Applied Physiology Response to Hypoxia

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Author:
frisbee
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79574
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Applied Physiology Response to Hypoxia
Updated:
2011-04-14 04:04:50
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Hypoxia Response
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The Integrated Response to Hypoxia
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  1. Why study Hypoxia?
    • Vertical integrative physiology.
    • Aviation and high altitude medicine.
    • Many conditions/diseases have hypoxic or ischemic components.
  2. Common abbreviations
    A -
    a -
    v -
    P or p -
    C -
    F -
    I or i -
    E or e -
    Q -
    • A - alveolar
    • a - arterial
    • v - venous
    • P or p - partial pressure
    • C - content
    • F - fraction
    • I or i - inspired
    • E or e - expired
    • Q - Cardiac output
  3. Meaning
    FiO2
    CaO2
    pO2
    pAO2
    • FiO2 - fraction of inspired air that is oxygen
    • CaO2 - arterial oxygen content
    • pO2 - partial pressure of oxygen
    • pAO2 - partial pressure of oxygen in the alveoli
  4. A Drop in barometric pressure results in...
    • -Drop in PiO2
    • -The reduced pO2 is transferred all the way down the oxygen cascade.
  5. What is the equation for the systemic oxygen delivery?
    DO2 = Q x CaO2

    Systemic Delivery of oxygen = cardiac output x oxygen content of arterial blood.

    When PiO2 falls, CaO2 also falls, reducing systemic oxygen delivery.
  6. Q = ?
    CaO2 = ?
    Q = HR x SV

    CaO2 = ([Hb] x SaO2 xH) + (PaO2 x S)
  7. CaO2 = ([Hb] x SaO2 xH) + (PaO2 x S)

    What does
    H = ?
    S = ?
    SaO2 = ?
    • H = Hufner's constant = 1.39 mL/g
    • S = Solubility coefficient
    • SaO2 = Solubility coefficient of O = 0.0225 mL/mm Hg
  8. What is the first line of defence against on arrival at high altitudes?
    Increase Cardiac Output (Q) via increasing Heart Rate and increasing Stroke Volume

    • (DO2 = Q x CaO2).
    • (Q = HR x SV)
  9. What is the next phase after first line of defence?
    • Restoration of CaO2.
  10. Restoration of CaO2
    What are the two mechanisms?
    • Ventilatory Adjustments
    • Haematological adjustments
  11. What are the two ventilatory compensation mechanisms?
    • The Hypoxic ventilatory response
    • The Hypercapnic ventilatory response
  12. What is the alveolar ventilation equation?
    PACO2 = k x (VCO2 / VA)
  13. What is the relationship between alveolar PCO2 and ventilatory rate?
    Alveolar PCO2 is inversely proportional to ventilatory rate. I.e. if ventilation doubles the PCO2 is halved.
  14. What is the Alveolar Gas Equation?
    PAO2 = PiO2 - (PACO2 / R)

    R = indication of the substrates utilised i.e. fatty acids, CHO. Equals about .71 for fats and about 1.0 for CHO. R is a essentially a ratio of amount of CO2 produced to O2.
  15. Why does Hyperventilation work?
    The alveolar ventilation and gas equations explain why hyperventilation works as an adaptation to high altitude. If you double ventilation rate the change in PCO2 and PAO2 is halved.
  16. If PiO2 drops from 150 mm Hg to 100 mm Hg. Using the alveolar gas equation what does PAO2 equal?
    Eq. PAO2 = PiO2 - (PACO2 / R)
    • Sea Level PAO2 = 150 - (40/0.8) = 100 mm Hg
    • Pikes Peak PAO2 = 150 - (40/0.8) = 50 mm Hg

    So PAO2 (and thus PaO2) has HALVED.
  17. Why does Hyperventilation work? (continued)
    If ventilation doubles PCO2 halves.
    Redo the calculations to get PAO2
    Sea Level PAO2 = 150 - (40/0.8) = 100 mm Hg Pikes Peak PAO2 = 150 - (40/0.8) = 75 mm Hg

    Therefore PAO2 is only reduced by 25% with hyperventilation.
  18. What drives the two different ventilatory responses at altitude?
    • The Hypoxic ventilatory response (to hyperventilate) is driven by low pO2.
    • The Hypercapnic ventilatory response (also to hyperventilate) is initially driven by high pCO2.
  19. What are the two different ventilatory responses detected by?
    • Hypoxic - sensors in the carotid and aortic bodies.
    • Hypercapnic - sensed by central chemoreceptors in the medulla.
  20. What happens when pCO2 falls below normal levels.
    • Normally at sea level this hypocapnia would cause breathing to slow.
    • However at altitude the need to correct arterial pO2 overwhelms the hypocapnic ventilatory response.
    • The Resulting condition is HYPERVENTILATION and HYPOCAPNIA. ( Respiratory alkalosis).
  21. Acid- Base effects
    What is the Henderson-Hasselbalch equation
    pH = pK + log{ [HCO3-] / s x PCO2)

    Decrease CO2 results in an increase in pH.
  22. Hyperventilation and Hypocapnia result in respiratory alkalaemia.
    How is it partially corrected?
    • By the kidneys - they excrete Bicarbonate.
    • Partially compensating for the respiratory alkalosis.
  23. Restoration of CaO2 - Haematological mechanisms

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