First, it would need to be determined the depth of sedation being administered. Minimal sedation/anxiolysis have no separate reporting or payment. If it was determined moderate sedation was, indeed, being administered, I would refer you to the Moderate Sedation codes and guidelines (starting with 99143). The ASA (American Society of Anesthesiologists) offers definitions on the continuum of sedation.
More importantly though, you might consider exploring the federal, state and local laws associated with nitrous oxide administration in an office setting.
Nitrous oxide is relatively contraindicated in pregnancy (not that he/she would be employing its use on their pregnant patients). It has known teratogenic effects and fetal toxic effects (with chronic use). However, the office staff would be exposed to what might be considered a chronic low dose.
There is also the consideration of scavenging the waste gas. Use of nitrous oxide in the dental chair is employed with nasal hoods that scavenge the waste gas with a suction system built into the infrastructure of the office.......not sure that an OB/GYN office would have this inherent capability.
Monitoring, use of pulse oximetry, etc are other considerations.
This is just a brief synopsis of things to think about. There is a wealth of published information and standards associated with N2O administration.
http://www.cdc.gov/niosh/docs/94-100/