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Chapter24
Acutemesentericischemia
MosheScheinandPaulN.Rogers
Vascularsurgeryispeculiarbecause,aboveall,itismainly
surgeryofruins.
CiddosSantos
Occlusionof themesenteric vesselsis regardedas one of
those conditions ofwhich the diagnosis is impossible, the
prognosishopeless,andthetreatmentalmostuseless.
A.Cokkins
Whichofyouhasn’tbeencalledbytheinternistsorERdocs,totheER
or medical floor, or the intensive care unit, to “rule out mesenteric
ischemia”insomeelderlypatient?Asarule,onsuchoccasionsyouwill
find a groaning patient with non-specific abdominal complaints and a
hospital chart that outweighs you. “Rule out mesentericischemia”—
easiersaidthandone!
Acutemesentericischemiausuallyinvolvestheregionsuppliedbythe
superior mesenteric artery (SMA). Thus, the small intestine is
predominantlyaffectedbuttheright colon,whichisalsosuppliedbythe
SMA, can be involved aswell. Isolatedischemia ofthe colon,which is
much less common,will be discussedseparately under the heading of
Ischemiccolitisin Chapter26.
Theproblem
The problem is a sudden reduction in arterial perfusion of the small
bowel,whichquicklyleadstocentralabdominalpain.Ifleftuntreated,the
processprogressivelyinvolvesthemuscularlayeroftheintestinesandit
is only after some hours, when the serosa is affected, that peritoneal
signsappear.Inanattempttosimplifymatters,letusdivideacutearterial
mesentericischemia(AMI)intothreetypes.
Thrombotic
: due to an acute arterial thrombosis (of a chronically diseased
artery), which usually occludes the orifice of the SMA, resulting in
massive
ischemiaoftheentiresmallbowelplustherightcolon
—
theareasuppliedbytheSMA.
Embolic
:duetoashowerofembolicmaterialoriginatingproximally—fromthe
heart (atrial fibrillation, post-myocardial infarction, diseased valve) or an
aneurysmaloratheroscleroticaorta.EmboliusuallylodgeintheproximalSMA,but
beyondtheexit of themiddlecolic artery; therefore, asarule, the most proximal
segment of proximal small bowel is spared, along with the transverse and
(probably) the right colon.
Emboli tend to fragment and re-
embolizedistally,producingapatchytypeofsmallbowel
ischemia
. Note that embolism can occur against a background of chronic
mesentericarterialdisease.
Non-occlusivemesenteric ischemia (NOMI)
: due to a ‘low-flow
state’, in the absence of documented arterial thrombosis or embolus. Note,
however, that underlying mesenteric atherosclerosis may be a
precipitating/contributory factor. The low-flow state is a product of low cardiac
output (e.g. cardiogenic shock), reduced mesenteric flow (e.g. intra-abdominal
hypertension)ormesentericvasoconstriction(e.g.administrationof vasopressors)
—
usually, however, it is due to a combination of these
factors,developinginthesettingofapre-existingcritical
illness
.
Mesenteric venous thrombosis can also produce small bowel
ischemia. The features and management of this entitydiffer drastically
fromtheotherthree.Itwillbediscussedseparatelybelow.
The problem is that in clinical practice, outside the textbook,
mesentericischemia is usuallyrecognizedwhenithas already led
to intestinalgangrene. At that stage the Pandora’s box of SIRS has
beenopenedandevenremovaloftheentiregangrenousintestinewillnot
always stop the progression to organ failure and death. Even if such
physiological consequences can be overcome, the patient commonly
becomesan‘intestinalcripple’,sufferingfromtheshortbowelsyndrome.
Havewedepressedyouenoughyet?
Assessingtheproblem
Typically, the early clinical picture is non-specific — the patient
complainsofsevereabdominal pain—ifheisabletocomplain at
all—andthedoctorfindslittleonphysicalexamination.
Textbooks will tell you that, early on, the abdominal pain is out of
proportiontotheclinicalfindings—thepatientshoutsandgroansbut
hisabdomenisinnocent!However,wehaveseenpatientswithalmosta
painless presentation — the (fully alert) patient presenting with what
appears as ‘ileus’, doesn’t complain about pain, but at laparotomy a
segmentofhissmallbowelisalreadynecrotic.Sothekeyisalwaysto
have the suspicion of AMI in your mind! Remember: occasionally
what the ER doc or internist admits as a “gastroenteritis”, or
“partialsmallbowelobstruction”or“ileus”wouldprove(oftentoo
late)tobeAMI. Thus,youmust resistthenaturaltemptationtoascribe
patients’non-specific symptoms to some other benign condition unless
theclinicalfeatures,includingimaging,forthealternativeexplanationare
fully present. And by the way — in the elderly — the diagnosis of
acutegastroenteritisisrarelythefinaldiagnosis;unlessitwasthe
wrongdiagnosis!
