Chapter 28: Hypoxemia

BACKGROUND

Hypoxia and hypoxemia are common reasons for hospital admission, usually in the context of a primary respiratory illness¹. An understanding of the evaluation, diagnosis, and monitoring of patients with hypoxemia can improve patient care and resource utilization.

PATHOPHYSIOLOGY

The Oxygen Cascade

The process of delivering oxygen (O₂) to tissues is complex, but knowledge of this process facilitates an understanding of the clinical aspects of hypoxemia and hypoxia. Providing adequate tissue oxygenation involves the pulmonary, cardiovascular, and hematology systems and is hampered by the relatively slow tissue diffusion of O₂. O₂ always moves down its partial pressure gradient from dry atmospheric air (partial pressure of O₂ [PO₂] 160 mmHg at sea level) to the mitochondria in tissues (PO₂ 3–20 mmHg). The gaseous waste product of aerobic metabolism, carbon dioxide (CO₂), is assisted in its transport to atmospheric air like O₂, but unlike O₂, diffuses readily across tissues.

Alveolar Oxygen

The fraction of inspired O₂ (Fio₂) in atmospheric air is 21%, but the partial pressure of inspired O₂ (P_IO2) is also dependent on barometric pressure (P_B), primarily as a function of altitude. O₂ entering the alveolus is “diluted” by humidification (PH₂O = 47 mmHg), and CO₂ in the alveolus. Arterial partial pressure of CO₂ (PaCO₂) approximately equals alveolar partial pressure of CO₂ (PAco₂) due to ready diffusion across capillary and alveolar membranes. PACO₂ is a function of energy metabolism (production of CO₂) and minute ventilatory volume (elimination of CO₂). CO₂ production reflects the respiratory quotient (RQ); RQ of a balanced diet is ~0.83. CO₂ elimination is reflected by the PACO₂. CO₂ is also involved in acid–base balance, which can impact its rate of elimination.

The simplified alveolar gas equation calculates an estimate of alveolar partial pressure of O₂ (PA_O2): PA_O2 ~ P_IO2 – (Pa_CO2/RQ). At sea level in atmospheric air, P_B is 760 mmHg. The resulting Pio₂ is Pio₂ = (F_IO2 *(P_B–PH₂O), or (0.21*(760 – 47)) = 149 mmHg. In the alveolus, CO₂ is present in proportion to the RQ and minute ventilation, and the P_IO2 is diminished by the factor Pa_CO2/RQ. With a PA_CO2 = paco₂ of 40 mmHg and an R of 0.83, the ideal PA_O2 at sea level is 149 mmHg – (40/0.83) = 101 mmHg. In contrast, in Salt Lake City, Utah, 1280 m above sea level, the P_B is 647 mmHG, and the ideal PA_O2 is 77 mmHG. Hyperventilation, environment, diet, and disease can influence ...