Hypoxia occurs when tissues do not receive adequate oxygen to meet their metabolic demands. It is nonspecific and can mean reduced arterial partial pressure of oxygen (PaO2; hypoxemic hypoxia), reduced hemoglobin for oxygen transportation (anemic hypoxia), decreased cardiac output or regional decrease in perfusion (stagnant hypoxia), disarray in cellular metabolism of oxygen such as in cyanide (histotoxic hypoxia), or mitochondrial dysfunction in sepsis (cytopathic dysoxia).1
Type 1 respiratory failure is defined as PaO2 less than 60 mmHg with an equivalent arterial oxygen saturation (SaO2) of 90% and normal or near or low arterial partial pressure of carbon dioxide (PaCO2). Type 2 respiratory failure involves hypercapnia and is defined as a PaCO2 of 46 mmHg or higher, regardless of oxygen saturation. Type 1 respiratory failure will be discussed in this chapter.
Two percent of oxygen is dissolved in plasma (PaO2) and 98% is bounded to hemoglobin (SaO2). One gram of hemoglobin (Hgb) carries 1.34 mL O2 per gram. Oxygen delivery is based on the formula DO2 = CaO2 × CO, where DO2 is the diffusing capacity of oxygen, CaO2 is the arterial concentration of oxygen, and CO is cardiac output. Oxygen content is derived from the formula CO2 = (1.39 × Hgb × SaO2/100) + (0.003 × PO2), where CO2 is the concentration of oxygen, Hgb is the hemoglobin, SaO2 is fraction of oxygen saturated hemoglobin over total hemoglobin, and PO2 is the partial pressure of oxygen. Oxygen consumption is derived from the formula V.O2 = CO × (CaO2 – CvO2), where CvO2 is the venous concentration of oxygen. The alveolar–arterial (A-a) oxygen gradient is PAO2 – PaO2, where PAO2 = (FiO2 × [PATM – PH2O]) – (PaCO2/R). In these formulas, PAO2 is the alveolar oxygen tension; PaO2 is the arterial pressure of oxygen; FiO2 is the fraction of inspired oxygen, which is 0.21 on room air; PATM is atmospheric pressure, which is 760 mmHg at sea level, PH2O is the partial pressure of water, which is 47 mmHg at 37°C; PaCO2 is arterial carbon dioxide tension; and R is 0.8 at steady state. The normal A-a gradient varies with age and is based on the formula 2.5 + 0.21 × age in years.
A common, noninvasive way to measure oxygen bounded to hemoglobin is oxygen saturation ...