Comparing Nasal End-Tidal Carbon Dioxide Measurement Variation and Agreement While Delivering Pulsed and Continuous Flow Oxygen in Volunteers and Patients
Authors: Burk Kyle M. , Sakata Derek J. , Kuck Kai , Orr Joseph A. 
Published: January 2019

BACKGROUND:
Supplemental oxygen is administered during procedural sedation to prevent hypoxemia. Continuous flow oxygen, the most widespread method, is generally adequate but distorts capnography. Pulsed flow oxygen is novel and ideally will not distort capnography. We have developed a prototype oxygen administration system designed to try to facilitate end-tidal carbon dioxide (ETco
2) measurement. We conducted a volunteer study (ClinicalTrials.gov, NCT02886312) to determine how much nasal ETco
2 measurements vary with oxygen flow rate. We also conducted a clinical study (NCT02962570) to determine the median difference and limits of agreement between ETco
2 measurements made with and without administering oxygen.

METHODS:
Both studies were conducted at the University of Utah and participants acted as their own control. Inclusion criteria were age 18 years and older with an American Society of Anesthesiologists physical status of I–III. Exclusion criteria included acute respiratory distress syndrome, pneumonia, lung or cardiovascular disease, nasal/bronchial congestion, pregnancy, oxygen saturation measured by pulse oximetry <93%, and a procedure scheduled for <20 minutes. For the volunteer study, pulsed and continuous flow was administered at rates from 2 to 10 L/min using a single sequence of technique and flow. The median absolute deviation from the median value was analyzed for the primary outcome of ETco
2. For the clinical study, ETco
2 measurements (the primary outcome) were collected while administering pulsed and continuous flow at rates between 1 and 5 L/min and were compared to measurements without oxygen flow. Due to institutional review board requirements for patient safety, this study was not randomized. After completing the study, measurements with and without administering oxygen were analyzed to determine median differences and 95% limits of agreement for each administration technique.

RESULTS:
Thirty volunteers and 60 patients participated in these studies which ended after enrolling the predetermined number of participants. In volunteers, the median absolute deviation for ETco
2 measurements made while administering pulsed flow oxygen (0.89; 25%–75% quantiles: 0.3–1.2) was smaller than while administering continuous flow oxygen (3.93; 25%–75% quantiles: 2.2–6.2). In sedated patients, the median difference was larger during continuous flow oxygen (−6.8 mm Hg; 25%–75% quantiles: −12.5 to −2.1) than during pulsed flow oxygen (0.1 mm Hg; 25%–75% quantiles: −0.5 to 1.5). The 95% limits of agreement were also narrower during pulsed flow oxygen (−2.4 to 4.5 vs −30.5 to 2.4 mm Hg).

CONCLUSIONS:
We have shown that nasal ETco
2 measurements while administering pulsed flow have little deviation and agree well with measurements made without administering oxygen. We have also demonstrated that ETco
2 measurements during continuous flow oxygen have large deviation and wide limits of agreement when compared with measurements made without administering oxygen.