Nitrous Oxide and Inhalation Sedation Flashcards

A patient scheduled for extraction of an impacted molar under nitrous oxide/oxygen sedation says that 10 days ago they had a vitrectomy for a retinal detachment, and the ophthalmologist placed a perfluoropropane (C3F8) gas bubble in the eye. What is the most appropriate action?

Answer: Don't use N2O, because it diffuses into the gas bubble faster than the bubble's gas can leave, which raises intraocular pressure and can cause permanent vision loss

N2O is about 34 times more soluble in blood than nitrogen, and much more soluble than SF6 or C3F8. It moves into any closed, gas-filled space faster than the gas inside can leave, so the space expands. In the eye this sharply raises intraocular pressure and can block the central retinal artery, causing permanent blindness. A C3F8 bubble can stay in the eye for 6–8 weeks or more, and SF6 for about 2 weeks. N2O is contraindicated until the ophthalmologist confirms the bubble has fully absorbed. Lowering the concentration does not remove the risk, and the gases do not react chemically.

During the morning equipment check, the pressure gauge on a nitrous oxide E-cylinder reads about 745 psi. What can the assistant correctly conclude?

Answer: The cylinder still holds some liquid N2O, but the gauge cannot show how much. Weighing the cylinder against its tare weight is the reliable way to estimate what is left

N2O is stored as a liquid with vapor above it. At room temperature the gauge shows the vapor pressure, about 745–750 psi, and that reading stays the same as long as any liquid is left. The pressure only starts to drop when roughly 25% of the contents remain and all of it is vapor. A steady 745 psi therefore tells you liquid is present, not how much. The contents can only be estimated by weighing the cylinder and subtracting the tare weight. Oxygen is a compressed gas, so its pressure falls in proportion to how much is used.

The Pin Index Safety System (PISS) keeps the wrong E-cylinder from being attached to a yoke. Which pin positions belong to nitrous oxide?

Answer: Pins 3 and 5

Under PISS, nitrous oxide uses pins 3 and 5 and oxygen uses pins 2 and 5. Air uses pins 1 and 5, and pins 2 and 6 are used for carbon dioxide mixtures. Because the pin positions are unique, an N2O cylinder physically cannot hang on an oxygen yoke. The Diameter Index Safety System (DISS) gives the same protection for pipeline and hose connections, but it uses different fitting diameters rather than pins.

Diffusion hypoxia can happen when nitrous oxide is stopped suddenly. Room air contains much more nitrogen than the N2O a patient breathes, yet nitrogen never causes the same effect when air breathing stops. Which property of N2O best explains the difference?

Answer: N2O is about 34 times more soluble in blood than nitrogen, so a large volume floods back into the alveoli and dilutes the alveolar oxygen

N2O's blood/gas partition coefficient is about 0.47, compared with about 0.015 for nitrogen, so far more N2O dissolves in the blood. When N2O delivery stops, that large dissolved volume moves quickly back into the alveoli, faster than nitrogen from room air can replace it. This dilutes the alveolar oxygen (the Fink effect). The fix is 100% oxygen for at least 3–5 minutes after N2O is stopped. Less than 0.004% of N2O is metabolized, and it does not bind hemoglobin.

A patient with untreated pernicious anemia is scheduled for long dental treatment and asks for nitrous oxide sedation. What is the main biochemical reason this is a concern?

Answer: N2O irreversibly oxidizes the cobalt atom in vitamin B12, which inactivates methionine synthase and impairs DNA synthesis and myelin formation

N2O oxidizes the cobalt in cobalamin (vitamin B12) from Co+ to Co3+, which irreversibly inactivates methionine synthase. That enzyme is needed to make methionine and tetrahydrofolate, which the body uses for DNA synthesis and myelin upkeep. A patient who is already B12-deficient, such as someone with pernicious anemia or an MTHFR defect, has little reserve. Even short exposures can trigger megaloblastic changes or subacute combined degeneration of the spinal cord. N2O does not affect intrinsic factor, cause hemolysis, or block folate receptors.

Chronic exposure to waste nitrous oxide is linked to reduced fertility and spontaneous abortion in dental staff. What is the NIOSH Recommended Exposure Limit (REL) for N2O?

Answer: 25 ppm as a time-weighted average during the period of anesthetic administration

NIOSH recommends a limit of 25 ppm for N2O, measured as a time-weighted average over the period of administration. The 2 ppm ceiling is the NIOSH limit for halogenated anesthetics used alone, not for N2O. Ways to meet the 25 ppm limit include a properly fitted scavenging nasal hood with vacuum suction of about 45 L/min, checking for leaks, keeping patient talking and mouth breathing to a minimum, and good room ventilation.