Medical gas cylinders (tanks) have been in use since 1888. Several medical/anesthetic gases, including pure (99%) oxygen, are packaged in cylinders, which are manufactured of seamless, high-quality, finely tempered steel. For safety, cylinders are color coded. Oxygen tanks are colored white (or green). White is the international color code for oxygen. Despite color coding, the contents of a cylinder should be verified by its tag/label. If there is no tag, do not use the cylinder.

Numerous federal and industrial regulatory bodies govern cylinder manufacturing specifications, labeling, transportation, storage, and handling of medical gases.

Oxygen cylinders are available in different sizes that contain various amounts of gas. Cylinder size is designated by single letters. For example, popular oxygen tanks are termed H (large) size and E (small) size. E-sized tanks are usually used for patient transport or in emergencies. H-sized tanks measure over 4½ ft (1.37 m) tall and are used more for stationary/long-term (bedside) applications. Oxygen cylinders are an expensive and bulky form of packaging for a relatively inexpensive substance.

Cylinders contain much more gas than their unpressurized internal volume. By pressurizing the gas into cylinders, H and E cylinders when full contain 244 ft³ (74.4 m³) and 22 ft³ (6.7 m³) of oxygen, respectively. The reader is reminded that 1 ft³ (0.30 m³) equates to 28.3 L. Regardless of the size of the oxygen cylinder, it is filled to a tremendous pressure (2,200 psi). As a comparison, most automobile tires contain less than 50 psi.

A means of controlling the release of oxygen from the cylinder is necessary to permit safe and adjustable delivery to the patient. Typically, this includes three components:

In most applications, the pressure-reducing valve and flowmeter are combined into a single assembly called a regulator.

Oxygen can be stored as a liquid. This requires very low temperatures (-183° C) maintained within a thermoslike cryogenic vessel. These systems operate under a
pressure of approximately 20 psi. As pressure is reduced, the liquid oxygen is converted into a gas. It is not possible to estimate the contents of a vessel containing liquid oxygen, because the vapor pressure remains constant until the tank is empty. This is analogous to estimating the amount of propane within a tank used with a domestic barbeque. The quantity of liquid gas contained within the vessel is measured in terms of weight, rather than pressure. The storage advantage is obvious when considering that a single cubic foot of liquid oxygen is equivalent to about 860 ft³ (262.1 m³) of oxygen gas. Liquid oxygen vessels can be transfilled, that is, one liquid system can be used to fill another. Patients requiring long-term oxygen treatment often use smaller, portable liquid oxygen vessels. Patients may arrive at the sleep center with portable liquid oxygen systems. Although the patient can easily be switched to the sleep center’s oxygen source, the technologist should be aware that the liquid oxygen will slowly vaporize and the vessel will empty even while not being used. This is similar to having a thermos of hot coffee that will slowly cool despite never being opened, the difference being that as liquid oxygen warms, it vaporizes to gas and escapes from the storage vessel.

Liquid oxygen is more expensive than compressed oxygen, and its primary advantage is storage. Most hospitals and other high-volume users store oxygen on-site in its liquid form.

These self-contained, somewhat portable devices use electricity to power a compressor and filtration system. Filtering of room air through a molecular sieve bed or permeable plastic membrane produces concentrated oxygen. Oxygen concentrators are not capable of delivering high flows or high concentrations of oxygen and are therefore not suited to emergency situations.

Modern oxygen concentrators are convenient, quiet, and safe and provide a reliable source of relatively inexpensive low-flow oxygen. Because of these benefits, concentrators are a good oxygen source for sleep centers located outside of hospitals. The reader is encouraged to become familiar with their respective oxygen concentrator in terms of flow rate capability and oxygen concentration specifications. Oxygen concentration should be checked as a part of the laboratory equipment quality assurance process.

Pure oxygen itself presents potential physical dangers. First, oxygen cylinders contain very high pressures. It must be ensured that tanks are secured from falling or tipping to avoid tank or regulator breakage. Oxygen cylinders must be stored away from heat sources. Liquid oxygen is of extremely low temperature, and the vaporizing liquid can cause low-temperature burns to the skin. Oxygen itself is not explosive but vigorously supports combustion. Oxygen and materials that are flammable make for a very dangerous mix. For example, collodion, which is both volatile and flammable, should not be used in the vicinity of oxygen.

Oxygen also poses physiologic consequences. Prolonged high alveolar oxygen levels (>60%) are associated with the production of free radicals and lung damage (oxygen toxicity). Alveolar oxygen concentrations of more than 80% (with special circumstances) can be associated with regional microlung collapse (absorption atelectasis).

The predominate stimulus to breathe is carbon dioxide level. There is also a secondary drive to breathe motivated by hypoxemia, and it is termed “hypoxic drive,” which is a normal mechanism and can be accentuated at high altitude.

Some chronic lung diseases cause chronic elevation of PaCO₂ (hypercarbia). Chronic hypercarbia, it is argued, causes normal sensitivity to carbon dioxide to become blunted, and hypoxia becomes a predominant stimulus to breathe. These patients are said to be dependent on their hypoxic drive; their drive to breathe is diminished when oxygen is administered, and they may become hypopneic or apneic.

Although patients with chronic carbon dioxide retention are predisposed to being more reliant on their hypoxic drive, the dependence is inconsistent. Patients should be closely monitored, and the technologist should be mindful of potential ventilatory drive changes.

Hypoxic drive is a term that is overused, poorly understood, and often misdiagnosed. Hypoxic drive is certainly not a valid argument for withholding oxygen administration when treating hypoxemia.