Background: The partial pressure of end-tidal CO2 (PETCO 2) is sometimes measured during a code situation to determine the effectiveness of chest compressions via bag-endotracheal tube ventilation. We sought to determine if the supplemental oxygen delivered to the bag during manual ventilation affected the capnometer reading. Secondarily we evaluated whether the application of PEEP had any effect on PETCO2. The null hypothesis is that neither high oxygen flow nor PEEP skew the capnometry reading during manual ventilation. Methods: A simulation model was created using a Michigan Instruments TTL test lung connected to either an Ambu Spur II or Mercury Medical CPR-2 bag. Compliance was set for 60 mL/cm H2O and resistance set for 5 cm H2O/L/s. A Hamilton Medical C6 Capnostat monitor was attached between the resuscitation bag and the test lung. CO2/air mixtures were introduced into the test lung to obtain a target PETCO2 of 20 mm Hg and 40 mm Hg. The patient was bagged by the same person delivering 20 cm H2O of pressure each time. Continuous bagging was performed for one minute, then a reading was recorded. Trials consisted of two flows (15 L/min and flush) and three PEEP levels (0, 10, and 20 cm H2O). All trials were performed three times, the average was taken, and the difference from target was recorded. Results: Ambu Spur II values at 15 L/min and flush were within ± 3 mm Hg of both PETCO2 targets and all PEEP levels except when the flow was set to flush with zero PEEP and the target was set to 40 mm Hg. In this scenario the measured value was 72.67 mm Hg, a 58% difference. Mercury Medical CPR-2 values at 15 L/min and flush were within ± 3 mm Hg of both PETCO2 targets only when PEEP was set to 20 cm H2O and when PEEP was set for 10 cm H2O and the target was 40 mm Hg. All other values ranged from 7-13 mm Hg of target with the highest variability occurring when the flow was set to flush, zero PEEP, and a target of 20 mm Hg. In this case the measured value was 33 mm Hg, a 49% difference. Conclusions: PETCO2 values tended to be more consistent when oxygen flow was set for 15 L/min regardless of the presence or absence of PEEP. However, when the flow was increased to flush and PEEP set to zero we found that there was a significant difference between the target and PETCO2 in both resuscitation bags but particularly in the Ambu Spur II bag when the target was 40 mm Hg. When bagging with high oxygen flow and zero PEEP, the capnometer may not reflect an accurate PETCO2.
Algazi et al. (Sun,) studied this question.