Key Points

  1. The oxygen cascade describes the process of declining oxygen tension from atmosphere to mitochondria.

  2. The amount of oxygen in the blood is calculated using the formula: [1.34 x Hb x (SaO2/100)] + 0.003 x PO2 = 20.8ml

  3. The delivery of oxygen to the tissues per minute is calculated from: DO2 = [1.39 x Hb x SaO2 + (0.003 x PaO2)] x Q

  4. Tissue  oxygen extraction is calculated by subtracting mixed venous oxygen content from arterial oxygen content.

  5. The Oxyhemoglobin dissociation curve describes the non-linear tendency for oxygen to bind to hemoglobin: below a SaO2 of 90%, small differences in hemoglobin saturation reflect large changes in PaO2 Right to left shunting causes hypoxemia resistant to oxygen therapy.

  6. Hypoxic Pulmonary Vasoconstriction is a physiologic protective mechanism which prevents right to left shunting of blood.

  7. Right to left shunt causes hypoxemia unresponsive to oxygen therapy Diffusion defects and ventilation perfusion mismatches cause hypoxemia, responsive to exogenous oxygen and positive pressure ventilation. Absorption atelectasis refers to the tendency for airways to collapse if proximally obstructed, gases are reabsorbed, this process is accelerated by nitrogen washout techniques.

  8. The objective of oxygen therapy is to give the patient as much oxygen as is required to return the PaO2 to what is normal for the particular patient.

  9. Oxygen is given thru fixed and variable performance devices.

  10. Fixed performance devices deliver a flow of oxygen equal to or in excess of peak inspiratory flow

  11. Variable performance devices use the deadspace of the nasopharynx or face masks as a reservoir of oxygen. They cannot deliver high inspired concentrations of oxygen.

  12. Hyperbaric oxygen therapy is used to increase the amount of oxygen dissolved in the plasma, by increasing the ambient pressure Carbon monoxide causes tissue ischemia by avidly binding to hemoglobin and displacing oxygen. The treatment is 100% oxygen, and possibly HBO therapy.

  13. High inspired oxygen concentrations cause toxicity by causing formation of oxygen free radicals (which damage tissues), and by causing absorption atelectasis and V/Q mismatch.

  14. Pulse oximeters measure the absorption of red and infrared light by pulsatile blood. They are inexpensive, continuous and portable. Accuracy declines below a SpO2 of 90%.


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