Apheresis is a technique in which blood is withdrawn from a donor, one or more substances are removed from the blood, and the blood is then returned to the donor. Most present-day apheresis equipment is based on separation of blood components by differential centrifugation. Apheresis has two major applications. One is the removal of certain blood components (e.g., platelets) to be used for transfusion into another patient. Apheresis has permitted collection of blood components in greater quantity and more frequently than ordinary phlebotomy. In addition, when all components come from the same donor, the risk of hepatitis is less than if blood from multiple donors is used. This technique is the major source of many rare blood group antibodies and certain blood components. The second major use of apheresis is the direct therapeutic removal of harmful substances or blood components from a patient. The most common application is removal of abnormal proteins from serum by plasmapheresis in patients with the hyperviscosity syndrome (associated with Waldenstцm’s macroglobulinemia or myeloma). Apheresis has also been used to remove immune complexes from patients with various disorders associated with autoimmune disease. However, use of apheresis in many of these conditions is considered experimental.

Blood transfusion filters

For many years it has been the custom during blood transfusion to place a filter with a 170-µ pore size between the blood donor bag and the patient. This trapped any blood clots large enough to be visible that might have formed in the donor bag. In the 1960s it was recognized that nonvisible clots or microaggregates of platelets, fibrin, degenerating leukocytes, and other debris frequently were present in stored bank blood and could produce microembolization with pulmonary and cerebral symptoms. The most severe condition was pulmonary insufficiency associated with open-heart surgery or massive blood transfusion for severe trauma. It was found that microfilters with pore sizes of 20-40 µ could trap most of these microaggregates. Some publications advocated using such a filter for every transfusion (the filter can accept 5-10 units of whole blood or packed cells before it must be replaced). Others believe that transfusions limited to one or two units do not subject the lungs to sufficient embolized material to necessitate use of a microfilter. Originally there was a flow rate problem with the 20 µ filters, but newer models have better flow characteristics. A substantial number of platelets are trapped by filter sizes less than 40 µ. However, blood more than 2 days old does not contain viable platelets.

Blood volume measurement

Blood volume measurement is useful in certain circumstances: (1) to differentiate anemia due to hemodilution from anemia due to RBC deficiency, (2) to differentiate polycythemia from dehydration, and (3) to quantitate blood volume for replacement or for therapeutic phlebotomy purposes. Blood volume measurement is discussed in this chapter since the most frequent indication for transfusion therapy is to replace depleted blood volume. This most commonly arises in association with surgery or from nonsurgical blood loss, acute or chronic. Immediately after an acute bleeding episode, the Hb and hematocrit values are unchanged (because whole blood has been lost), even though the total blood volume may be greatly reduced, even to the point of circulatory collapse (shock). With the passage of time, extracellular fluid begins to diffuse into the vascular system to partially restore total blood volume. Since the hematocrit is simply the percentage of RBCs compared with total blood volume (total blood volume being the RBC mass plus the plasma volume), this dilution of the blood by extracellular fluid means that the hematocrit value eventually decreases, even while total blood volume is being increased (by extracellular fluid increasing plasma volume). Hemodilution (and thus the drop in hematocrit) may be hastened if the patient has been receiving intravenous (IV) fluids. Serial hematocrit determinations (once every 2-4 hours) may thus be used as a rough indication of blood volume changes. It usually takes at least 2 hours after an acute bleeding episode for a significant drop in hematocrit value to be demonstrated. Sometimes it takes longer, even as long as 6-12 hours. The larger the extracellular blood loss, the sooner a significant hematocrit value change (> 2%) is likely to appear.

Previous dehydration, a low plasma protein level, or both will tend to delay a hematocrit drop. Besides the uncertainty introduced by time lag, other conditions may affect the hematocrit value and thus influence its interpretation as a parameter of blood volume. Anemias due to RBC hemolysis or hematopoietic factor deficiencies such as iron may decrease RBC mass without decreasing plasma or total blood volume. Similarly, plasma volume may be altered in many situations involving fluid and electrolyte imbalance without changing the RBC mass. Obviously, a need exists for accurate methods of measuring blood volume.

