When the Proportions of Nitrous Oxide and Oxygen Are Adjusted

When the proportions of nitrous oxide and oxygen are adjusted to achieve the desired level of analgesia, each adjustment should be at the rate of which of the following?

A) 0.5 L nitrous oxide for at least 30 seconds
B) 0.5 to 1 L nitrous oxide for at least 1 minute
C) 1 L nitrous oxide for at least 30 seconds
D) 1 to 1.5 L nitrous oxide for at least 1 minute
E) 1.5 L nitrous oxide for at least 30 seconds