Rectal Bleeding (cont.)
Radionuclide scans
There
are two types of radionuclide scans that are used for determining the site of
gastrointestinal bleeding; Meckel's scan, and tagged red blood cell (RBC) scan.
The Meckel's scan is a scan for detecting a Meckel's diverticulum. A radioactive
chemical is injected into the patient's vein, and a nuclear camera (like a
Geiger counter) is used to scan the patient's abdomen. The radioactive chemical
will be picked up and concentrated by the acid-secreting tissue in the Meckel's
diverticulum and will appear as a "hot" area in the right lower abdomen on the
scan.
Tagged RBC scans are used to determine the location of the
gastrointestinal bleeding. After drawing blood from the bleeding patient, a
radioactive chemical is attached to the patient's red blood cells and the
"tagged" red blood cells then are injected back into the patient's vein. If
there is active gastrointestinal bleeding, the radioactive red blood cells leak
into the intestine where the bleeding is occurring and will appear as a hot area
on the scan. A weakness of the tagged RBC scan is that it will not show a hot
area if there is no active bleeding at the time of the scan. Thus, it can fail
to diagnose the site of bleeding if bleeding is intermittent and the scan is
done between bleeding episodes. A second weakness of the scan is that it
requires a minimum amount of bleeding to form a hot area. Thus, it can fail to
diagnose the site of bleeding if bleeding is too slow. The tagged RBC scan is
safe, and can be done quickly and without discomfort.
Unfortunately, the tagged
RBC scans are not very accurate in defining the exact location of the bleeding;
there is often a poor correlation between where the tagged RBC scan shows the
bleeding to be and the actual site of bleeding found at the time of surgery.
Therefore, tagged RBC scans cannot be relied upon to help surgeons decide what
area of the gastrointestinal tract to remove in the event bleeding is severe or
persistent and requires surgery. However, if the scan shows a hot area, it
usually means there is active bleeding, and the patient may be a candidate for a
visceral angiogram to more accurately locate the site of bleeding.
Visceral
angiogram
A visceral angiogram is an x-ray study of the blood vessels of the
gastrointestinal tract. The doctor (usually a specially trained radiologist)
will insert a thin, long catheter into a blood vessel in the groin and, under
x-ray guidance, will advance the tip of the catheter into one of the mesenteric
arteries (arteries that supply blood to the gastrointestinal tract).
Radio-opaque dye then is injected through the catheter and into the mesenteric
artery. If there is active bleeding, the dye can be seen leaking into the
gastrointestinal tract on the x-ray film. Visceral angiograms are accurate in
locating rapid bleeding in the gastrointestinal tract, but it is not useful if
the bleeding is slow or has stopped at the time of angiogram.
The visceral
angiogram is not widely used because of its potential complications such as
kidney damage from the dye, allergic reactions to the dye, and the formation of
blood clots in the mesenteric arteries. It is reserved for patients who have
severe and continuous bleeding and in whom colonoscopy cannot locate the site of
the bleeding.
Next: Blood tests »
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