Proper theoretical analysis of the oxygen wash-in kinetics of circle breathing systems

Abstract

Objective: To correctly define the theoretical treatment of oxygen wash-in kinetics in circle breathing systems and reevaluate previously published results for the rate of change of oxygen concentration in a large animal circle breathing system.

Study design: Theoretical analysis of previously published data.

Animals: None.

Methods: Previously published data for the rate of change of oxygen concentration in a large animal circle breathing system with two reservoir bag sizes (40 and 20 L) at three different flow rates (3, 6, and 10 L minute(-1)) were obtained from the original publication, via digital extraction, and analyzed to determine system time constants. The results of this analysis were compared to those originally reported and the level of mathematical agreement between experiment and theory was quantified.

Results: Theoretical time constants for the system with 40 L reservoir bag with flow rates of 3, 6, and 10 L minute(-1) were 16.4, 8.2, and 4.9 minutes, respectively. Experimentally derived time constants for this system with flow rates of 3, 6, and 10 L minute(-1) were 18.1, 9.2, and 5.4 minutes, respectively. Percent differences between experimental and theoretical time constants for this system with flow rates of 3, 6, and 10 L minute(-1) were 10.4, 12.2, and 10.2%, respectively. For the system with a 20 L reservoir bag and 6 L minute(-1) flow rate, the theoretical and experimentally derived time constants were 5.5 and 5.6 minutes, respectively, with a 1.8% difference. The average relative deviations between theory and experiment for the system at 6 L minute(-1) flow with a 40 or 20 L reservoir bag were 1.3% and 3.0%, respectively.

Conclusions and clinical relevance: Proper theoretical analysis of experimentally obtained data for the wash-in kinetics of oxygen into a large animal circle breathing system leads to improved mathematical agreement between theory and experiment when compared to the originally published results. Application of this method should allow more accurate prediction of the rate of change of oxygen concentration in anesthetic circuits.