This chapter is relevant to 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 ... including autoregulation... These include, but not limited to, the cerebral and spinal cord, hepatic and splanchnic, coronary, renal and utero-placental circulations". The renal, coronary and hepatic circulatory systems have appeared in the exam so many times that these questions crowd out all the others. In contrast, the splanchnic circulation is forgotten by the examiners, which is puzzling because of its significant importance in our clinical work, where it often kills people by mesenteric ischaemia. The college have made up for this by asking many detailed questions about the mesenteric circulation in the fellowship exam. Moreover, the anatomy of the gastrointestinal blood supply was asked in Question 7 from the first paper of 2018.
- Splanchnic vascular anatomy
- There are three main vessels: coeliac trunk, superior mesenteric artery and inferior mesenteric artery
- There is an extensive collateral circulation which protects against ischaemia
- The venous drainage is into the portal vein, and then into the liver
- The blood volume in this circulatory region is about 20% of the total
- Splanchnic blood flow
- At rest, the total blood flow is 30ml/100g/min (10-20% of total cardiac output)
- Postprandial blood flow can be up to 35% of total cardiac output
- Oxygen extraction ratio is low (~ 10%)
- The splanchnic organs tend to extract more oxygen as flow decreases, and autoregulatory input into blood flow is therefore minimal under normal circumstances
- Regulation of splanchnic blood flow:
- Intrinsic autoregulation:
- Myogenic autoregulation (stretch-mediated)
- Metabolic autoregulation (likely mediated by adenosine)
- Rapid, but
- Autonomic regulation
- Sympathetic vasoconstriction (noreadrenergic α-1 effect)
- Parasympathetic vasodilation (acetylcholine-mediated NO release)
- Humoural and hormonal regulation
- Vasoactive mediators (of which there are many)
- Exogenous drugs
As is often the case, in this topic one name keeps appearing in the bylines of the papers, and for splanchnic blood flow that is D. Neil Granger - pretty much anything from the 1980s seems to be perfect for this topic, and much of it seems to be available as free full text. If one had to pick a representative piece, one would have to pick Intestinal Blood Flow (Granger et al, 1980). Fraser et al (1991) is also excellent, even though it is ostensibly supposed to be only about the autonomic nervous system of the gut.
Following from the ancient Greek origin of the word σπλαγχνικός, which basically means "entrails", the casual anatomist will use this term interchangeably with "mesenteric" to describe the blood vessels which supply the digestive tract. However, this is wrong. When you search for "splanchnic circulation" in professional literature, the results generally yield articles like Parks et al (1985), which confidently define it as follows:
"The splanchnic circulation is composed of gastric, small intestinal, colonic, pancreatic, hepatic, and splenic circulations, arranged in parallel with one another".
The attentive reader will note that this definition includes the liver. However, when they were writing the Syllabus Document, the college examiners clearly meant mesenteric instead of splanchnic, because they listed the hepatic circulation separately. Moreover, all the hepatic circulation SAQs do not require any discussion of mesenteric blood supply in their answer. As a result, the circulatory system of the liver gets its own chapter, and this page ends up having to use continue misusing the word "splanchnic", cringing awkwardly at the incorrectness.
The major vessels and their branches are:
The structures supplied by each branch are:
It is of course completely pointless to accurately describe the layout of the mesenteric anatomy to the CICM exam candidate, as they should never have to encounter it in the wild. Occasionally, the interpretation of imaging might be called for, or the discussion of embolised branches, but realistically the ICU trainee would almost never be called upon to make diagnostic or management decisions on the basis of raw untreated anatomy. From this, it follows that even a discussion of the splanchnic circulation which represents the vessels schematically, or which just lists the names of the arterial and venous branches, would be educationally meaningless. It does not help that these vessels are normally subject to endless individual variation. In case a detailed anatomical review of these variations is required, one can find authoritative answers in excellent reference resources such as Kornblith et al (1992) or Rosenblum et al (1997). Instead, the following broad generalisations or unique features can be mentioned here, as they may actually have relevance to the intensive care practitioner:
The discussion of splanchnic blood flow must necessarily be split into the discussion of blood flow through the three major arteries, as they are rather different. Of these, the coeliac trunk is the largest, most proximal, and has the highest blood flow, followed by the SMA. The IMA is the smallest and has the lowest blood flow. Two important points need to be made, which have some sort of vague critical care relevance:
Most textbooks seem to quote a total blood flow of 30ml/100g/min, but of course the total flow of blood to the splanchnic circulation can vary considerably, depending on what and when you ate, and can vary from 10% to 35% of the total cardiac output. To illustrate the point, Rosenblum et al (1997) report coeliac axis flows varying from 300ml/min to 1200 ml/min. Perko et al (1998) reported total splanchnic blood flow of around 1600ml/min on average for resting adults, and around 3000ml/min for the same adults after they horfed down a bucket of Swedish glog. In short, splanchnic blood flow is highly variable. Which brings us to...
Clearly, one of the characteristics of the splanchnic circulation is the ability to change its vascular resistance and invite influence the rate of splanchnic blood flow according to some change in conditions. This can, for lack of a better word, be called "autoregulation", in the sense that it appears as if the blood vessels of the gut are being influenced by something the gut is doing, but in fact most of the mechanisms of control are external, and most of them are designed to optimise the benefit to the rest of the organism rather than catering for the metabolic demands of abdominal organs. This topic is covered in just the right amount of detail by Parks & Jacobson (1985).
In brief, there are three main levels of control over the splanchnic circulation:
Yes, the gut has some mechanisms to ensure that it maintains some sort of stable flow, in the event that systemic blood pressure is fluctuating wildly, or if its metabolic needs are not being met. These mechanisms are not especially unique. There are two main ones: myogenic regulation and metabolic regulation. They are common to all (well, most) arterial circulatory regions, and are described well enough in the chapter which deals with the mechanisms of peripheral vascular resistance. In summary:
"... it is not the intense phenomenon seen in other organs (e.g. kidney, brain) since a reduction in perfusion pressure is usually accompanied by a reduction in blood flow, while resistance falls by a modest amount"
So, don't expect the intestine to attentively self-manage like the brain. Fortunately, it does not need to, because of its ascetic disinterest in metabolic substrate. Under normal circumstances, according to Granger & Norris (1980) at rest the oxygen extraction ratio of the gut is about 5-10%. This means, it really does not need to do very much to autoregulate its blood flow, and if perfusion pressure drops it can just extract more oxygen. Thus, when the investigators dropped the MAP to 30 mmHg, the oxygen extraction ratio increased to 80% (i.e. tenfold) with only 30% change in vascular resistance.
To summarise, there is some intrinsic (myogenic, metabolic) autoregulation of blood flow in the mesenteric circulation, but under most conditions it is a rather relaxed affair, as the gut is not viewed as a mission-critical organ. However, everything changes as soon as you have a meal.
The greatest changes which occur in splanchnic blood flow during routine use are due to eating. This is handled largely by the sympathetic and parasympathetic fibres which innervate the blood vessels of the gut. Fraser et al (1991) offer the best overview for this topic. The main role of the autonomic nervous system is to increase blood flow to the gut for digestion, and to redirect it away from the gut during times of increased haemodynamic demand. Thus:
There turns out to be a vast array of circulating vasoactive substances, and all of them have some effect on the splanchnic circulation. It would literally be easier to list hormones which don't influence the splanchnic circulation. An excellent table is offered by Harper (2016), and it is reproduced here with zero modification. Theoretically, it would possible to track down references for each of these influences, but it did not seem essential, considering especially the fact that CICM have never asked about this in any of the past papers.