Objective - To describe the effects of changes in circuit volume and oxygen inflow rate on inspired oxygen concentration for a large animal circle anesthetic system. Study population - A large animal circle anesthetic system, a 10 L/min flowmeter, and 20- and 40-L breathing bags. Procedure - Circuit volume was determined by a carbon dioxide dilution technique. Oxygen flow rates of 3, 6, and 10 L/min were delivered to the circuit with the large breathing bag, and a flow rate of 6 L/min was used with the small bag. Gas samples were collected during a 20-minute period. The time constant (τ) and half-time (T1/2) were calculated and compared with measured values. Results - Mean ± SEM volume of the breathing circuit with a 20- and 40-L breathing bag was 32.97 ± 0.91 L and 49.26 ± 0.58 L, respectively. The τ from measurements was 11.97, 6.10, and 3.60 minutes at oxygen flow rates of 3, 6, and 10 L/min, respectively, for the large breathing bag and 3.73 minutes at a flow rate of 6 L/min for the small breathing bag. The T1/2 was 8.29, 4.22, and 2.49 minutes at oxygen flow rates of 3, 6, and 10 L/min, respectively, for the large breathing bag and 2.58 minutes for the small breathing bag. Conclusions and clinical relevance - This study emphasizes that there are delays in the rate of increase in the inspired oxygen concentration that accompany use of conventional large animal circle anesthetic systems and low rates of inflow for fresh oxygen.
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