Oxygen Therapy and Basic Respiratory Care

Oxygen in air exists in a diatomic molecular form (O2, molecular weight [MW] 16 g/mol) that, at standard temperature and pressure, is a colorless, odorless gas. The diatomic molecular state is the form that we administer to our patients as a respiratory gas, either pure oxygen or in mixtures with air or helium (heliox), and is that form most commonly referred to as oxygen. While the gaseous state of oxygen is most clinically relevant, it can be found in a liquid and solid state as well under appropriate conditions. However, molecular oxygen has additional chemical states that expand its impact on human physiology. The diatomic form (“oxygen”) is essential for aerobic metabolism in animals but is toxic to obligate anaerobic organisms (a fact exploited in hyperbaric oxygen therapy). The triatomic form, ozone, is produced continuously by ultraviolet (UV) radiation in the upper layers of the atmosphere, and serves to shield the earth’s surface from UV radiation. However, ozone is also produced by the immune system as an antimicrobial defense. Finally, singlet oxygen (several different forms exist) is a high-energy state of molecular oxygen in which all the electron spins are paired, resulting in extreme instability and exceptional reactivity toward common organic molecules. Such oxygen-free radicals, as they are collectively termed, have varied and increasingly important physiologic roles; therefore, their role in both normal and pathologic processes must be recognized and understood to allow the correct application of oxygen therapy. In this chapter, we will discuss the various modalities for oxygen delivery in the critical care setting. In addition, we will address oxygen’s diverse physiologic and possible pathologic effects, but in our discussion of oxygen-free radicals, we will confine it to their role in oxidative lung injury, while being cognizant of their potential impact on all tissues.