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

Abstract

Background: Critical care ventilators are frequently used to provide noninvasive ventilation (NIV) support to critically ill patients. Questions remain regarding carbon dioxide (CO 2 ) 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 CO 2 for full-face and oronasal masks when using a dual limb circuit. Method: 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 H 2 O) and pressure support of 5 cm H 2 O. 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. Results: 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). Conclusion:A PEEP level of at least 5 cm H 2 O associated with the reported leak was required to minimize the likelihood of CO 2 rebreathing while using a critical care ventilator to provide NIV with a double limb circuit and full-face or oronasal masks.

Rebreathing of carbon dioxide during non-invasive ventilation. Is PEEP the final solution?

“…We have read the article published by Al Hussain and Vines [ 1 ] with keen interest. This was a simulated experiment on healthy volunteers examining the effect of varying levels of positive end-expiratory pressure (PEEP) on the level of rebreathing of carbon dioxide (CO 2 ) during non-invasive ventilation (NIV) with a dual limb critical care ventilator [ 1 ].…”

Response: Rebreathing of carbon dioxide during noninvasive ventilation. Is PEEP the final solution?

“…We would like to thank you for the opportunity to respond to Dr. Dhar et al and provide clarity to issues of concern with our recent paper, “Potential rebreathing of carbon dioxide during noninvasive ventilation provided by critical care ventilator” [ 1 ].…”

“…As in the clinical setting, masks were secured but not overtightened. The volunteer in figure 1 notably does have a light beard [ 1 ]. We did not consider this an issue as the critical care ventilator normally triggered and cycled with all participants, which indicated the leak was acceptable [ 2 ].…”

“…Every mask started at zero PEEP and increased to 5 cm H 2 O, and as observed, CO 2 rebreathing significantly decreased. These are normal volunteers, so regardless of the baseline CO 2 at each PEEP level, it appears that 5 min is enough time to clear CO 2 at higher levels of PEEP, which is evident in figures 2 and 3 [ 1 ].…”