Tricky gut ischemia, the uselessness of lactate, and the importance of clinical suspicion

You are called to see Ms A, an 80 year old woman on the surgical ward, due to worsening abdominal pain and tachycardia. She was admitted 8 hours ago with the same abdominal pain and diarrhea and had a CT abdomen, which the radiology registrar has provisionally reported as showing non-specific pericolonic fat stranding. She has been treated as an infectious colitis. She has a history of ischemic heart disease, atrial fibrillation and claudication. Examination of her abdomen shows diffuse tenderness but no peritonism.

You call the surgical registrar to express your concern this lady might have ischemic gut. He informs you he is reassured by the CT findings, the lack of peritonism and the normal lactate, which you had decided to check because you have recently heard about the association between gut ischemia and elevated lactate. When you arrive at work the next morning you discover that overnight she had become septic, spiked her lactate to 8 and been taken for a laparotomy, where extensively necrotic bowel was found. She was palliated.

Ischemic gut is one of those diagnoses that is always tricky to make, as there is no lab test to help you and the examination findings can be non-specific, although “pain out of proportion to the exam” is what you might find in the textbooks.  Age confers an exponentially increasing risk, and past the age of 75 it becomes more likely than appendicitis or ruptured AAA (1). This is a fact which I certainly hadn’t appreciated and I suspect many people don’t, given the frequency with which we query the latter two on CT requests and the infrequency with which we query ischemic gut.

The first point to make abundantly clear is that peritonism is a late sign of extensive bowel necrosis so is not reassuring. The same applies to the finding of portal venous gas on an abdominal Xray, pictured below (2). The whole point is to diagnose the condition early enough that you can do something about it (either open or endovascular revacularisation). By the time these signs develop, the proverbial train has left the station.

portal venous gas

The other thing that people put too much weight on is the lactate. It seems about half of patients have a normal lactate, and when it is raised, it is again associated with extensive bowel necrosis (2,3). Yet, time and time again people will reassure themselves with a normal lactate. Following on from this, a significant proportion of patients present with elevated troponin levels, resulting in inappropriate referral to a cardiologist and increased mortality (3). Elevated amylase levels can also be found.

When it comes to CT imaging, the most accurate scan requires contrast in both arterial and venous phases. Unfortunately your standard abdominal pain contrast CT will be taken in the venous phase (2). When the ‘biphasic’ contrast CT is taken, the sensitivity reported in the literature is 89-100%. However, the majority of the patients in these studies had advanced bowel ischemia, which is usually a pre-terminal finding. What we really want to know is the accuracy of CT to find something when it will actually make a difference to the patient.

In this regard, the accuracy of CT is much lower. In one series the reported sensitivity was between 67-85% for patients being scanned for abdominal pain of unclear cause, however many of these scans would not have been dual contrast enhanced (2). Ischemia specific findings (visible thrombus, bowel wall hypoenhancement, and gas in the bowel wall) are present less often than non-specific bowel findings such as bowel dilation, bowel wall thickening and fat stranding, which can be present in a variety of conditions (4).

It is certainly disquieting to learn that we can’t rely completely on a CT scan. What then can we do? Well, it is all about clinical suspicion. In this study (1) the CT report was correct 97% of the time when the referring clinician expressed a suspicion of ischemic bowel but only 81% correct when they did not. Radiologists seem to look more closely for signs of a disease if you tell them to look for it. This highlights why it is always important to tell the radiologist what your differential diagnosis is. Keep in mind also that afterhours it is the radiology registrar reporting the scans and not a consultant- the report may change in the morning.

So, conclusions?

  • Maintain a high index of suspicion based on age, cardiovascular risk factors, non-specific exam findings and history that may sound like chronic intestinal ischemia (post prandial abdominal pain)
  • Pay no attention to lack of peritonism or a normal lactate
  • Tell the radiologist you are suspicious of ischemic gut (this applies to all conditions) so they can do the right contrast sequence and look more closely for the relevant findings
  • The CT may be falsely negative or non-specific, especially early on

 

  1. Detecting acute mesenteric ischemia in CT of the acute abdomen is dependent on clinical suspicion: Review of 95 consecutive patients. Tiina T. Lehtimäki, Jussi M. Kärkkäinenb, Petri Saari, Hannu Manninena, Hannu Paajanenb, Ritva Vanninen. European Journal of Radiology 84 (2015) 2444–2453
  2. Acute mesenteric ischemia (part I) Incidence, etiologies, and how to improve early diagnosis Jussi M. Karkkainen and Stefan Acosta. Best Practice & Research Clinical Gastroenterology 31 (2017)15e25.
  3. DIAGNOSTIC PITFALLS AT ADMISSION IN PATIENTS WITH ACUTE SUPERIOR MESENTERIC ARTERY OCCLUSION Stefan Acosta, Tomas Block, Steinarr Bjornsson, Timothy Resch, Martin Bjorck, Torbjorn Nilsson. The Journal of Emergency Medicine, Vol. 42, No. 6, pp. 635–641, 2012
  4. Interpretation of Abdominal CT Findings in Patients Who Develop Acute on Chronic Mesenteric Ischemia. Kärkkäinen, Jussi M; Saari, Petri; Kettunen, Hannu-pekka; Lehtimäki, Tiina T; Vanninen, Ritva; et al. Journal of Gastrointestinal Surgery; New York Vol. 20, Iss. 4, (Apr 2016): 791-802.  DOI:10.1007/s11605-015-3013-y

 

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