number of holes per square inch of a screen. The hydroxide salts are irritating to the skin and mucous membranes. Increasing the hardness of soda lime by adding silica minimizes the risk of inhalation of sodium hydroxide dust and also decreases resistance of gas flow. Additional water is added to absorbent during packaging to provide optimal conditions for carbonic acid formation. Commercial soda lime has a water content of 14% to 19%.
Table 3-5 Indicator Dye Changes Signaling Absorbent Exhaustion. ||Download (.pdf)
Table 3-5 Indicator Dye Changes Signaling Absorbent Exhaustion.
|Indicator||Color when Fresh||Color when Exhausted|
Absorbent granules can absorb and later release medically important amounts of volatile anesthetic. This property can be responsible for modest delays of induction or emergence.
The drier the soda lime, the more likely it will absorb and degrade volatile anesthetics. Volatile anesthetics can be broken down to carbon monoxide by dry absorbent (eg, sodium or potassium hydroxide) to such a degree that it is capable of causing clinically significant carbon monoxide poisoning. The formation of carbon monoxide is highest with desflurane
; with sevoflurane
, it occurs at a higher temperature.
Amsorb is a CO2 absorbent consisting of calcium hydroxide and calcium chloride (with calcium sulfate and polyvinylpyrrolidone added to increase hardness). It possesses greater inertness than soda lime, resulting in less degradation of volatile anesthetics (eg, sevoflurane into compound A or desflurane into carbon monoxide).
Compound A is one of the by-products of degradation of sevoflurane by absorbent. Higher concentrations of sevoflurane, prolonged exposure, and low-flow anesthetic technique seem to increase the formation of Compound A. Compound A has been shown to produce nephrotoxicity in animals,
The granules of absorbent are contained within one or two canisters that fit snugly between a head and base plate. Together, this unit is called an absorber (Figure 3-9). Although bulky, double canisters permit more complete CO2 absorption, less frequent absorbent changes, and lower gas flow resistance. To ensure complete absorption, a patient’s tidal volume should not exceed the air space between absorbent granules, which is roughly equal to 50% of the absorber’s capacity. Indicator dye color is monitored through the absorber’s transparent walls. Absorbent exhaustion typically occurs first where exhaled gas enters the absorber and along the canister’s smooth inner walls. Channeling through areas of loosely packed granules is minimized by a baffle system, which directs gas flow through the center, thereby allowing greater utilization of the absorbent. A trap at the base of the absorber collects dust and moisture. Newer absorbers are used until CO2 is found in the inhaled gas on the anesthetic-gas monitor, at which time the canister(s) are replaced.
A carbon dioxide absorber.
Unidirectional valves, which function as check valves, contain a ceramic or mica disk resting horizontally on an annular valve seat (Figure 3-10). Forward flow displaces the disk upward, permitting the gas to proceed through the circuit. Reverse flow pushes the disk against its seat, preventing reflux. Valve incompetence is usually due to a warped disk or seat irregularities. The expiratory valve is exposed to the humidity of alveolar gas. Condensation and resultant moisture formation may prevent upward displacement of the disks, resulting in incomplete escape of expired gases and rebreathing.
Inhalation opens the inspiratory valve, allowing the patient to breathe a mixture of fresh and exhaled gas that has passed through the CO2 absorber. Simultaneously, the expiratory valve closes to prevent rebreathing of exhaled gas that still contains CO2. The subsequent flow of gas away from the patient during exhalation opens the expiratory valve. This gas is vented through the APL valve or rebreathed by the patient after passing through the absorber. Closure of the inspiratory valve during exhalation prevents expiratory gas from mixing with fresh gas in the inspiratory limb.
Malfunction of either unidirectional valve may allow rebreathing of CO2
, resulting in hypercapnia.
