Viva G4(ii)g

This viva tests Section G4(ii) of the 2017 CICM Primary Syllabus, which expects the exam candidate to "describe the distribution of blood volume and flow in the various regional circulations and explain the factors that influence them, including autoregulation. These include, but not limited to, the cerebral and spinal cord, hepatic and splanchnic, coronary, renal and uteroplacental circulations."

Specifically, this viva is all about regional skeletal muscle blood flow.

Describe the blood supply of the skeletal muscle.

• Resting skeletal muscle receives minimal blood flow:  1-4 ml/min/100g, or about 14-15% of the total cardiac output (700-750ml)
• Skeletal muscle blood flow scales massively to meet demand, more than any other regional circulatory system. 
• During exercise,  90% of the cardiac output can be directed to the skeletal muscle
• During states of shock, blood flow to muscle can decrease by 90% from the resting baseline.
• Thus, the regional circulation of skeletal muscle can alter its blood flow over three factors of magnitude, from 0.1 to 100 times the normal value. 

How is skeletal muscle blood flow regulated?

• Intrinsic vascular mechanisms
  • Myogenic control, an intrinsic property of all vascular smooth muscle
  • Vessel wall stretch produces a calcium-mediated reflex vasoconstriction
• Vasoactive substrates and products of muscle metabolism
  • Muscle hypoxia produces vasodilation
  • Metabolic byproducts (CO₂, lactate, hydrogen peroxide and potassium ions) act as vasodilators
  • Regional decreases in pH produce vasodilation independently of CO₂ and lactate
• Vasoactive mediators released by the endothelium can alter skeletal muscle blood flow, though they do not seem to be essential for exercise-induced hyperemia:
  • Nitric oxide (NO)
  • Adenosite triphosphate (ATP)
  • Adenosine
  • Prostaglandins
  • Endothelium-derived hyperpolarization factors (EDHFs)
• Regulation by the autonomic nervous system
  • α-1 receptor activation leads to skeletal muscle vasoconstriction
    • Sympathetic innervation vasconstricts skeletal muscle arterioles to maintain a high resting vessel tone for inactive muscle
    • In haemorrhagic shock, α-1 receptor activation helps redistribute blood flow away from muscle
  • β-2 receptor activation leads to skeletal muscle vasodilation
    • Systemic adrenaline release increases muscle blood flow for "fight or flight" responses, in addition to increasing cardiac output, increasing the capacity for muscle activity
  • There is a resting sympathetic vasoconstrictor tone which normally maintains a low blood flow to resting muscle