1 What are the different types of anesthesia breathing circuits?

Breathing circuits are usually classified as open, semiopen, semiclosed, or closed. They include various components configured to allow the patient to breathe (or be ventilated) with a gas mixture that differs from room air.

2 Give an example of an open circuit

An open circuit is the method by which the first true anesthetics were given 160 years ago. A bit of cloth saturated with ether or chloroform was held over the patient’s face. The patient breathed the vapors and became anesthetized. The depth of anesthesia was controlled by the amount of liquid anesthetic on the cloth; thus it took a great deal of trial and error to become good at the technique.

3 Give an example of a semiopen circuit

The various semiopen circuits were fully described by Mapleson and are commonly known as the Mapleson A, B, C, D, E, and F circuits (Figure 19-1). All have in common a source of fresh gas, corrugated tubing (more resistant to kinking), and a pop-off or adjustable pressure-limiting valve. Differences among the circuits include the locations of the pop-off valve, fresh gas input, and whether or not a gas reservoir bag is present. Advantages of the Mapleson series are simplicity of design, ability to change the depth of anesthesia rapidly, portability, and lack of rebreathing of exhaled gases (provided the fresh gas flow is adequate). Disadvantages include lack of conservation of heat and moisture, limited ability to scavenge waste gases, and high requirements for fresh gas flow. These are rarely used today except for patient transport.

Figure 19-1 Mapleson A, B, C, D, E, and F circuits. FGF, Fresh gas flow.

(From Willis BA, Pender JW, Mapleson WW: Rebreathing in a T-piece: Volunteer and theoretical studies of the Jackson-Rees modification of Ayre’s T-piece during spontaneous respiration, Br J Anaesth 47:1239–1246, 1975, with permission.)

4 Give an example of a semiclosed circuit

The prototypical semiclosed circuit is the circle system, which is found in most operating rooms in the United States (Figure 19-2). Every semiclosed system contains an inspiratory limb, expiratory limb, unidirectional valves, carbon dioxide (CO₂) absorber, gas reservoir bag, and a pop-off valve on the expiratory limb. Advantages of a circle system include conservation of heat and moisture, the ability to use low flows of fresh gas (thereby conserving volatile anesthetic and the ozone layer), and the ability to scavenge waste gases. A disadvantage is its complex design; it has approximately 10 connections, each of which has the potential for disconnection.

Figure 19-2 Circle system.

(From Andrews JJ: Inhaled anesthetic delivery system. In Miller RD, editor: Anesthesia, ed 4, New York, 1994, Churchill Livingstone, pp 185–228.)

5 Give an example of a closed circuit

Like the semiclosed circuit, the closed circuit is a circle system adjusted so the inflow of fresh gas just matches the patient’s oxygen (O₂) consumption and anesthetic agent uptake. The CO₂ is eliminated by the absorber.

6 Rank the Mapleson circuits in order of efficiency for controlled and spontaneous ventilation

Controlled: D > B > C > A (Pneumonic: Dog Bites Can Ache)

Spontaneous: A > D > C > B (Pneumonic: All Dogs Can Bite)

7 What circuit is most commonly used in anesthesia delivery systems today?

Almost every anesthesia manufacturer supplies a circle system with its equipment. When compared with other available circuits, the circle system provides the most advantages.

8 How is a breathing circuit disconnection detected during delivery of an anesthetic?

Breath sounds are no longer detected with an esophageal or precordial stethoscope, and, if the parameters are properly set, the airway pressure monitor and tidal volume–minute volume monitor alarm will sound. The capnograph no longer detects CO₂, and soon thereafter the O₂ saturation begins to decline. Exhaled CO₂ is probably the best monitor to detect disconnections; a decrease or absence of CO₂ is sensitive although not specific for disconnection.

9 How is CO₂ eliminated from a circle system?

The exhaled gases pass through a canister containing a CO₂ absorbent such as soda lime or Baralyme. Soda lime consists of calcium hydroxide (Ca[OH]₂) with lesser quantities of sodium hydroxide (NaOH) and potassium hydroxide (KOH). Baralyme substitutes barium for calcium. Both soda lime and Baralyme react with CO₂ to form heat, water, and the corresponding carbonate. The soda lime reaction is as follows:

10 How much CO₂ can the absorbent neutralize? What factors affect its efficiency?

Soda lime is the most common absorber, and at most can absorb 23 L of CO₂ per 100 g of absorbent. However, the average absorber eliminates 10 to 15 L of CO₂ per 100 g absorbent in a single-chamber system and 18 to 20 L of CO₂ in a dual-chamber system. Factors affecting efficiency include the size of the absorber canister (the patient’s tidal volume should be accommodated entirely within the void space of the canister), the size of the absorbent granule (optimal size is 2.5 mm or between 4 and 8 mesh), and the presence or absence of channeling (loose packing allowing exhaled gases to bypass absorber granules in the canister).

11 How do you know when the absorbent has been exhausted? What adverse reactions can occur between volatile anesthetic and CO₂ absorbants?

