ACE Question

Carbon dioxide (CO₂) concentration is MOST likely to be highest at which of the following locations within a properly functioning anesthesia circle system?

• □ (A) Reservoir bag
• □ (B) Unidirectional inspiratory valve
• □ (C) Fresh gas outlet

The modern anesthesia circle system is designed to maximize the efficiency of volatile anesthetic agents – some of the most expensive drugs that anesthesiologists use. To do so, the circle system recirculates expired gases back to the patient. To prevent the development of dangerous levels of hypercarbia, carbon dioxide (CO₂) must first be removed from the expired gas before these anesthetic gases can be recycled.

The highest concentration of CO₂ is found within the tracheal tube (or other airway device) during expiration (Figure). Once these gases reach the Y-piece of the circuit, they are mixed and diluted by fresh and recycled gases that are devoid of CO₂ and coming from the inspiratory limb.

The patient reservoir bag is commonly located on the expiratory side of the circuit, before gases are passed through the CO₂ absorber. Gases at many locations on the expiratory side have essentially identical concentrations of CO₂: those within the unidirectional expiratory valve, within the patient reservoir bag, exiting the circle system en route to the scavenging system, and those entering the CO₂ absorber canister. While multiple variations of the circle system are possible – dependent on placement of the CO₂ absorber, the reservoir bag, and the fresh gas outlet – the arrangement depicted in the Figure is perhaps the most common.

Unless the CO₂ absorbent is completely exhausted, there will be a gradient across the canister as CO₂ is absorbed. There should be a markedly reduced concentration of CO₂ in the middle of the CO₂ absorber canister compared to that which entered the canister. At the outlet of the CO₂ absorber, the concentration should be negligible.

The recycled gases coming out of the CO₂ absorber mix with fresh gases coming from the fresh gas outlet of the anesthesia machine. These gases should have negligible concentrations of CO₂ within the inspiratory limb up to the Y-piece, including the unidirectional inspiratory valve (nonshaded portions of the circle system in the Figure).