QUARTERLY BEAT / DECEMBER 2025
• 25 kg dog using a rebreathing circuit FGF of 20 mL/kg/ min equals a FGF rate of 500 mL/min. • 5 kg cat using a non-rebreathing circuit FGF of 200 mL/ kg/min equals a FGF rate of 1000 mL/min or 1 L/min. Keep in mind that most anesthetic precision vaporizers require minimum FGF rates of at least 500 mL/min or 0.5 L/min to ensure accurate inhalant output. 3. REDUCE DEAD SPACE Reduce patient dead space where possible. Use appropriate length endotracheal tubes. Endotracheal tubes can be cut down to size if the portion removed is before the cuff pilot line (see Figure 7). Other methods for reducing dead space include using pediatric tubing for small patients or consider using Mapleson (or non-rebreathing) circuits and pediatric capnograph adaptors.
Figure 6. Circle systems hanging. Pediatric 15 mm ID tubing circle system (left); Traditional 22 mm ID tubing circle system (right). Image courtesy of Amanda M. Shelby. SO, HOW DO YOU PREVENT REBREATHING OF CO 2 IN YOUR ANESTHETIZED PATIENTS? 1. PREFORM LEAK TESTS AND ROUTINE ANESTHETIC EQUIPMENT CHECK OUT PROCEDURES BEFORE EACH CASE. Good checkout procedures involving routine inspection of all anesthetic equipment, especially breathing circuits with co-axial tubing, and performing positive pressure machine checks before use will eliminate many of the common causes for rebreathing of CO 2 . If using a circle system ensure unidirectional valves are functioning during the positive pressure checkout — while filling the circuit with positive pressure the inspiratory valve should open, expiratory valve should remain closed, when the positive pressure is released from the circuit by opening the adjustable pressure limiting (APL or ‘pop-off’ valve), the expiratory valve should open and inspiratory valve should remain closed. Additionally ensure the CO 2 absorption granules are functional. This could include ensuring they crumble when crushed between your fingers. The absence of a color change may not always be an indicator that the CO 2 absorption granules are ok to use, especially when a machine is infrequently used. For more information on how to set up and pressure check your anesthetic machine and select the patient breathing circuit, check out this VETgirl webinar! 2. CALCULATE APPROPRIATE FGF RATES BASED ON THE PATIENT BREATHING CIRCUIT in use (rebreathing vs. non-rebreathing) specific to the patient’s requirement. See example:
Figure 7. The red line denotes the lowest point towards the patient end, where this endotracheal tube could be cut to reduce dead space. The cuff pilot line enters to the right of the red line. Image courtesy of Amanda M. Shelby. TIPS TO HELP YOU IDENTIFY THE CAUSE FOR HIGH FICO 2 Lastly, what should you do if your experiencing high FiCO 2 and can’t identify the cause? An FiCO 2 above 5 mmHg should be concerning. If this continues to elevate and the cause cannot be identified, use of the machine should be discontinued until it can be properly serviced. Since the most often causes are related to unidirectional valves, expired CO 2 absorbing granules or the patient breathing circuits, often the causes are easy to identify and resolve. When the issue is related to the patient breathing circuit, regardless of the machine used, rebreathing will continue to occur. If these patient breathing circuits cannot be repaired, it is best to discard and replace them. For me personally, the hardest source for rebreathing CO 2 situations I have experienced has been with leaks in the inner tubes of co-axial circuits (both on Modified Bain circuits and F-circuits) and warped/cracked unidirectional valves. Otherwise, identifying the cause for rebreathing of CO 2 is typically straightforward if you following the causes identified in Figure 3. CONCLUSION When in doubt, it is very important that veterinarians and veterinary technicians be aware of how to prevent rebreathing of carbon dioxide (CO 2 ) in intubated veterinary patients. Most of the cause for rebreathing of CO 2 can be prevented with similar anesthetic check out procedures that evaluate each component of the anesthetic machine and patient breathing circuit. Being able to prevent, recognize, and treat hypercapnia, we can improve the outcome and safety of anesthesia in our veterinary patients.
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