Anesth Analg 2008; 106:1603-1604
© 2008 International Anesthesia Research Society
doi: 10.1213/ane.0b013e31816bbd17
EDITORIAL
Blood, Plasma, and Red Blood Cell Volumes in Intensive Care Unit Patients
Robert Hahn, MD, PhD
From the Clinical Research Center, Södertälje County, 152 86 Södertälje, Sweden.
Address correspondence and reprint requests to Robert G. Hahn, MD, PhD, Head, Clinical Research Center, Södertälje County, 152 86 Södertälje, Sweden. Address e-mail to r.hahn{at}telia.com.
Most clinicians hesitate to measure the blood volume in intensive care unit (ICU) patients, since the procedure requires much time, multiple blood sampling, and the handling of complex equipment and/or radioactive material. Instead, the volume of the circulating blood is usually inferred from central hemodynamics, such as cardiac output. An attempt to popularize direct measurement was made in the late 1990s following the introduction of indocyanine pulse spectrophotometry.1 Recently, a New York-based company, Daxor Corporation, has begun to market a reasonably simple kit and apparatus, BVA-100, which quantifies the plasma and blood volumes in a more classical way.
Their approach is to assess the dilution of radioactive iodine tagged to albumin. The BVA-100 measures the plasma volume from centrifuged blood and then estimates the blood volume based on subsequent measurement of the peripheral hematocrit. The plasma volume is obtained as the ratio between the injected tracer dose and its activity in the plasma at the time of injection. The latter parameter cannot be obtained directly, but must be calculated based on linear regression using the log-transformed tracer activity in five blood samples taken over 30 min after the injection.2 The reason for this difficulty is that between 5% and 10% of the plasma albumin is extravasated per hour, a rate that is much higher in inflammatory states associated with "capillary leakage"3 and in certain diseases such as diabetes.4 The rate of disappearance of the albumin per se provides information about the ICU patient that may occasionally be even more interesting than the blood volume. This is an asset of the iodine method, which has yet to be realized by the majority of clinicians.
Shortcomings of tracer methods like iodine-tagged albumin include the fact that the blood volume cannot be allowed to change much during the 30-min period of equilibration. If it gradually decreases, the figure for blood volume will be falsely large and the rate of disappearance of albumin too low. Expansion of the blood volume creates the opposite errors. Moreover, the half-life of the isotope is several days, which makes it problematic to repeat the test in sick patients too often.
The BVA-100 was used in the study by Takanishi et al. published in the present issue of Anesthesia & Analgesia.5 The question of whether the blood volume adds important information in the care of ICU patients is indeed relevant. However, the Hawaii-based group chose to study an associated issue: namely, whether the body hematocrit is assessed correctly by routine laboratory tests. If it is not correctly assessed, erroneous choices can be made between providing patients with clear fluids or erythrocytes.
Takanishi et al. studied 40 ICU patients with varying diagnoses and found a marked difference between the hematocrit inferred from the red blood cell volume and the hematocrit measured directly in peripheral blood. The mean difference was 3.4% points, with a standard deviation of 7.8. Thus, the measured peripheral hematocrit showed systematically lower values, and there was considerable variability between the two ways of assessing this parameter. The difference increased for higher hematocrits, and vice versa. Thus, if the hematocrit derived from the red blood cell volume is correct and the peripheral hematocrit incorrect, we would under-transfuse ICU patients with low hematocrits.
The observation that the peripheral hematocrit differs from the total body hematocrit is old. Many studies performed in the 1950s reported that deducing circulating red blood cell volume from measurement of plasma volume and peripheral venous hematocrit yielded a higher value than when red cell volume was measured by a tracer.6 The difference, which is usually called the f ratio, amounted to 9% and varied little among studies. However, the BVA-100 automatically corrects for this discrepancy, which otherwise would operate to reduce the difference reported by Takanishi et al. Another error is "plasma trapping," which distorts the hematocrit when assessed by centrifugation of whole blood. This is not a problem when the hematocrit is calculated as the product of the red cell count and mean cell volume, as is the case in modern chemistry laboratories. The BVA-100 also corrects for plasma trapping, since, in fact, the blood is centrifuged to plasma before being analyzed.
The article by Takanishi et al. deserves some criticism. A weakness is that the BVA-100 does not measure the red cell volume. To evaluate whether the peripheral hematocrit is "correct," the authors compared the measured value with a theoretical hematocrit inferred from the plasma volume and anthropometric data derived from healthy adults having an ideal body weight. Although reasonably stable relationships between the plasma and red cell volumes may exist outside the hospital, a variety of factors involved in the pathophysiology and therapy of severe disease may disturb these correlations. For example, most anesthesia drugs induce vasodilatation, which expands the plasma volume within 15–20 min, in particular, if clear fluids are also provided.7 Changing from the sitting to the supine position expands the plasma volume by 10%, and the multiple blood sampling performed in ICU patients sooner or later creates anemia. Such factors impair the ability of reference tables to provide reliable figures for the blood, plasma, and red cell volumes in ICU patients. Frankly, a single IV infusion of clear fluid would be enough to create an acute positive difference between measured hematocrit and the normalized hematocrit presented here.
Despite all controversies, the return of blood volume as a clinical parameter in the ICU would be most welcome. In the years before the pulmonary artery catheter, when the blood volume was frequently a part of patient monitoring, the scope for research was much more limited than it is today. Therefore, several potential uses of its measurement must remain to be explored.
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Footnotes
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Accepted for publication January 17, 2008.
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REFERENCES
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