KEY POINTS

1. The causes of hypoxemia are low PIO₂ , elevated PACO₂ , ventilation-perfusion (V̇A/) mismatching, right-to-left shunt, and diffusion nonequilibrium.
2. The clinical approximation to the alveolar gas equation for O₂ is given by PAO₂ (P_barometric - 47) × FIO₂ - 1.2 × PCO₂ .
3. The A-a gradient = PAO₂ - PaO₂ = 0.21 × (age in years + 2.5); this gradient increases when supplemental O₂ is administered. The a/A ratio = PaO₂ /PAO₂ (normal value of 0.8 to 0.85); this ratio does not significantly change when supplemental O₂ is given.
4. To assess the adequacy of O₂ exchange, use the a/A ratio or PaO₂ /FIO₂ ratio (i.e., P/F ratio). An a/A ratio that is less than 0.8 or a P/F ratio that is less than 350 mm Hg implies abnormal gas exchange.
5. To assess the adequacy of CO₂ exchange, multiply minute ventilation (V̇E) by the arterial PCO₂ . Normal V̇E × PCO₂ is typically about 200 L/min/mm Hg during spontaneous breathing and 300 to 400 L/min/mm Hg during mechanical ventilation.
6. When PO₂ values are inexplicably low in the presence of high leukocyte or platelet counts, the clinician should consider an artifact caused by O₂ consumption by cells within the sample. Inhibition of cellular O₂ consumption may be accomplished by adding sodium fluoride to the sample.
7. In the presence of high carboxyhemoglobin levels, each 8% increase in carboxyhemoglobin causes only a 1% decrease in the SpO₂ value. With a high methemoglobin concentration, the measured SpO₂ approaches 85%, independent of the actual arterial oxygenation.
8. When the pulse oximeter reading is difficult to obtain because of hypothermic vasoconstriction, the anesthesiologist should consider using a digital nerve block or topical application of a local anesthetic cream (e.g., EMLA). When the patient is cold, rapid changes in SpO₂ are more quickly detected using an ear or forehead probe than a finger probe.
9. A sudden drop in PETCO₂ most likely results from a decrease in cardiac output, regional hypoperfusion of the lung due to pulmonary embolism, or an airway problem. A rise in the PETCO₂ value can occur only because of increased CO₂ production (e.g., fever, seizure, bicarbonate-hydrogen ion buffering) or hypoventilation.
10. When arterial blood is fully saturated, the shunt fraction can be approximated from the arterial and mixed venous saturation according to the following equation: (S/T) ≅ [(1 - SaO₂ )/1 - Sv̄O₂ )].
11. Although narcotic administration usually slows the respiratory rate, severe respiratory depression can occur in the face of a normal respiratory rate. However, narcotic overdosage is always associated with somnolence.