patients with chronic renal failure receiving hemodialysis experience hypoxemia during routine dialysis. First reported, to our knowledge, by Johnson et al1 in 1970, the fall in Pao2 is typically 10% to 20% of baseline. This decrease is usually well tolerated,2 but may be of critical importance in patients with limited pulmonary or cardiac reserve. We will review clinical evidence for pathophysiologic alterations—pulmonary and metabolic—that contribute to production of this hypoxemia.
PHYSIOLOGIC APPROACH TO HYPOXEMIA
Mechanisms of Arterial Hypoxemia
Hypoxemia may result from five causes listed in the Table. Data needed to differentiate between these are shown. Categorization of hypoxemia physiologically aids in determining its cause, and provides a rational basis for choosing therapy.
Alveolar Gas Equation and Arterial-Alveolar Po2 Difference
In the Table, calculation of the arterial-alveolar PO2 difference (aADO2) provides the basis for distinguishing reduced fraction of oxygen in inspired air (FiO2) and hypoventilation