Hypoxemia Alveolar gas equation predicts the change in PAO2 that will occur for a given change in PACO2 PAO2 = alveolar PO2 PIO2 = PO2 in inspired air normally, PIO2 = 150 mm Hg PACO2 = alveolar PCO2 R = respiratory exchange ratio = (CO2 production) / (O2 consumption) normally, R = 0.80 A-a Gradient alveolar PO2 (PAO2) - systemic arterial PO2 (PaO2) normal A-a gradient = 15 mm Hg (~Age/4 +4) equilibration of O2 across alveolar gas and pulmonary capillary blood pulmonary capillary blood becomes systemic arterial blood increases with age hypoxemia → ↑ A-a gradient e.g., shunt, V/Q mismatch, pulmonary fibrosis (diffusion defect) Causes of Hypoxemia (↓ PaO2) High Altitude (normal A-a gradient) a decrease in barometric pressure (PB) decreases the PIO2 of inspired air FiO2 doesn't change (21%) ↓ PB → ↓ PIO2 → ↓ PAO2 → ↓ PaO2 Hypoventilation (normal A-a gradient) a decrease in minute ventilation (respiratory rate x tidal volume) decreases PAO2 ↓ ventilation → ↑ PACO2 → ↓ PAO2 FiO2 and PB don't change in alveolar gas equation V/Q Mismatch (↑ A-a gradient) Diffusion Limitation (↑ A-a gradient) e.g., pulmonary fibrosis ↑ diffusion distance for diffusion e.g., pulmonary edema ↓ surface area for diffusion, ↑ diffusion distance Right-to-Left Shunt (↑ A-a gradient) low O2 shunted blood mixes with and dilutes high O2 non-shunted blood Hypoxia Decreased Oxygen Delivery to Tissues O2 content of blood = (O2 binding capacity) x (% saturation) + dissolved O2 (O2 binding capacity) x (% saturation) = O2 bound to hemoglobin O2 delivery = (cardiac output) x (O2 content of blood) Causes of Hypoxia (↓ O2 Delivery to Tissues) circulatory (↓ Cardiac Output) ↓ blood flow hypoxemic ↓ PaO2 → ↓ % saturation → ↓ O2 content of blood anemic (see CO poisoning below) ↓ concentration of hemoglobin→ ↓ O2 content of blood histotoxic (example: cyanide poisoning) ↓ O2 utilization by tissues (mitochondrial poison) CO Poisoning ↓ O2 bound to hemoglobin → ↓ O2 content of blood, ↓ O2 delivery to tissues CO binds to hemoglobin with 200x greater affinity than O2 oxygen-hemoglobin dissociation curve left shift early sign of exposure is headache; significant exposure causes coma, death classical presentation at autopsy is cherry-red appearance of skin not common in living patients Ischemia Causes of Ischemia (↓ Blood Flow) ↓ arterial blood flow e.g., atherosclerosis ↓ venous return e.g., Budd-Chiari syndrome