Viva G4(ii)f

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 splanchnic blood flow.

Describe the blood supply of the abdominal viscera.

The major vessels and their branches are:

• Coeliac trunk
  • Left gastric artery
  • Common hepatic artery
  • Splenic artery
• Superior mesenteric artery
  • Inferior pancreaticoduodenal artery
  • Intestinal (jejunal and ileal) arteries
  • Ileocolic artery
  • Right colic artery
  • Middle colic artery
• Inferior mesenteric artery
  • Left colic artery
  • Sigmoid arteries
  • Superior rectal artery

The structures supplied by each branch are:

• Coeliac trunk:
  • Abdominal part of the oesophagus
  • Stomach
  • Superior half of the duodenum
  • Liver
  • Superior half of the pancreas
  • Spleen
• Superior mesenteric artery:
  • Intestine, from the lower half of the duodenum down to the splenic flexure of the large intestine
• Inferior mesenteric artery:
  • Colon, from the splenic flexure down to the sigmoid and superior portion of the rectum

Venous structures:

• Oesophagus is drained by branches of the azygous veins and the inferior thyroid vein
• Mesenteric circulation drains via the superior and inferior mesenteric vein
  • These two vessels are then joined by the splenic vein
  • These form the portal vein
  • This then splits to form the right and left branches in the liver
  • From the portal vein, blood drains via the hepatic vein into the inferior vena cava
• Lower third of the rectum and anus drain into the middle rectal vein, which drins directly into the IVC

How is blood flow distributed in the splanchnic circulation?

• The splanchnic blood flow can vary from 10% to 35% of the total cardiac output
• This circulatory system can contain up to 20% of the total circulating blood volume
• There is significant collateral circulation (extensive anastomosis exists between arterial branches)
• The flow is distributed unequally:
  • The main recipients of the blood flow, among the organs, are the pancreas and small intestine.
  • The stomach and large bowel are comparatively underperfused.
  • Absorptive surfaces get more blood flow. In each hollow viscus, the mucosa and submucosa enjoy greater perfusion than the muscularis layer.
• The venous drainage is into the liver, rather than back into the heart. This system supplies 75% of hepatic blood flow and 50% of hepatic oxygen. 

What are the unique features of the intestinal microcirculation?

• The microcirculation of the villus features a counter-current mechanism, which allows arterial oxygen to "escape" into the countercurrent venous blood, thereby predisposing the intestinal mucosa to ischaemia.

How is splanchnic blood flow regulated?

• Intrinsic autoregulation:
  • Myogenic autoregulation (stretch-mediated) 
  • Metabolic autoregulation (likely mediated by adenosine)
• Autonomic regulation
  • Sympathetic vasoconstriction (noradrenergic α-1 effect)
  • Parasympathetic vasodilation (acetylcholine-mediated NO release)
• Humoural and hormonal regulation
  • Vasoactive mediators (of which there are many)
  • Exogenous drugs

Mostly, the blood flow of the gut is not related to the metabolic demands of the gut:

• At rest the oxygen extraction ratio of the gut is only about 5-10%. 
• It can increase up to 80% if the flow decreases

How does the autonomic nervous system influence the blood flow to the intestine?

• Sympathetic stimulation:
  • Decreases gastric (and especially mucosal) blood flow
  • Markedly decreases intestinal blood flow (an α-1 mediated effect); though after a time some sort "autoregulatory escape" occurs, which is probably mediated by the release of vasodilator peptides in a negative feedback response
  • Decreases blood flow to the colon
• Parasympathetic stimulation:
  • Increases gastric blood flow (though this effect does not appear to be acetylcholine-mediated)
  • Increases intestinal blood flow, and this is clearly a parasympathetic phenomenon, but it is not by means of vagal stimulation. The vagus apparently does not play any role in this, as direct vagal stimulation has no vasodilatory effect, but there definitely is postprandial vasodilation happening, and it is definitely blocked by atropine.
  • Increases colonic and rectal blood flow