Although modern operating room ventilators are typically large and complex, the basic components are fairly simple. Figure 12-1 shows the basic breathing system components: (1) carbon dioxide (CO2) absorbent; (2) two unidirectional valves; (3) fresh gas inlet; (4) Y-connector; (5) reservoir bag; (6) adjustable pressure-limiting (APL) valve; and (7) low-resistance tubing.
A circle system. APL, adjustable pressure-limiting (valve) (Reproduced with permission from Butterworth JF, Mackey DC, Wasnick JD, Morgan and Mikhail’s Clinical Anesthesiology, 5th ed. McGraw-Hill; 2013.)
CARBON DIOXIDE ABSORPTION
Carbon dioxide absorbance is vital to preventing hypercarbia with rebreathed tidal volumes. Absorbents remove CO2 from the circuit’s expiratory limb, allowing anesthetic gas to be recycled, thereby making a closed system possible. Soda lime, Baralyme, and Amsorb are the most common substances used for CO2 extraction.
Soda lime is predominantly made up of calcium hydroxide (Ca(OH)2) with smaller amounts of sodium hydroxide and potassium hydroxide. Silica is added to decrease dust formation. The soda lime reaction is:
H2CO3 + 2 NaOH → Na2CO3 + 2 H2O + Heat
Na2CO3 + Ca(OH)2 → CaCO3 + 2 NaOH
In the first equation, exhaled CO2 reacts with water to form carbonic acid. In the second equation, carbonic acid reacts with the hydroxide salts of barium, calcium, potassium, or sodium to form water, heat, and carbonates of barium, calcium, potassium, or sodium. The third equation shows the carbonates reacting with calcium hydroxide to form calcium carbonate and hydroxides of barium, calcium, potassium, or sodium.
As can be seen from the equation, water and heat are produced, adding humidity and heat to the breathing circuit. Soda lime can absorb 23-26 L of CO2 per 100 g of absorbent. When soda lime absorbent is exhausted, a color change occurs due to a pH-sensitive reaction.
Note: Baralyme is made up of barium hydroxide and calcium hydroxide. Water in Baralyme’s structure obviates the need for silica. Baralyme was withdrawn from the US market in 2005.
One of the most important aspects of absorbents is the size of the granules. Smaller granules have greater surface area for absorption, but increased resistance to air flow. Granule size has been carefully engineered to maximize surface area and absorption while minimizing resistance. Typical granule size is 4-8 mesh (ie, will pass through a mesh of 4-8 strands per inch in each axis, or 2.36-4.75 mm).
In addition to CO2, granules also absorb volatile anesthetics. Dry granules can break down desflurane or isoflurane into carbon monoxide (CO), whereas ...