This is a common enough event in the ICU that one might have expected a serious discussion of ischaemic gut to appear in the exam with some frequency, and indeed it has now appeared three times:
These SAQs asked for a detailed exploration of mesenteric ischaemia, specifically asking candidates to "outline the pathophysiology, diagnosis and treatment". Additionally, Question 8 from the second paper of 2021 went into some considerable detail about vascular supply anatomy and the difference between venous and arterial ischaemia.
What does an intensivist need to know about this for their day-to-day practice? Not much, if Oh's Manual is anything to go by. Mesenteric ischaemia comes up once as a quarter-page discussion in Jamie Cooper's chapter on lactic acidosis (p/160 of the 7th edition), where much is made of how difficult it is to diagnose, and how one must have a high index of suspicion. Nothing in the way of concrete recommendations or instructions is offered, which is characteristic of the Manual (and defies the definition of a manual).
For good quality reading on this topic, the time-poor candidate is directed to either this 2014 review by Schofield et al or this NEJM article by Clair and Beach (2016). Each has helpful exam-ready headings like "Pathophysiology" and "Diagnosis". These articles were remixed into the summary which follows. Most of the discussion will be about acute mesenteric ischaemia as it is seen more commonly in critical care environments, but chronic mesenteric ischaemia is also a thing - more akin to claudication of the gut - which will receive some minimum of attention here. For more detail about this very different beast, the exam candidate with infinite time resources is directed to the excellent review by Pecoraro et al (2012), or to the Guidelines of the World Society of Emergency Surgey (Bala et al, 2017)
The splanchnic circulation is described in greater detail in the CICM First Part Exam study notes. In summary:
The major vessels and their branches are:
The structures supplied by each major branch are:
The mesenteric circulation enjoys extensive collateral blood supply, which generally means that it is resistant to ischaemia. For example, anastomoses between arteries from different parts of the mesenteric circulation mesh freely and form a shared capillary network. Thus, none of these arteries are end-arteries, which means they should be relatively resistant to emboli. Still, emboli can be highly destructive if they block a proximal large vessel.
Also, the areas of greatest collateralisation are areas which are the most susceptible to non-occlusive mesenteric ischaemia (NOMI). These watershed areas are:
There are several major ways this thing can happen:
Obviously, the risk factors for different kinds of mesenteric ischaemia are going to be different.
Occlusive acute mesenteric ischaemia tends to result from embolic phenomena or atheroma rupture, so the risk factors tend to be things which create mobile thrombi or which agitate atheromae. Some risk factors, like cardiac surgery, are a source of both emboli (eg. air bubbles) as well as atheroma fragmentation (eg. when the surgeon stabs your aorta).
Non-occlusive acute mesenteric ischaemia is a phenomenon where an otherwise relatively normal mesenteric vascular rbed is subjected to some sort of sudden increase in vascular resistance, or a sudden decrease in blood flow, or some combination of both.
Venous mesenteric ischaemia due to mesenteric venous thrombosis has risk factors which resemble the risk factors for DVT and VTE in general. Additionally, there are a few risk factors unique to the mesenteric venous circulation which are probably of interest to the intensivist. Hmoud et al (2014) list quite a number of different risk factors, which can be arbitrarily grouped into these two categories:
The World Society of Emergency Surgery suggest that "severe abdominal pain out of proportion to physical examination findings should be assumed to be AMI until disproven". This does not seem to be specific to any particular cause of ischaemia. According to Park et a (2002), the presenting features usually are:
The clinical scenario is said to differentiate the causes of mesenteric ischaemia, as the clinical findings are typically similar for all of them (wait long enough and ultimately everything starts looking like severe septic shock and multiorgan system failure). In short, the abdomen is usually the same on examination, and the way you come to the conclusion that this must be embolic or venous, is by discovering that the patient is in AF, or has recently had a bowel resection for a colonic carcinoma, or something along those lines. Still, Hmoud et al (2014) were able to create this table of clinical findings and historical features which may help differentiate arterial and venous mesenteric ischaemia:
|Arterial mesenteric ischaemia||Venous mesenteric ischaemia|
|Risk factor||Acute intra-abdominal process||Atrial fibrillation|
|Thrombophilia||Cardiomyopathy and CHF|
|Malignancy||Valvular heart disease|
|History of DVT||Absent||Present in 20–40%|
|CT||Diagnostic in 40–50% only||In more than 90%|
|Angiogram||Diagnostic in most||In 50–60% only|
|Acute presentation||Most of the time||Can be subacute or chronic|
|Bowel infarction||Likely if not relieved in 12 h||Not usual if diagnosed and AC started|
|Treatment||Embolectomy or IA papaverine||Anticoagulation, possible thrombolysis, systemic or directed|
|Need for surgery||Frequent||Less likely if AC started early|
|Ischemic to normal transition||Abrupt||Gradual|
|Mortality||More common||Less common with CT and AC availability|
|Chronicity||Rare||Can occur with portal hypertension and varices|
Broadly, one could outline the management in a simplified way, but one would also have to admit that a slightly different strategy would be required depending on what sort of mesenteric ischaemia is involved.
Like with everything in ICU, the prognosis of mesenteric ischaemia is mainly related to whatever else is going on. The prognosis of a promptly detected arterial embolic event in a patient with AF and nothing else is obviously going to be different to the prognosis of the patient who developed mesenteric ischaemia in the course of severe septic shock with multiorgan system failure. Looking at an undifferentiated case series of 780 patients, Leone et al (2018) found an in-ICU mortality rate of 58%. They could not determine which was occlusive or which was venous; but they assumed that the 14% of patients who survived with conservative management probably had the venous version, which generally does better. On the other hand, where the ischaemia is non-occlusive and due to something like cardiogenic shock, it should be viewed as uniformly fatal: nobody is going to operate on these people to remove the infarcted bowel, which means they will surely die. Thus, it would be rather difficult to produce precise numbers to describe the mortality for each subtype of mesenteric ischaemia.