Outline the physiological detection and response to an acute decrease in PaO2

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College Answer

Candidates were expected to focus on the physiology (including mechanisms) involved with 
detection of hypoxia (e.g. peripheral chemoreceptor stimulation) as well as response. 
Examples of response included sympathetic stimulation, the respiratory centre and organ 
specific (e.g. cardiac, CNS, blood, cellular, etc) responses. This topic is well covered within a 
number of fundamental texts, in particular, Nunn’s Respiratory Physiology. In general 
candidates’ answers were not “broad” enough, lacked detail and were too focused on 
chemoreceptors only. 

Discussion

Detection of hypoxaemia

  • Sensors: peripheral chemoreceptors at the carotid glomus and in the aortic arch
  • Afferents: vagus nerve and glossopharyngeal nerve
  • Controller: central medullary and pontine respiratory control centres
  • Efferents: phrenic nerve and spinal innervation of respiratory skeletal muscles
  • Effectors: diaphragm, intercostal muscles, scalenes and abdominal muscles 

Response to hypoxaemia

  • Ventilatory response
    • Decreasing PaO2 causes an increase in minute ventilation.
    • The relationship between oxygen tension and minute volume can be described as a hyperbolic curve
    • The inflexion point for this relationship is approximately a PaO2 of 50-60 mmHg; beyond this threshold value the minute volume increases steeply
  • Cardiovascular effects of hypoxaemia 
    • Hypoxic vasoconstriction in the pulmonary circulation
    • Hypoxic vasodilatation in the systemic circulation
  • Autonomic effects of hypoxaemia
    • Increased sympathetic and vagal tone, but the sympathetic increase dominates
    • Increased cardiac output and tachycardia are the net result
    • Hypoxic systemic vasodilation prevents hypertension
  • Cellular effects of hypoxaemia
    • Absence of oxygen facilitates the switch to anaerobic metabolism of glucose
  • Hypoxia-inducible factors
    • These are transcriptional activators which meidate adaptive responses to low oxygen availability (Semenza, 2012)
    • Their effects include:
      • Increased haemopoiesis
      • Decreased cell differentiation
      • Accelerated angiogenesis

References

References

Semenza, Gregg L. "Hypoxia-inducible factors in physiology and medicine." Cell 148.3 (2012): 399-408.

Ward, D. S., S. B. Karan, and J. J. Pandit. "Hypoxia: developments in basic science, physiology and clinical studies." Anaesthesia 66 (2011): 19-268.

Fukuda, Yasuichiro, et al. "Autonomic nerve and cardiovascular responses to changing blood oxygen and carbon dioxide levels in the rat." Journal of the autonomic nervous system 28.1 (1989): 61-74.