Hypoxemia

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  1. Oxygen carrying capacity of blood = 1.34 x Hgb x saturation + .03 x PaO2 (0.03 x PaO2 reflects the oxygen carrying capacity of crystalloid)
  2. 100cc of oxygenated blood carries 20cc of oxygen
  3. at around 80% saturation or PaO2 of 50 mm Hg, the oxyhemoglobin dissociation curve precipitously falls
    1. factors that shift the oxyhemoglobin dissociation curve to the right (making oxygen less tightly bound to hemoglobin) are acidosis, increased temperature, increased PaCO2, increased hemoglobin concentration, and increased 2,3DPG
    2. factors that shift the oxyhemoglobin dissociation curve to the left making oxygen more tightly bound to hemoglobin include alkalosis, decreased temperature, decreased PaCO2, decreased hemoglobin concentration, and decreased 2,3 DPG
  4. pathophysiologic causes of arterial hypoxemia include VQ mismatch or venous admixture from regional alveolar hypoventilation, decreased mixed venous oxygen content (due to decreased cardiac output)
  5. a physiologic shunt causing hypoxia as occurs in ARDS does not respond to increases in FIO2 due to low saturation of the admixing venous blood; neither does shunting produce hypercapnia since small increases in PaCO2 affect chemoreceptor induced increases in ventilation
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