Optimization of Circle System Design
Although the major components of the circle system (unidirectional valves, fresh gas inlet, APL valve, CO2 absorber, and a reservoir bag) can be placed in several configurations, the following arrangement is preferred (Figure 3-8):
- Unidirectional valves are relatively close to the patient to prevent backflow into the inspiratory limb if a circuit leak develops. However, unidirectional valves are not placed in the Y-piece, as that makes it difficult to confirm proper orientation and intraoperative function.
- The fresh gas inlet is placed between the absorber and the inspiratory valve. Positioning it downstream from the inspiratory valve would allow fresh gas to bypass the patient during exhalation and be wasted. Fresh gas introduced between the expiratory valve and the absorber would be diluted by recirculating gas. Furthermore, inhalation anesthetics may be absorbed or released by soda lime granules, thus slowing induction and emergence.
- The APL valve is usually placed between the absorber and the expiratory valve and close to the reservoir bag. Positioning of the APL valve in this location (ie, before the absorber) helps to conserve absorption capacity and minimizes the venting of fresh gas.
- Resistance to exhalation is decreased by locating the reservoir bag in the expiratory limb. Bag compression during controlled ventilation will vent expired gas through the APL valve, conserving absorbent.
Performance Characteristics of the Circle System
With an absorber, the circle system prevents rebreathing of CO2
at reduced fresh gas flows (≤1 L) or even fresh gas flows equal to the uptake of anesthetic gases and oxygen
by the patient and the circuit itself (closed-system anesthesia). At fresh gas flows greater than 5 L/min, rebreathing is so minimal that a CO2
absorber is usually unnecessary.
With low fresh gas flows, concentrations of oxygen and inhalation anesthetics can vary markedly between fresh gas (ie, gas in the fresh gas inlet) and inspired gas (ie, gas in the inspiratory limb of the breathing tubes). The latter is a mixture of fresh gas and exhaled gas that has passed through the absorber. The greater the fresh gas flow rate, the less time it will take for a change in fresh gas anesthetic concentration to be reflected in a change in inspired gas anesthetic concentration. Higher flows speed induction and recovery, compensate for leaks in the circuit, and decrease the risks of unanticipated gas mixtures.
That part of a tidal volume that does not undergo alveolar ventilation is referred to as dead space. Thus, any increase in dead space must be accompanied by a corresponding increase in tidal volume, if alveolar ventilation is to remain unchanged.
Because of the unidirectional valves, apparatus dead space in a circle system is limited to the area distal to the point of inspiratory and expiratory gas mixing at the Y-piece. Unlike Mapleson circuits, the circle system tube length does not affect dead space. Like Mapleson circuits, length does affect circuit compliance and thus the amount of tidal volume lost to the circuit during positive-pressure ventilation. Pediatric circle systems may have both a septum dividing the inspiratory and expiratory gas in the Y-piece and low-compliance breathing tubes to further reduce dead space, and are lighter in weight.
The unidirectional valves and absorber increase circle system resistance, especially at high respiratory rates and large tidal volumes. Nonetheless, even premature neonates can be successfully ventilated using a circle system.
Humidity and Heat Conservation
Medical gas delivery systems supply dehumidified gases to the anesthesia circuit at room temperature. Exhaled gas, on the other hand, is saturated with water at body temperature. Therefore, the heat and humidity of inspired gas depend on the relative proportion of rebreathed gas to fresh gas. High flows are accompanied by low relative humidity, whereas low flows allow greater water saturation. Absorbent granules provide a significant source of heat and moisture in the circle system.
The minimal risk of microorganism retention in circle system components could theoretically lead to respiratory infections in subsequent patients. For this reason, bacterial filters are sometimes incorporated into the inspiratory or expiratory breathing tubes or at the Y-piece.
Disadvantages of the Circle System
Although most of the problems of Mapleson circuits are solved by the circle system, the improvements have led to other disadvantages: greater size and less portability; increased complexity, resulting in a higher risk of disconnection or malfunction; complications related to use of absorbent; and the difficulty of predicting inspired gas concentrations during low fresh gas flows.