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.
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.
Cobalt-60
Vitamin B₁₂ is a cobalt compound. Normally the few milligrams of B₁₂ in the body are stored in the liver and released to the blood stream as needed. In pernicious anemia, a potentially fatal but curable disease, the B₁₂ content of the blood falls from the usual level of 300-900 micromicrograms per milliliter (ml) to 0 to 100 micromicrograms per ml. The administration of massive doses of B₁₂ is the only known remedy for this condition.