Blood volume methods. The first widely used direct blood volume measurement technique was Evan’s Blue dye (T-1824). After IV injection of the dye, the amount of dilution produced by the patient’s plasma was measured, and from this the plasma volume was calculated. At present, radioisotopes are the procedure of choice. These methods also are based on the dilution principle. Chromium 51 can be used to tag RBCs; a measured amount of the tagged RBCs is then injected into the patient. After equilibration for 15 minutes, a blood sample is obtained and radioactivity is measured. Since the tagged RBCs have mixed with the patient’s RBCs, comparison of the radioactivity in the patient’s RBCs with the original isotope specimen that was injected reveals the amount that the original isotope specimen has been diluted by the patient’s RBCs; thus, the patient’s total RBC mass (RBC volume) may be calculated. If the RBC mass is known, the plasma volume and total blood volume may be derived using the hematocrit value of the patient’s blood. Another widely used method uses serum albumin labeled with radioactive iodine (RISA). This substance circulates in the plasma along with the other plasma proteins. Again, a measured amount is injected, a blood sample is withdrawn after a short period of equilibration, and the dilution of the original injected specimen is determined by counting the radioactivity of the patient’s plasma. Plasma volume is provided by RISA measurement; RBC volume must be calculated using the patient’s hematocrit value. There is no doubt that isotope techniques are much more accurate than the hematocrit value for estimating blood volume. Nevertheless, there are certain limitations to isotope techniques in general and specific limitations to both 51Cr and RISA (see box below).

Assay problems. The main drawback of blood volume techniques is the lack of satisfactory reference values. Attempts have been made to establish reference values for males and females in terms of height, weight, surface area, or lean body mass. Unfortunately, when one tries to apply these formulas to an individual patient, there is never any guarantee that the patient fits whatever category of normal persons that the formula was calculated from. The only way to be certain is to have a blood volume measurement from a time when the patient was healthy or before the bleeding episode. Unfortunately, this information is usually not available. Another drawback is the fact that no dilution method can detect bleeding that is going on during the test itself. This is so because whole blood lost during the test contains isotope in the same proportion as the blood remaining in the vascular system (a diminished isotope dose in a

Factors That Can Adversely Affect the Conditions Necessary for Accurate Blood Volume Results
1. All isotope must be delivered into the fluid volume being measured:
Extravasation outside vein (falsely increased result)

2.Need uniform mixing of isotope throughout fluid volume being measured:

Capillary sequestration in shock
Slow mixing in congestive heart failure
Focal sequestration in splenomegaly

3. Need uniform mixing of isotope throughout fluid volume being measured:
Male vs. female
Obesity or malnourishment

diminished blood volume) in contrast to the situation that would prevail if bleeding were not going on, when the entire isotope dose would remain in a diminished blood volume. Fortunately, such active bleeding would have to be severe for test results to be materially affected. Another problem is dependence on hematocrit value when results for RISA are used to calculate RBC mass or data from 51Cr are used to obtain plasma volume. It is well established that hematocrit values from different body areas or different-sized vessels can vary considerably, and disease may accentuate this variation. The venous hematocrit value may therefore not be representative of the average vascular hematocrit value.

Single-tag versus double-tag methods. All authorities in the field agree that blood volume determination combining independent measurement of RBC mass by 51Cr and plasma volume by RISA is more accurate than the use of either isotope alone. Nevertheless, because both isotopes must be counted separately with special equipment, most laboratories use only a single isotope technique. Most authorities concede that 51Cr has a slightly better overall accuracy than RISA, although some dispute this strongly. However, most 51Cr techniques call for an extra venipuncture to obtain RBCs from the patient for tagging, plus an extra 30-minute wait while the actual tagging of the cells takes place. In addition, tagged RBCs (and their radioactivity) remain in the circulation during the life span of the cells. The main advantages of RISA are the need for one less venipuncture than 51Cr and the fact that RISA procedures can be done in less than half the time of 51Cr studies. The error with RISA blood volume has reached 300 ml in some studies, although most determinations come much closer to double-isotope results. In patients with markedly increased vascular permeability, significant quantities of RISA may be lost from blood vessels during the test, which may produce additional error. Severe edema is an example of such a situation. Nevertheless, even under adverse conditions, RISA (and 51Cr) represents a decided advance over hematocrit values for estimating blood volume.

Central venous pressure measurement. Central venous pressure (CVP) is frequently used as an estimate of blood volume status. However, CVP is affected by cardiac function and by vascular resistance as well as by blood volume. Circumstances in which the CVP does not accurately reflect the relationship between blood quantity and vascular capacity include pulmonary hypertension (emphysema, embolization, mitral stenosis), left ventricular failure, and technical artifacts due to defects in catheter placement and maintenance.