Quantification of reservoir bags as airway pressure-limiting devices in a nonrebreathing system

Objective: To evaluate the influence of reservoir bag types, volumes and previous use on the peak pressures (Pmax) and the times to develop 30 cmH ₂O pressure (P30) within a nonrebreathing system with a closed adjustable pressure-limiting (APL) valve. Study design: In vitro study using three-way factorial design with repeated measure on one factor. Subjects: A total of 75 new anesthesia reservoir bags (five types, three volumes, five bags from each type × volume). The bag types were reusable latex (RL), disposable latex (DL) and three disposable neoprene (DN-1, DN-2 and DN-3). Methods: Each bag was tested three times (treatments): new, after prestretching and 1 week later. The bags were attached to a Bain system and anesthesia machine with closed APL valve and patient port with O ₂ flow 2 L minute ^–1 until Pmax was reached. The Pmax and time to reach P30 values were determined from recorded pressure traces. General linear mixed model analysis was used to examine the effects of bag type, volume and treatment. One-sided 95% upper prediction limits of Pmax were calculated to test the null hypothesis that predicted Pmax of new bags would be ≥ 50 cmH ₂O for each factor combination. Results: RL bags were the least and DN-3 bags were the most compliant. Prestretching increased compliance in all bag types. Smaller bags of RL, DL and DN-1 were less compliant than larger ones. The predicted Pmax values were < 50 cmH ₂O only for DN-3 bags after prestretching. The time to reach P30 was critically low when using 0.5 L bags (median 17 seconds). Conclusions and clinical relevance: To minimize the risk of barotrauma, highly compliant reservoir bags (e.g. DN-3) are recommended and reusable bags should be avoided. Bags should be prestretched before first use, 0.5 L bags should be avoided and fresh gas flow minimized.