What potential complication could occur from using a circle circuit?

• A. Hyperventilation
• B. Airway obstruction
• C. Hypercapnia
• D. Hypoventilation

Answer: (D)

Circle circuits recycle exhaled gas after CO2 removal, which adds dead space and some resistance to breathing. Because of this, if fresh gas flow isn’t high enough or the animal isn’t ventilating adequately, there isn’t enough effective alveolar ventilation. Gas exchange then becomes inefficient and CO2 isn’t cleared properly, leading to hypoventilation. Hyperventilation would mean excessive CO2 loss, which isn’t the typical issue with this circuit. Airway obstruction can happen with any airway setup but isn’t specific to circle circuits, and while hypercapnia can result from hypoventilation or rebreathing, the most characteristic risk from the circuit design itself is reduced ventilation efficiency, i.e., hypoventilation.