There may have been preceding symptoms of a similar sort of pain
developingwithmealsandaccompaniedbyweightloss,suggestingpre-
existing mesenteric angina. A history or evidence of systemic
atherosclerotic vascular disease is almost the rule in patients with
mesenteric thrombosis, while a source for emboli, such as atrial
fibrillation,isusuallypresentinpatientswithmesentericembolism.Low-
flow state patients are commonly moribund due to underlying critical
diseaseandarenotinfrequentlyfoundinthecardiacsurgeryICU.
Nausea, vomiting, diarrhea (caused by ‘reflex bowel emptying’) and
hematochezia may come late, if ever, and are, again, non-specific.
Physicalexaminationintheearlystagesoftheprocessistreacherously
benign;peritonealirritation appears too late, when the bowel isalready
dead.
Laboratory tests are not much help here either. Evidence of
compromised bowel such as elevated serum lactic acid, C-reactive
protein (CRP), amylase or a negative base excess are suggestive but
non-specific.However,extremeleukocytosisisnotuncommon even
beforetheintestinedies.
PlainabdominalX-raysearlyinthe courseoftheillness arenormal.
Later, there may be a pattern of adynamic ileus, with visible loops of
small bowel and fluid levels, but with gas and feces seen within the
normalcolonandrectum.
Thebottom line isthatinitiallyin acute mesentericischemiathe
physicalexaminationandallcommonlyavailableX-raysandblood
testsmaybenormal.
At this stage, entertaining the diagnosis of mesentericischemia, you
havetwooptions:thefirstistoenterinthechart“abdominalexamination
normal; mesenteric ischemia cannot be ruled out; will reassess later”.
This,youwilldoonlyifyouarealazymoron,which, we trust, you are
not.Thesecondoption—therightone—istoorderaCTscanwith
intravenous contrast, which has replaced mesenteric angiography as
the initial, screening imaging modality in AMI. Although angiography is
morespecific andaccurate,surgeons havebeen reluctanttooffersuch
an invasive procedure in patients with a non-specific clinical picture.
Unfortunately,thefirstoptionisstillcommoninsomeplaces—leadingto
procrastination, late diagnosis and treatment, and a very high mortality
rate.
Computedtomography
To be diagnostic, the examination should include appropiately
timed intravenous contrast (‘CT angio’), with the focus on two
areas:thebowelwallandmesentericvessels.Thecommonestfinding
isbowelwallthickening,whichis,however,non-specific.Thebowelwall
may appear low in attenuation due to edema or, when submucosal
hemorrhage is present, it may appear of high attenuation due to the
bloodproducts.Visualizationofthedynamicenhancementpatternofthe
affectedbowelloopsmay improvediagnosis.Affectedbowel loops may
demonstrate absence of enhancement, delay in enhancement, or
persistent enhancement when compared to unaffected loops.
Pneumatosisandportalveingasareuncommonbutspecific,albeitlate,
signs—duetointraluminalgasdissectingintothefriablebowelwalland
then into the portal venous tributaries. CT angio can also visualize
acutechangessuchasemboliwithintheSMAorthrombosisat its
origin;itcan also demonstrate chronic changes, such as stenosis
or occlusion of the chief collateral sources of mesenteric blood
supply — the celiac and/or inferior mesenteric artery. Often,
however, eventhe CT findings inthis condition are subtle and easy to
miss.
Magneticresonanceangiography(MRA)couldbesuperbinimaging
mesentericvessels(withreducedriskofcontrastnephrotoxicity)but,like
CTangio,isfar inferiortoconventional angiographyin visualizingdistal
branches.Inaddition,inhowmanyhospitalswouldMRAbeavailablein
themiddleofthenight?
