Can nitrous oxide be administered effectively by nasal cannula? A preliminary report.

STUDY OBJECTIVE

To predict the inspired concentrations achieved when nitrous oxide (N2O)/oxygen mixtures are administered to patients by way of a nasal cannula.

DESIGN

The method used for estimating the FiN2O is based on one employed to calculate the FiO2 obtained with a nasal cannula. We assume a tidal volume of 500 ml, a respiratory rate of 20 breaths per minute, an inspiratory time of 1 second, an expiratory time of 2 seconds, and an anatomic reservoir volume of 50 ml. The reservoir consists of the nose, the nasopharynx, and the oropharynx. Its volume is assumed to be one-third of the anatomic dead space. It is also assumed that during the last 0.5 second of expiration, there is negligible flow of expired respiratory gases. A 6 L/min flow from the cannula will completely fill the reservoir. The FiO2 or FiN2O is then calculated by assuming that during the 1 second inspiratory time period, the gases in the anatomic reservoir that are provided by the nasal cannula and a volume of air such that the sum of the components of the tidal volume equals 500 ml are inspired.

SETTING

Research laboratory of a university-affiliated metropolitan medical center.

MEASUREMENTS AND MAIN RESULTS

The calculated FiO2 values for 100% oxygen delivered by nasal cannula agree with those determined by others. The FiN2Os estimated were directly proportional to the cannula flow rate and the fraction of N2O delivered. At the maximum total flow rate considered, 6L/min flow, with 70% N2O (remainder O2) delivered to the nasal cannula, an FiN2O of only 0.21 was estimated due to the large volume of air inspired. The FiO2 under these conditions would only be 0.23.

CONCLUSIONS

Our analysis shows that the maximum FiN2O achievable by using a nasal cannula is limited to 0.21 even with a 6 L/min flow of 70% N2O for the defined respiratory parameters.