QUARTERLY BEAT / DECEMBER 2025
COMMON CAUSES FOR ELEVATED FICO 2 A normal inspiratory to expiratory ratio (I:E) is 1:2 (e.g., one second of inspiration in, two seconds of expiration). Regardless of patient breathing system, an inappropriate inspiratory to expiratory ratio (I:E) like that experienced when a patient pants (i.e., I:E of 1:1) can result in rebreathing of CO 2 . Common causes for an inappropriate I:E ratio of 1:1 could be iatrogenic (i.e. caused by the anesthetist through inappropriate ventilator settings), a patient that becomes light under anesthesia resulting in tachypnea (i.e. panting) or other causes of tachypnea (e.g. hypoxemia). More commonly causes of rebreathing of CO 2 involve failure of a part of the anesthetic machine or patient breathing circuit. Figure 3 breaks down general causes unique to the categorical types of patient breathing systems used in veterinary medicine.
CIRCLE SYSTEMS (REBREATHING FGF RATES)
MAPLESON SYSTEMS (NON-REBREATHING FGF RATES)
Figure 4. Area within the red circle is dead space, and, while not excessive in this patient, could become excessive by the addition of large capnograph adaptors, excessively long endotracheal tubes and other breathing circuit adaptors (e.g. elbows). Image courtesy of Amanda M. Shelby. Regardless of which patient breathing circuit in use, when patients have inappropriate I:E ratios less than 1:2, excessive dead space which is any space inspiratory and expiratory gases can mix (See Figure 4 as an example) or a leak between co-axial tubes. Co-axial tubes are breathing circuits designed with the inspiratory tubing inside the expiratory tubing (e.g. modified Bain circuit designed to be used with non-rebreathing FGF rates or the F-circuit circle system designed to be used as a rebreathing circuit) (See Figure 5 and 6). A thorough understanding of the patient breathing circuits’ leading causes for an increase in FiCO 2 assists with rapid identification of the cause.
• Inappropriate I:E ratio (<1:2) • Leak between co-axial tubing
• Inappropriate I:E ratio (<1:2) • Leak between co-axial tubing
• Excessive dead space
• Malfunctioning
• Excessive dead space
unidirectional valve
• Insufficient FGF rate
• Exhausted CO 2
absorption granules
Figure 3. Causes of rebreathing CO 2 in patient breathing systems. Understanding the impact of fresh gas flow (FGF) rates on determining the degree of rebreathing that can occur in the patient breathing circuit is a foundational step to performing anesthesia. Less rebreathing of expired gases occurs with the use of higher FGF rates. In veterinary medicine, FGF rates are primarily 100% oxygen with a small percentage of halogenated gases (i.e. inhalant anesthetics). Perhaps inappropriately, breathing circuits are commonly referred to as rebreathing, more appropriately termed circle systems and non-breathing systems, more appropriately called Mapleson circuits. When circle systems use oxygen flow rates of 10-30 mL/kg/min, rebreathing of expired gases can occur.(1) Benefits of using a circle (or rebreathing) patient breathing circuit include conservation of resources such as patient body heat, humidity, inhalant use, minimize environmental impact of waste anesthetic gases (WAGs) and oxygen use. Unlike circle systems, Mapleson (or non-rebreathing) circuits are designed to only be used with high oxygen flow rates (i.e. 200 mL/kg/min or greater).(1) Circle systems have unidirectional valves that designate inspiration and expiration hosing as well as CO 2 absorption granules that remove CO 2 from expired gases before a portion is recycled into the inspiratory limp of the circle system. When a Mapleson system is selected, the absence of unidirectional valves and CO 2 absorber makes this patient breathing system dependent on fresh gas flow rate to prevent rebreathing of CO 2 , in other words, unidirectional flow of gases in the circuit hosing.
Figure 5. Mapleson patient breathing circuits designed to be used with non- rebreathing FGF rates. Mapleson circuit (left); Modified Bain circuit (right). Image courtesy of Amanda M. Shelby.
21
VETGIRL BEAT EMAGAZINE | VETGIRLONTHERUN.COM
Made with FlippingBook - Online Brochure Maker