Nitrous oxide concentrations in the posterior nasopharynx during administration by nasal mask.
OBJECTIVE
Nitrous oxide (N2O) administration with nasal mask produces variable outcomes in dental patients. This study describes a novel sampling method to measure actual inspired/expired N2O concentrations ([N2O]).


METHODS
Fifteen adult volunteers (32.5 +/- 8.5 years) underwent placement of a nasopharyngeal probe. With a nasal mask, 100% oxygen (O2) was administered for 2 minutes. N2O was introduced incrementally every 2 minutes for a final flowmeter [N2O] of 50% and subsequently decreased in the same manner. Anesthesia gas monitors analyzed inspired/expired [N2O], [O2], and PETCO2 from the nasopharynx and end-inspired/expired [N2O] in the mask. Data were measured every 20 seconds and analyzed. Inspired/expired nasopharyngeal and nasal mask [N2O] and [O2] were expressed as the median value at each time point for all subjects and plotted against flowmeter settings.


RESULTS
Average inspired nasal mask [N2O] was 31% lower than flowmeter settings and decreased by another 19% on the way to the nasopharyngeal sampling site. During the phase of increasing N2O, average expired nasopharyngeal [N2O] was 22% lower than inspired [N2O]. When N2O was decreased, the effect was reversed and average expired [N2O] was 18% higher than inspired. Expired [N2O] was on average 51% lower than flowmeter settings. Mean PETCO2 was 39.7 +/- 1.4 mm Hg.


CONCLUSIONS
Nasopharyngeal end-expired [N2O] varied markedly from flowmeter settings. Correlation of PETCO2 with expected physiologic values validates sampling methodology. This method allows accurate, continuous, and actual measurements of inhaled/exhaled gases in awake patients as well as decision-making/analysis of effectiveness of mask type to determine average [N2O] during administration by nasal mask.
Department of Pediatric Dentistry, The Children's Hospital, Denver, Colorado, USA. Klein.Ulrich@tchden.org
Ulrich
U
Klein
Brenda A
BA
Bucklin
Thomas J
TJ
Poulton
Daniel
D
Bozinov
D000328:Adult; D000766:Anesthesia, Dental / Q000379:methods*; D018685:Anesthetics, Inhalation / Q000032:analysis*; D045853:Exhalation; D005260:Female; D045268:Flowmeters; D006801:Humans; D001239:Inhalation; D008297:Male; D008397:Masks; D009305:Nasopharynx; D009609:Nitrous Oxide / Q000032:analysis*
D018685:Anesthetics, Inhalation; D009609:Nitrous Oxide