Hypoxia

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  • Hypoxemia is defined as a decrease in arterial PO2
  • Hypoxia is defined as a decrease in O2 delivery to, or utilization by, the tissues.
  • Hypoxemia is one cause of tissue hypoxia

Hypoxemia

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  • A – a gradient, or A – a difference
    • useful tool for comparing causes of hypoxemia
    • A – a gradient is the difference between the PO2 of alveolar gas (PAO 2 ) and the PO2 of systemic arterial blood (PaO2).

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  • A a gradient describes whether there has been equilibration of O2 between alveolar gas and pulmonary capillary blood (which becomes systemic arterial blood).
  • Normally, O2 equilibrates across the alveolar-pulmonary capillary barrier
  • Normally ,A a gradient is close to zero.
  • In some, but not all causes of hypoxemia, the A- a gradient is increased, or widened, signifying a defect in O2 equilibration.(MCQ)
  • High altitude
    • causes hypoxemia because barometric pressure (PB) is decreased, which decreases the PO2 of inspired air (PIO2) and of alveolar air (PAO2)
    • Equilibration of O2 across the alveolar/pulmonary capillary barrier is normal, and systemic arterial blood achieves the same (lower) PO2 as alveolar air.
    • Because PAO2 and PaO2 are nearly equal, the A a gradient is normal.(MCQ)
    • At high altitude, breathing supplemental O2raises arterial PO2 by raising inspired and alveolar PO2.
  • Hypoventilation
    • causes hypoxemia by decreasing alveolar PO2 (less fresh inspired air is brought into alveoli).
    • Equilibration of O2 is normal, and systemic arterial blood achieves the same (lower) PO2 as alveolar air.
    • PAO2 and PaO2 are nearly equal
    • A- aO2 gradient is normal.(MCQ)
    • In hypoventilation, breathing supplemental O2raises arterial PO2 by raising the alveolar PO2.(MCQ)
  • Diffusion defects (e.g., fibrosis, pulmonary edema)
    • cause hypoxemia by
      • increasing diffusion distance or
      • decreasing surface area for diffusion.
    • Equilibration of O2 is impaired
    • PaO2 is less than PAO2,
    • A a gradient is increased, or widened.(MCQ)
    • With diffusion defects, breathing supplemental O2raises arterial PO2 by raising alveolar PO2 and increasing the driving force for O2 diffusion.(MCQ)
  • V_/Q_ defects
    • always cause hypoxemia with increased A a gradient.(MCQ)
    • In V_/Q_ defects, supplemental O2 can be helpful, primarily because it raises the PO2 of low V_/Q_ regions where blood flow is highest.(MCQ)
  • Right-to-left shunts (right-to-left cardiac shunts, intrapulmonary shunts)
    • always cause hypoxemia and increased A- a gradient(MCQ)
    • Shunted blood completely bypasses ventilated alveoli and cannot be oxygenated. Because shunted blood mixes with, and dilutes, normally oxygenated blood (nonshunted blood), the PO2 of blood leaving the lungs must be lower than normal.
    • Why supplemental O2 has a limited effect on the PO2 of systemic arterial blood ?(MCQ)
      • becausesupplemental O2 can only raise the PO2 of normal nonshunted blood; the shunted blood continues to have a dilutional effect.
  • HYPOXIA
    • Hypoxia is decreased O2 delivery to the tissues
    • O2 delivery is the product of cardiac output and O2 content of blood
    • hypoxia is caused by
      • decreased cardiac output (blood flow)
      • decreased O2 content of blood.
    • O2 content of blood is determined primarily by the amount of O2-hemoglobin.

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  • Carbon monoxide (CO)poisoning
    • causes hypoxia because CO occupies binding sites on hemoglobin that normally are occupied by O2(MCQ)
    • CO decreases the O2 content of blood.(MCQ)
  • Cyanide poisoning
    • interfereswith O2 utilization of tissue
    • it is one cause of hypoxia that does not involve decreased blood flow or decreased O2 content of blood.(MCQ)


Hypoxia and Hypoxemia: Mechanisms and Etiologies (ABG Interpretation – Lesson 18)
A review of the mechanisms and etiologies of hypoxemia, focusing on impaired diffusion, ventilation/perfusion mismatch, and shunting.
Hypoxia 14, Physiological response to hypoxia