Discuss the physiological causes of early post operative hypoxaemia.

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College Answer

For a good mark it was expected that candidates define hypoxaemia and, in a structured manner,
discuss the physiological causes of early postoperative hypoxaemia. Candidates should always try
and begin their answer with a definition of the term that is to be discussed. Many candidates
invoked clinical disease related causes and not physiological. This did not score marks. Similarly,
factors leading to tissue hypoxia, such as anaemia or low 2,3-dpg were not given marks.
Candidates may have done so because they confused “hypoxaemia” with “hypoxia”.
Better answers addressed different physiological causes of hypoxaemia, including areas of low
V/Q; hypoventilation; increased oxygen consumption; loss of hypoxic pulmonary vasoconstriction
and closing capacity exceeding functional residual capacity.
Syllabus: B1a2b, B1e,f,g
References: Nunn’s respiratory physiology – various sections

Discussion

Definitions of hypoxaemia and hypoxia:

  • Hypoxaemia is an abnormally low concentration of oxygen in arterial blood
  • Hypoxia is the failure of tissue oxygenation

Causes of post-operative hypoxaemia:

  • Decreased alveolar oxygen
    • Alveolar hypoventilation, due to
      • Opiate analgesia
      • Residual anaesthetic agent
      • Incomplete reversal of NMJ blocker
    • Decreased inspired oxygen content (eg. high altitude) - but then, why are you operating up there?...
  • Decreased perfusion of gas exchange surfaces
    • V/Q mismatch
      • atelectasis due to decreased FRC (supine position, anaesthesia)
      • atelectasis due to increased closing capacity
      • atelectasis due to the use of supplemental oxygen ("absorption atelectasis")
    • Increased dead space (due to positive pressure ventilation)
  • Decreased mixed venous oxygen content 
    • Increased O2 consumption, due to:
      • an increased metabolic rate, eg. malignant hyperthermia
    • Decreased cardiac output, i.e. increased oxygen extraction ratio, due to the cardiotoxic effects of anaesthetic drugs

References

Jones, J. G., D. J. Sapsford, and R. G. Wheatley. "Postoperative hypoxaemia: mechanisms and time course." Anaesthesia 45.7 (1990): 566-573.

Howard, R.P. "Rapid response to: Oxygen transport—1. Basic principles" BMJ 1998; 317 doi: https://doi.org/10.1136/bmj.317.7168.1302 (Published 07 November 1998) 

Treacher, D. F., and R. M. Leach. "Oxygen transport—1. Basic principles." Bmj 317.7168 (1998): 1302-1306.

Tipping, Robert, Robin Berry, and Ian Nesbitt. "Mechanisms of hypoxaemia and the interpretation of arterial blood gases." Surgery (Oxford) 30.10 (2012): 505-511.