It is exceedingly important for anesthesia personnel to understand and become proficient at using a variety of breathing systems. Fortunately, anesthesia breathing systems represent very simple and logical devices and are consequently quite easy to learn. For the purpose of this chapter, anesthesia breathing apparatuses are defined as devices that enhance the ability to breathe a desired gas or vapor (including air or oxygen). The reader should note that this discussion does not include a detailed discussion of the anesthesia machine itself, but only the breathing circuit. Ventilators that assist breathing and masks and other aids for breathing (such as masks and nasal prongs) are discussed in Chapter 30 and Chapter 35, respectively. This chapter focuses on the development of anesthesia breathing systems from a functional perspective.

One of the most straightforward breathing systems was demonstrated in an early Tarzan film when the “ape man” cut a long reed through which he was able to breathe while submerged in a pond, to elude the natives pursuing him. The reed functioned in a manner similar to a modernday snorkel. Thus, the simplest form of a breathing system may still be one of the most commonly employed, the snorkel. Today’s “standard” snorkel is about 50 cm in length, with an internal diameter of 2 cm, resulting in a capacity, or dead space, of about 150 mL. At a length of 7.5 ft and an internal diameter of 1 cm, Tarzan’s reed would have contained the same volume. He could have easily disappeared from view in a murky pond while submerged at over 7 ft. Although it helped Tarzan hide, snorkel-type breathing tubes suffer from two limitations: (1) as you further decrease the diameter of the breathing tube, there will be significantly increased resistance to breathing (try breathing through a standard soda straw for any length of time) and (2) as you increase the volume of the breathing tube by increasing either the internal diameter or the length of the breathing tube, it increases the dead space in the breathing system. Most of us intuitively understand the concept of dead space. Imagine trying to breathe through a very long garden hose. How long would you be able to survive?

Dead space in a breathing circuit is defined as the volume from the patient end to the point at which exhaled gas can be flushed out of the system and exchanged for fresh gas. Imagine that Tarzan is breathing with a tidal volume of 500 mL (volume of each breath). As Tarzan exhales through his 150-mL reed, it fills with his exhaled breath until it is flushed out at the end. Just before Tarzan inhales his next breath, the 150 mL volume of the reed (dead space) is filled with his last exhaled breath. When he begins to inhale, the first 150 mL he breathes in will be his previously exhaled breath, depleted of oxygen and containing carbon dioxide (CO₂). The next 350 mL of his inhalation will come from the fresh jungle air. This 350 mL is plenty of air for Tarzan. Now imagine that Tarzan is hiding on the bottom of a lake and is using a reed that is 30 ft long and 1 cm in diameter. The dead space volume of the reed is now 600 mL. If Tarzan were to exhale a 500-mL tidal breath, it would fill the reed. When Tarzan inhaled, he would breathe back in his exhaled breath from the dead space in the reed and never get any fresh air! A large dead space would have prevented Tarzan from breathing fresh air. The exhaled gas in the dead space contains residual oxygen and large amounts of CO₂. Rebreathing this gas would rapidly prevent Tarzan from eliminating CO₂ from his lungs. For a short period of time he could utilize the residual oxygen that he
exhales and reinhales. Therefore, if Tarzan were breathing through a system with a large dead space, he would rapidly become hypercarbic and soon become hypoxic.

Fortunately, Tarzan learned that he could breathe quite comfortably through a reed that was very long, and thus contained a tremendous dead space, by inhaling though his mouth and then exhaling through his nose. In this way, the reed always contained fresh air. As illustrated in our Tarzan example, rebreathing of exhaled gases can be prevented by inhaling through the mouth and exhaling through the nose. The same thing can be accomplished mechanically, by adding a one-way valve to a snorkel that converts an open breathing system to a semiclosed system. This is also what is done when you change from breathing air from the atmosphere through a snorkel to breathing air from a tank, using a self-contained underwater breathing apparatus (SCUBA). The valve allows inspired air to flow into the lungs, and then diverts exhaled air into the water. These examples allow us to understand the critical elements of an effective anesthesia breathing system. The device must have the following:

To design an effective anesthesia breathing system, one must identify the ideal goals, evaluate the theoretical limitations of the device or modifications being proposed, and make appropriate accommodations to meet the need. This is indeed what occurred historically in the development of breathing devices used in anesthesia. So, why in reality were these devices not originally based on the simple breathing tube described above? As stated previously, specific breathing devices were designed to meet specific needs. In Tarzan’s case, the need was to use the breathing system to breathe air from the atmosphere while submerged in a shallow pond. For the purpose of delivering anesthetic agents, the goal is to deliver known mixtures of gases and vapors (e.g., oxygen and anesthetic agents), delivered not from the atmosphere, but usually from a pressurized source of gas and an anesthesia machine.

Now let’s design a breathing system to anesthetize Tarzan rather than enable him to breathe underwater, by modifying a simple snorkel. Rather than inserting the tube into his mouth, we will place the breathing apparatus over his mouth and nose using a mask. We can now examine the most effective way to connect this simple breathing apparatus to the gas source. The simplest way to connect the gas source to the breathing tube is to run the gas outlet hose from the gas source (from the anesthesia machine) and connect it to the breathing tube (Fig. 29.1