Potential rebreathing of carbon dioxide during noninvasive ventilation provided by critical care ventilator

Critical care ventilators are frequently used to provide noninvasive ventilation (NIV) support to critically ill patients. Questions remain regarding carbon dioxide (CO₂) clearance while using a critical care ventilator and dual limb circuit with various patient interfaces. The purpose of this study is to determine the positive end expiratory pressure (PEEP) level required to effectively washout CO₂ for full-face and oronasal masks when using a dual limb circuit.

This randomized crossover trial was conducted at an academic medical center in the Midwest United States. After obtaining informed consent, eight healthy volunteers were placed on a 980 Puritan Bennett (Medtronic, Minneapolis, MN) ventilator operating in the NIV mode. All subjects performed 20 min of breathing on four levels of PEEP (0, 2, 4, and 5 cm H₂O) and pressure support of 5 cm H₂O. NIV settings were applied to four masks (two oronasal and two full-face masks) that were randomly selected with a 5-min washout period between each mask. The fraction of inspired carbon dioxide (F _ICO2) was sampled/monitored with a nasal cannula using a Capnostream 20p monitor (Medtronic, Minneapolis, MN) and reported as percentages. A Kruskal-Wallis test was used to reveal significant differences across PEEP levels. Pairwise comparisons of the groups were made using Mann-Whitney tests with a family-wise error correction.

Median (IQR) F _ICO2 was significantly lower 0.0% (0%-0.92%) at PEEP of 5 compared to 1.83% (0.66%-4.0%; p < 0.001) at PEEP of 0 or 1.0% (0.33%-2.66%; p = 0.002) at PEEP of 2. F _ICO2 was significantly lower 0.5% (0%-1.92%) at PEEP of 4 compared to PEEP of 0 (p = 0.001).

A PEEP level of at least 5 cm H₂O associated with the reported leak was required to minimize the likelihood of CO₂ rebreathing while using a critical care ventilator to provide NIV with a double limb circuit and full-face or oronasal masks.