Mesentericangiography
To bebeneficial the angiogram should be performedbefore the
bowel has become gangrenous. The clock is ticking; every passing
minutereducesthechancesofthebowelandthepatientsurviving.Note
thatanacuteabdomenwithperitonealsignsisacontraindicationto
angiography, for such patients belong in the operating room. So
thesedays angiography is obtainedusuallyafter the ‘CT-angio’ to
establish which revascularization option, if any, is advisable. The
radiologistshouldstart withbiplanar angiography(i.e.includingalateral
view to show the originsof theSMA and the celiac axis). Anoccluded
ostium of the SMA denotes thrombosis and calls for an immediate
revascularization procedure (radiological or surgical) — unless there is
evidence of a good collateral inflow — the angiography providing the
road map for vascular reconstruction. When the ostium is patent the
radiologistadvancesthecatheterintotheSMA.Embolilodgedistaltothe
take-offofthemiddlecolicartery,produceasmoothfillingdefectonthe
backgroundofanormalSMA,andcanbemultiple.
Non-operativetreatment
Intheabsenceofperitonealsigns,attemptsatnon-operativetreatment
arejustified—tailoredtotheclinical/CT/angiographicfindings.Selective
diagnostic angiography can now become therapeutic — infusing a
thrombolytic agent to lyse the thrombus or embolus with or without
adding papaverine torelieve the associated mesenteric vasospasm. At
thesametimestents canbeinsertedtoalleviatesignificantstenosis of
the SMA and, if necessary, the celiac artery. Cessation of abdominal
symptoms together with angiographic resolution means that the
emergencyisover.
Intheeventofnon-occlusivemesentericischemia,theapproach
involves attempts at restoring compromised hemodynamics. To
relieve associated arteriospasm, a selective intra-arterial infusion of a
vasodilator, such as papaverine, has been advocated. The few
championsofthismethodhavereported“favorableresponses”although
that20-year-oldretrospectivestudyfromNewYorkwasneverconfirmed
byfirmdata, so we may be dealingwitha myth. When emboli are the
cause, after successfultranscatheter therapy, long-term anticoagulation
is indicated. A final point — while rushing to the arteriography suite,
remember to ensure adequate hydration of your patient to oppose the
nephrotoxiceffectofthecontrastmedia.
Operativetreatment
Aswe havealreadytold you—peritoneal signs,considered together
with a suggestive clinical (and CT) picture, are an indication not to do
arteriography but to operate; the same applies to failure of the nonoperativeregimendiscussedabove.
Figure24.1.“HowmuchshouldIresect?”
Through a midline incision assess the viability of the intestine. In
generaltherearetwomainpossiblescenarios:oneisthatthebowel
isfranklygangrenous(dead);thesecondiswhenthebowelappears
ischemic(dusky)andofquestionableviability:
•
Frank gangrene of the entire small bowel is usually combined
with the same problem in the right colon and signifies SMA
thrombosis. Theoretically, a sporadic patient could survive
resection of the entire small bowel and right colon. He may even
tolerate a duodenocolic anastomosis while being nutritionally
supported at home with total parenteral nutrition (TPN). But the
eventual mortality of such an exercise in the average elderly
vasculopath approaches 100% and the cost is immense. Our
recommendation to you when involved in a similar situation is to
walkouttotalktothefamily,andexplainthatanythingdonewillonly
increasethe sufferingof theirbeloved. Ifthefamily wishessothen
returnto close the abdomen over thedeadbowel. Provide a lot of
morphine and comfort. As with everything in life — there are
exceptions:inarelativelyyoungandactivepatient,andwhenlocal
circumstances are favorable (we doubt that there are facilities for
homeTPN in rural Afghanistan orevenEastern Ukraine), you and
thefamilymaywanttostriveforlong-termsurvival.
•
Frankgangreneofashortersegment,ormultiplesegmentsof
small bowel, usuallydenotes embolism. After excising all dead
segments carefully examine the remainingbowel. Measure it:how
longisit?Onlyabouthalfofpatientsleftwithlessthan1meter
(3 feet) of small bowel will live without TPN (saving the
ileocecal valve improves the prognosis). Now, observe the
remaining bowel. Is it truly non-compromised? Are the mesenteric
arcadespulsatingwell? Feel the SMAat its root — is it vigorously
pulsating?
•
Dusky bowel. When you are not happy with the remaining
bowel,orwhenthebowelisnotdeadbutappearsischemicand
of questionable viability from the start, proceed as follows.
First,ifpossible,revascularizeusingthetechniquesdescribedinthe
next section. Then wrap the bowel in warm, saline-moistened
spongesandwait15minutes.Unscrubandhaveacoffee;surgeons
cannot stare at an inactive field for that long without starting to
fiddle.Failureoftheboweltopink-upmandatesitsresection.When
the length of remainingnormal-looking bowel reduces towards 1.5
meters (5 feet)it may beadvisable to leave the doubtful bowel in
situ, to be re-examined during a relook operation (see below).
