Breathing circuit compliance and accuracy of displayed tidal volume during pressure-controlled ventilation of infants: A quality improvement project

Summary

Introduction

Anesthesia machines have evolved to deliver desired tidal volumes more accurately by measuring breathing circuit compliance during a preuse self-test and then incorporating the compliance value when calculating expired tidal volume. The initial compliance value is utilized in tidal volume calculation regardless of whether the actual compliance of the breathing circuit changes during a case, as happens when corrugated circuit tubing is manually expanded after the preuse self-test but before patient use. We noticed that the anesthesia machine preuse self-test was usually performed on nonexpanded pediatric circuit tubing, and then the breathing circuit was subsequently expanded for clinical use. We aimed to demonstrate that performing the preuse self-test in that manner could lead to incorrectly displayed tidal volume on the anesthesia machine monitor. The goal of this quality improvement project was to change the usual practice and improve the accuracy of displayed tidal volume in infants undergoing general anesthesia.

Methods

There were four stages of the project: (i) gathering baseline data about the performance of the preuse self-test and using infant and adult test lungs to measure discrepancies of displayed tidal volumes when breathing circuit compliance was changed after the initial preuse self-test; (ii) gathering clinical data during pressure-controlled ventilation comparing anesthesia machine displayed tidal volume with actual spirometry tidal volume in patients less than 10 kg before (machine preuse self-test performed while the breathing circuit was nonexpanded) and after an intervention (machine preuse self-test performed after the breathing circuit was fully expanded); (iii) performing department-wide education to help implement practice change; (iv) gathering postintervention data to determine the prevalence of proper machine preuse self-test.

Results

At constant pressure-controlled ventilation through fully expanded circuit tubing, displayed tidal volume was 83% greater in the infant test lung (mean±SD TV 15±5 vs 9±4 mL; mean [95% CI] difference=6.3 [5.6, 7.1] mL, P<.0001) and 3% greater in the adult test lung (245±74 vs 241±72 mL; difference=5 [1, 10] mL, P=.0905) when circuit compliance had been measured with nonexpanded tubing compared to when circuit compliance was measured with fully expanded tubing. The clinical data in infants demonstrated that displayed tidal volume was 41% greater than actual tidal volume (difference of 10.4 [8.6, 12.2] mL) when the circuit was expanded after the preuse self-test (preintervention) and 7% greater (difference of 2.5 [0.7, 4.2] mL) in subjects when the circuit was expanded prior to the preuse self-test (postintervention) (P<.0001). Clinical practice was changed following an intervention of departmental education: the preuse self-test was performed on expanded circuit tubing 11% of the time prior to the intervention and 100% following the intervention.

Conclusion

Performing a preuse self-test on a nonexpanded pediatric circuit that is then expanded leads to falsely elevated displayed tidal volume in infants less than 10 kg during pressure-controlled ventilation. Overestimation of reported tidal volume can be avoided by expanding the breathing circuit tubing to the length which will be used during a case prior to performing the anesthesia machine preuse self-test. After department-wide education and implementation, performing a correct preuse self-test is now the standard practice in our cardiac operating rooms.

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