Applied Respiratory Physiology — Hypoxaemia Response

Hypoxaemia

Hypoxaemia = Acute ↓ PaO₂ = (low partial pressure of oxygen in arterial blood)

Overall response: Hypoxaemia → ↑ peripheral vasosmotor tone, ↑rate and depth of respiration

Local response to ↓ PaO₂:

• ↑ local vasodilation → ↓ SVR and ↓ BP
• ↓ BP → Baroreceptors → SNS activation → ↑ HR and ↑ C.O.
• PaO2 < 50-60 mmHg
  • → peripheral chemoreceptors stimulation (see below)
  • → ↑ventilation
    • If ↓ PaO₂ is Gradual ↑ MV PaO2 < 500mmHg,
    • If ↓ PaO₂ is Rapid ↑MV PaO2 < 50 mmHg
• Severe hypoxaemia
  • ↓ HR and BP → shock
  • ↑ CBF (regardless of PaCO2)

Chemoreceptor (CR) Reflex:

Two types of CR:

• Central CR
  • CR location: Below the ventral surface of the medulla
  • Stimuli: Δ ECF [H+] whereby ↑ ECF [H+] stimulates ↑ ventilation
  • Rapid Response:
    • CO2 rapidly diffuses into CSF
    • Low CSF protein and HCO3 (c.f. plasma)
    • Poor buffering capacity
    • ΔCSF pH per ΔPaCO2 is GREATER than that of blood
• Peripheral CR
  • CR location:
    • “Carotid bodies” (more important)
      • at bifurcation point of common carotid arteries
      • supplied by CN IX
    • “Aortic bodies” (less important)
      • above and below the aortic arch
      • supplied by CN X
  • Composition:
    • PCRs found in “Glomus cells”
      • ↓ PaO2 – via inhibition of O2 sensitive K-channels
      • ↑ PaCO2 and/or ↓pH – via effect on pH sensitive K-channels
      • Type I cells (rich in NAd, DA, ACh)
        • Hypoxia causes release of NTs
          • NAd/ACh – ↑ AP firing rate of AB or CB afferent fibres
          • DA – Damping of type 2 cell responses
      • Type II cells (rich in capillary supply)
        • ↓ PaO2, ↑ PaCO2 and/or ↓pH
        • ↓ IC [ATP] which leads to ↑ NT production and release
        • ↑ AP firing rate of AB or CB afferent fibres
  • Response:
    • ↓’d perfusion → ↑[H]/↓PaO2 → CR stimulation
    • Afferent signal → Sinus nerve (of herring) / Vagus Nerve → Chemosensitive area of medulla
    • Vasomotor area → ↑ peripheral vasoconstriction
    • Respiratroy center → ↑rate and depth of respiration → ↑ venous return

Chronic Response

• Adrenal medullary hypoxia → EPO
  • ↑ erythropoiesis (chronic)

Gladwin 2016

Answer by Gladwin