Salvaging even a short segment of small bowel may improve the
chancesofpreservingalifeworthliving.Someauthorsrecommend
theuse of hand-held Dopplertoexamine the perfusion oftheantimesentericsideofthebowel;othersuseintra-operativefluorescein
angiography. More practical appears to be a pulse oximeter —
sterile probes are now available. You may choose to use such
modalities if available to you but your clinical judgment should be
justasgoodasanygimmick( Figure24.1).
Adjunctivevascularprocedures
Theidealsettingforsurgicalimprovementoftheperfusionofischemic
smallboweliswhentheoperationfollowsemergencyarteriography(plus
failed angiographic therapy) and the bowel is viable or doubtful.
Obviously, when the bowel is dead it cannot be revived! Arteriography
servesasaroadmap;whentheSMAisthrombosedatitsorigin,avein
orother graftbypass,antegrade orretrograde, is indicatedto reperfuse
the SMA. The two main options here are autogenous vein bypass or
externally supported polytetrafluoroethylene (PTFE) inserted in a ‘lazy
loop’fashion.Theoriginofthesebypassgraftsisoftendeterminedbythe
patternofdiseaseaffectingtheotherintra-abdominalvessels.Usuallythe
iliacvesselsarethebestsourceofpulsatileflowbutcaremustbetaken
whenveinisused,toseethatitdoesnotkinkwhenthegutisreturnedto
itsnormalposition.
The above scenario is, however, rare; more commonly you’ll
encounter a picture of SMA embolism. Feel for the SMA just at the
base of the mesocolon; if non-pulsatile you’ll find it, after incising the
peritoneum,to theright (asyoulook atitfrom below)of thelarge,blue
superior mesenteric vein. After obtaining control, open the artery
transversely and pass up and down a small Fogarty embolectomy
balloon catheter. You may conclude the procedure with a shot of
urokinaseinjected distallyto lysethe clotsin thedistalbranches,which
are inaccessible to your embolectomy balloon catheter. Heparinize the
vesselsbeforerepairingthearteriotomy.
Ifyouwork at an ‘ivory tower’ the option of endovascular therapies
maybeavailableaswell.Earlylysisandstentingofan‘acuteonchronic’
occlusion of the SMA may obviate the need for laparotomy altogether.
Even if bowel resection is needed, the patient can be transfered
directyfromtheORtotheradiologysuiteforstentingoftheSMA
(and/orceliacartery)toimprovethebloodsupplytotheremaining,
marginallyperfusedintestine.
Toanastomoseornot?
You should be very selective in attempting an anastomosis
followinganyresectionofdevitalizedintestine.Thepatienthastobe
hemodynamically stable and his nutritional status at least fair. To be
hooked-up,theremainingbowelhastobeunquestionablyviableandthe
peritonealcavityfreeofestablishedinfection.Mostcrucially,thecause
ofischemiahastobedefinitivelysolved.
Anotherfactorstronglybearingonyourdecisionisthelengthof
theremainingbowelanditspredictedpostoperativefunction.When
more than half of the small bowel is resected, the resection is
considered ‘massive’. Restoring intestinal continuity in such cases
would lead to poorly tolerated and intractable diarrhea. And finally, the
chief reason not toanastomose the bowel is the possibility that further
ischemiamaydevelop.Inaddition,astomaprovidesanexternalwindow,
albeitnotaperfectone,allowingyoutoassessviabilityoftheremaining
bowel.
We recommend, therefore, that whenever the above-mentioned
favorablefactorsareabsent,orwhenresectionismassive,thetwoends
oftheresectedbowelshouldbeexteriorizedasanend-enterostomyand
mucous fistula — if possible via one abdominal wall site as a ‘doublebarrel stoma’. This will allow a subsequent elective reanastomosis —
after the patient has reached optimal nutritional status, and the bowel
remnantachieveditsmaximaladaptation—withoutamajorlaparotomy.
Thepostoperative appearanceof the stomaswill, in mostcases, reflect
the status of the remaining bowel — but not always: in patients with
patchyintestinalinjury—usuallycausedbyashowerofmicroemboli—
aperfect looking stomamaycoexisistwith apatchofgangrenein
thebowelaboveorbelowit!
Second-lookoperations?
A routine planned ‘second-look’ reoperation allows direct
reassessment of intestinal viability at an early stage, before
additionalmediatorsofSIRShavebeenreleased,andinawaythat
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