• A-a gradient = PAO2 – PaO2: normal (on room air) = “4 + age/4” or “2.5 + (0.2 × age)” hypoxemia + normal A-a gradient → problem is excess PaCO2 (ie, hypoventilation)
• V/Q mismatch and shunt represent spectrum w/ both coexisting in alveolar disease
100% O2 can overcome V/Q mismatch but not large shunt b/c sigmoidal Hg-O2 curve
Figure 2-6 Workup of acute hypoxemia
• Cyanosis: seen when >4 g/dL of reduced Hb in blood vessels of skin/mucous membranes central: ↓ SaO2 (pulm disease, shunt); abnl Hb [metHb, sulfHb, COHb (not true cyanosis)] peripheral: ↓ blood flow → ↑ O2 extraction (eg, ↓ CO, cold, arterial or venous obstruction)
CO binds to Hb more avidly than does O2. Pulse oximeter (Ox) misreads COHb as HbO2 → falsely nl sat. Oxidizing drugs Δ Hb (ferrous) to MetHb (ferric), which cannot carry O2. Pulse ox misreads MetHb as HbO2. Cyanide inhibits mitochondrial O2 use → cellular hypoxia but pink skin and ↑ venous O2 sat.