Mechanism 1 Myocardial Diffusion

• Mechanism 1 pt1:

  • The myocardium is extremely efficient at extracting O2 from the coronary blood supply
  • This efficiency can be demonstrated by examining the resting a-vO2 difference of coronary blood and comparing it to the a-vO2 difference of skeletal muscle blood
  • a-vO2 difference is the difference in O2 content in arterial compared to venous blood
  • When the arterial O2 content supplying a tissue is recorded directly and compared to the O2 content of venous blood leaving the tissue, the difference corresponds entirely to the amount of O2 extracted and utilised by that tissue
• Mechanism 1 pt2:

  • The a-vO2 difference in blood from skeletal muscle at rest is 25% i.e. only 25% of the O2 content of arterial blood passing through skeletal muscle tissues is extracted and used at rest
  • Venous blood is therefore still very oxygenated (~ 75%)
  • However, in the coronary arteries the a-vO2 difference is 65 – 70% at rest, this means that at rest the myocardium extracts and uses 65 - 70% of the O2 on offer in the coronary arteries
• Mechanism 1 pt3:

  • This wide a-vO2 difference demonstrates how extremely efficient the heart muscle is at extracting and utilising O2
  • So even at rest coronary venous blood is almost desaturated – and there is only a little O2 reserve that the myocardium could “tap into” if require
  • Therefore on exercise the myocardium must be supplied with more O2 or it will become ischaemic, potentially causing electrical and contractile dysfunction and possibly causing cardiac arrest or infarction. So here we see the second factor coming into play