A pH-sensitive dye added to the granules changes color in the presence of carbonic acid, an intermediary in the CO₂ absorption chemical reaction. The most common dye in the United States is ethyl violet, which is white when fresh and turns violet when the absorbent is exhausted. Adverse reactions between volatile anesthetics and absorbants are discussed in Chapter 10.

12 How can you check the competency of a circle system?

You should close the pop-off valve, occlude the Y-piece, and press the O₂ flush valve until the pressure is 30 cm H₂O. The pressure will not decline if there are no leaks. Then you should open the pop-off valve to ensure that it is in working order. In addition, you should check the function of the unidirectional valves by breathing down each limb individually, making sure that you cannot inhale from the expiratory limb or exhale down the inspiratory limb.

13 How do anesthesia ventilators differ from intensive care unit ventilators?

Intensive care unit (ICU) ventilators usually are more powerful (allowing greater inspiratory pressures and tidal volumes, which is important in patients with decreased pulmonary compliance) and support more modes of ventilation. However, new generations of anesthesia machines continue to offer more ventilatory modes. See Chapter 21.

14 What gas is used to drive the bellows in an anesthesia ventilator?

O₂ is usually used for this purpose because it is cheap and always available. On modern ascending bellows ventilators the pressure inside the bellows is always slightly higher than in the chamber outside the bellows because of the weight of the bellows itself and amounts to 1 to 2 cm H₂O. Where there is a bellows leak, any net gas flow would be out of (not into) the bellows and would not change the composition of the breathing mixture. When the bellows reaches the top of its ascent, an additional 1 to 2 cm H₂O pressure causes the exhaust valve to open.

15 What is the status of the scavenger system when the bellows is below the top of its excursion?

When the ventilator is engaged and the bellows is below the top of its excursion, the breathing circuit is completely closed. Excess gas can escape from the breathing circuit only when a special pneumatic value is activated by the bellows as it reaches the top of its range.

16 What is the effect of the extra pressure required to open the exhaust valve on the patient?

A patient being ventilated with an ascending bellows ventilator is usually subjected to 2.5 to 3 cm H₂O of positive end-expiratory pressure (PEEP). Most experts agree that this addition of PEEP is actually more physiologic than ventilating the patient with no PEEP.

17 What parameters can be adjusted on an anesthesia ventilator?

Most anesthesia ventilators allow adjustment of:

Tidal volume (or minute volume)

Respiratory rate

Inspiratory-to-expiratory (I:E) ratio

Sometimes inspiratory pause

PEEP

Some newer anesthesia ventilators allow other adjustments, including the selection of different ventilation modes such as pressure support and synchronized intermittent mandatory ventilation.

18 Why have descending bellows been abandoned in favor of ascending bellows?

Bellows are classified according to their movement during expiration (i.e., their position when the ventilator is not ventilating the patient). Hanging or descending bellows are considered unsafe for two reasons. First, if a circuit disconnection occurs, the bellows will fill with room air; and, although its movement may appear normal, the ventilator will be ventilating the room rather than the patient. Second, since the weight of the bellows creates a slight negative pressure in the circuit, this can cause negative end-expiratory pressure and can suck room air backward through the scavenger, interfering with the anesthesiologist’s control of gas concentrations breathed by the patient.

19 What would be the cause when the bellows fails to rise completely between each breath?

The most obvious reason for this is that a leak exists in the breathing circuit, a disconnect has occurred, or the patient has become extubated. If the fresh gas flow is too low, it is possible for the patient to use O₂ from the circuit faster than it is replenished.

20 How does fresh gas flow rate contribute to tidal volume?

The flow during the inspiration phase of the ventilatory cycle adds to the tidal volume. Let us assume that the respiratory rate is set at 10 breaths/min with an I:E ratio of 1:2 and a tidal volume of 1000 ml. Each breath cycle is then 6 seconds long, 2 seconds for inspiration and 4 seconds for expiration. If the fresh gas flow is 6 L/min, 2/60 × 6000 = 200 ml of fresh gas is added to each inspiration. Most modern anesthesia ventilators automatically compensate for this addition to tidal volume.

21 How and where is tidal volume measured? Why are different measures frequently not equal?

Tidal volume is measured using several techniques and at several sites in the breathing circuit. Common measures include the setting on the ventilator control panel, bellows excursion, and flow through the inspiratory or expiratory limbs of the circuit. These measures frequently differ because they may or may not include the contribution of the inspiratory flow, are measured at different pressures, and compensate differently for flow rates. Since each measure can in theory be an accurate measure of a different parameter, it is more important to record a consistent measure of tidal volume than to debate which measure is correct.

22 When using very low flows of fresh gas, why is there sometimes a discrepancy between inspired oxygen concentration and fresh gas concentration?

At low fresh gas flows concentrations in the circuit are slow to change. However, this question refers to the fact that the patient will take up different gases (removing them from the circuit) at rates different from the rates at which the gases are added to the circuit. In the case of O₂, an average adult patient consumes (permanently removing from the circuit) approximately 200 to 300 ml/min of O₂. If nitrogen or nitrous oxide is supplied along with the O₂, the patient continues to consume O₂ while the nitrogen or nitrous oxide builds up in the circuit. It is possible for a hypoxic mixture to develop in the circuit, even though the fresh gas contains 50% or more O₂.