Several questions arise when considering the issue of preoxygenation. The first is “What is preoxygenation?” Preoxygenation is the removal of nitrogen from the body by oxygen. “Is preoxygenation the same as denitrogenation ?” The answer is not exactly. With preoxygenation, oxygen is used as the sole agent, while denitrogenation is performed with oxygen combined with another gas in substantial concentration, usually nitrous oxide 50–60 %. “How much nitrogen has to be removed for the average patient to complete preoxygenation?” This value can be calculated reasonably accurately based on certain assumptions related to where the nitrogen is located.

The largest volume of nitrogen in the body is located in the lungs. Assuming a functional residual capacity (FRC) of 4 L and the lungs are filled with air that is 79 % nitrogen, the total volume of nitrogen would be 3160 mL. Nitrogen is dissolved in plasma in a volume of 1.28 mL/100 mL plasma at one atmosphere (760 mmHg) and 37 °C. If we assume a blood volume of 5 L and a hematocrit of 40, the volume of nitrogen in plasma would be 3000 mL × 1.28 mL/100 or 384 mL at a normal body temperature and atmospheric pressure. There is a small amount of air in the sinuses; middle ear and bowel, the exact amount of which would vary depending upon patient size and air swallowing, but the total would not exceed 20–30 mL. Most of the gas in the bowel is in the form of methane, carbon dioxide and hydrogen. So, the total quantity of retrievable nitrogen would less than 4 L in the average patient with 90 % of it being in the lungs.