Because chromium, in the molecule sodium chromate, attaches itself to
red blood cells, it is useful in several kinds of tests. The procedures
are slightly complicated, but yield useful information. In one, a sample
of the patient’s blood is withdrawn, stabilized with heparin (to prevent
clotting) and incubated with a tracer of radioactive sodium chromate.
Excess chromate that is not taken up by the cells is reduced and washed
away. Then the radioactivity of the cells is measured, just before
injection into the patient. After a suitable time to permit thorough
mixing of the added material throughout the blood stream, a new blood
sample is taken and its radioactivity is measured. The total volume of
red blood cells then can be calculated by dividing the total
radioactivity of the injected sample by the activity per milliliter of
the second sample.
[Illustration: _Spleen scans made with red blood cells, which had been
altered by heat treatment and tagged with chromium-51. Such damaged
cells are selectively removed by the spleen. A is a normal spleen. B
shows an abscess in the spleen. Note dark ring of radioactivity
surrounding the lighter area of decreased activity at the central
portion of spleen._]
In certain types of anemia the patient’s red blood cells die before
completing the usual red-cell lifetime of about 120 days. To diagnose
this, red cells are tagged with chromium-51 (⁵¹Cr) in the manner just
described. Then some of them are injected back into the patient and an
identical sample is injected into a compatible normal individual. If the
tracer shows that the cells’ survival time is too short in both
recipients to the same degree, the conclusion is that the red cells
themselves must be abnormal. On the other hand, if the cell-survival
time is normal in the normal individual and too short in the patient,
the diagnosis is that the patient’s blood contains some substance that
destroys the red cells.
When chromium trichloride, CrCl₃, is used as the tagging agent, the
chromium is bound almost exclusively to plasma proteins, rather than the
red cells. Chromium-51 may thus be used for estimating the volume of
plasma circulating in the heart and blood vessels. The same type of
computation is carried on for red cells (after correction for a small
amount of chromium taken up by the red blood cells). This procedure is
easy to carry out because the radioactive chromium chloride is injected
directly into a vein.
An ingenious automatic device has been devised for computing a patient’s
total blood volume using the ⁵¹Cr measurement of the red blood cell
volume as its basis. This determination of total blood volume is of
course necessary in deciding whether blood or plasma transfusions are
needed in cases involving bleeding, burns, or surgical shock. This ⁵¹Cr
procedure was used during the Korean War to determine how much blood had
been lost by wounded patients, and helped to save many, many lives.
For several years, iodine-131 has been used as a tracer in determining
cardiac output, which is the rate of blood flow from the heart. It has
appeared recently that red blood cells tagged with ⁵¹Cr are more
satisfactory for this measurement than iodine-labeled albumin in the
blood serum. It is obvious that the blood-flow rate is an extremely
important physiological quantity, and a doctor must know it to treat
either heart ailments or circulatory disturbances.
In contrast to the iodine-131 procedure, which requires that an artery
be punctured and blood samples be removed regularly for measurement,
chromium labeling merely requires that a radiation counter be mounted on
the outside of the chest over the aorta (main artery leaving the heart).
A sample of labeled red blood cells is introduced into a vein, and the
recording device counts the radioactivity appearing in the aorta as a
function of time. Eventually, of course, the counting rate (the number
of radioactive disintegrations per second) levels off when the indicator
sample has become mixed uniformly in the blood stream. From the shape of
the curve on which the data are recorded during the measurements taken
before that time, the operator calculates the heart output per second.
[Illustration: _In this cardiac output study a probe is positioned over
the heart and the passage of iodine-131 labeled human serum albumin
through this area is recorded._]
Obstetricians caring for expectant mothers use red cells tagged with
⁵¹Cr to find the exact location of the placenta. For example, in the
condition known as _placenta previa_, the placenta—the organ within the
uterus by which nourishment is transferred from the mother’s blood to
that of the unborn child—may be placed in such a position that fatal
bleeding can occur. A radiation-counting instrument placed over the
lower abdomen gives information about the exact location of the
placenta. If an abnormal situation exists, the attending physician is
then alert and ready to cope with it. The advantages of chromium over
iodine-131, which has also been used, are that smaller doses are
required, and that there is no transfer of radioactivity to the fetal
circulation.
Still another common measurement using ⁵¹Cr-labeled red blood cells is
the determination of the amount and location of bleeding from the
gastrointestinal tract (the stomach and bowels). The amount is found by
simple measurement of chromium in the blood that appears in the stools.
To find the location is slightly more complicated. The intestinal
contents are sampled at different levels through an inserted tube, and
the radiation of the samples determined separately.
Finally, gastrointestinal loss of protein can be measured with the aid
of ⁵¹Cr-labeled blood serum. The serum is treated with CrCl₃ and then
injected into a vein. In several very serious ailments there is serious
loss of blood protein through the intestines. In these conditions the
⁵¹Cr level in the intestinal excretions is high, and this alerts the
doctor to apply remedial measures.