Statistical variations of ultrasound signals backscattered from flowing blood

The statistical distributions of ultrasonic signals backscattered from blood have recently been used to characterize hemodynamic properties, such as red blood cell (RBC) aggregation and blood coagulation. However, a thorough understanding of the relationship between blood properties and the statistical behavior of signals backscattered from flowing blood is still lacking. This prompted us to use the statistical parameter to characterize signals backscattered from both whole blood and RBC suspensions at different flow velocities (from 10 to 60 cm/s) and hematocrits (from 20% to 50%) under a steady laminar flow condition. The Nakagami parameter, scaling parameter, backscatter amplitude profile and flow velocity profile across a flow tube were acquired using a 10 MHz focused ultrasonic transducer. The backscattered signal peaked approximately at the centerline of the flow tube due to the effects of RBC aggregation, with the peak value increasing as the flow velocity of whole blood decreased. The Nakagami parameter increased from 0.45 to 0.78 as the flow velocity increased from 10 to 60 cm/s. The probability density function (PDF) of signals backscattered from flowing whole blood conformed with a pre-Rayleigh distribution. The Nakagami parameter was close to 1 for signals backscattered from RBC suspensions at all the flow velocities and hematocrits tested, for which the PDF was Rayleigh distributed. These differences in the statistical distributions of backscattered signals between whole blood and RBC suspensions suggest that variations in the size of dynamic scatterers in the flow affect the shape of the backscattered signal envelope, which should be considered in future statistical models used to characterize blood properties. (E-mail: j648816n@ms23.hinet.net and shyhhau@cycu.edu.tw)     

On the use of photoacoustics to detect red blood cell aggregation

The feasibility of detecting red blood cell (RBC) aggregation with photoacoustics (PAs) was investigated theoretically and experimentally using human and porcine RBCs. The theoretical PA signals and spectra generated from such samples were examined for several hematocrit levels and aggregates sizes. The effect of a finite transducer bandwidth on the received PA signal was also examined. The simulation results suggest that the dominant frequency of the PA signals from non-aggregated RBCs decreases towards clinical frequency ranges as the aggregate size increases. The experimentally measured mean spectral power increased by ~6 dB for the largest aggregate compared to the non-aggregated samples. Such results confirm the theoretical predictions and illustrate the potential of using PA imaging for detecting RBC aggregation.OCIS codes: (110.5125) Photoacoustics, (170.1470) Blood or tissue constituent monitoring     

Preoperative red cell production in patients undergoing weekly autologous blood donation

BACKGROUND: Modest autologous blood donation programs involving weekly phlebotomy and threshold hematocrits for blood donation higher than 33 percent are frequently used in patients scheduled for elective cardiac surgery. This study was performed to determine the gain in red cells (RBCs) obtained with such a program. STUDY DESIGN AND METHODS: The blood bank and medical records of 225 adult patients (194 men, 31 women; mean age, 57 years [range, 18–77]) who donated blood for autologous use in elective cardiac surgery during a 3-year period were reviewed. Preoperative RBC production was estimated by the total volume of RBCs donated minus the change in circulating RBC volume between the first donation and the day before surgery. RESULTS: A total of 604 blood units were donated (2.7 units/patient; range, 1–3). The mean volume of RBCs donated was 522 mL (range, 171–732). Mean RBC production (over baseline RBC production) was 351 mL (range, 9–719), or 19 percent (range, 0.5-40) of the circulating RBC volume at baseline. CONCLUSION: A modest autologous blood donation program using three phlebotomies at weekly intervals and a threshold hematocrit for blood donation of 36 percent yields an average of 351 mL (range, 9–719) of RBCs. This is equivalent to 2 units (range, 0.5-4) of allogeneic packed RBCs at 180 mL per unit.     

Measurement of the Doppler Power of Flowing Blood Using Ultrasound Doppler Devices

Measurement of the Doppler power of signals backscattered from flowing blood (henceforth referred to as the Doppler power of flowing blood) and the echogenicity of flowing blood have been used widely to assess the degree of red blood cell (RBC) aggregation for more than 20 y. Many studies have used Doppler flowmeters based on an analogue circuit design to obtain the Doppler shifts in the signals backscattered from flowing blood; however, some recent studies have mentioned that the analogue Doppler flowmeter exhibits a frequency-response problem whereby the backscattered energy is lost at higher Doppler shift frequencies. Therefore, the measured Doppler power of flowing blood and evaluations of RBC aggregation obtained using an analogue Doppler device may be inaccurate. To overcome this problem, the present study implemented a field-programmable gate array-based digital pulsed-wave Doppler flowmeter to measure the Doppler power of flowing blood, in the aim of providing more accurate assessments of RBC aggregation. A clinical duplex ultrasound imaging system that can acquire pulsed-wave Doppler spectrograms is now available, but its usefulness for estimating the ultrasound scattering properties of blood is still in doubt. Therefore, the echogenicity and Doppler power of flowing blood under the same flow conditions were measured using a laboratory pulser–receiver system and a clinical ultrasound system, respectively, for comparisons. The experiments were carried out using porcine blood under steady laminar flow with both RBC suspensions and whole blood. The experimental results indicated that a clinical ultrasound system used to measure the Doppler spectrograms is not suitable for quantifying Doppler power. However, the Doppler power measured using a digital Doppler flowmeter can reveal the relationship between backscattering signals and the properties of blood cells because the effects of frequency response are eliminated. The measurements of the Doppler power and echogenicity of flowing blood were compared with those obtained in several previous studies.     

Effect of inositol hexaphosphate–loaded red blood cells (RBCs) on the rheology of sickle RBCs

BACKGROUND: The recent in vitro demonstration that inositol hexaphosphate–loaded red blood cells (IHP‐RBCs) may reduce the risks of sickling of sickle RBCs (SS RBCs) exposed to hypoxia make these modified RBCs potentially useful in transfused sickle cell anemia (SCA) patients. STUDY DESIGN AND METHODS: Hemorheologic properties of IHP‐RBCs, normal RBCs (AA RBCs), SS RBCs, SS RBCs plus AA RBCs, and SS RBCs plus IHP‐RBCs were compared under normoxia and/or after hypoxic challenges. RESULTS: Although IHP‐RBCs have reduced deformability compared with SS RBCs or AA RBCs, IHP‐RBCs exhibited lower aggregability than AA RBCs and SS RBCs and, when mixed with SS RBCs, the aggregation level was below the one of SS RBCs alone or SS RBCs plus AA RBCs. Blood viscosity of SS RBC plus IHP‐RBC suspension was lower than the viscosity of SS RBCs alone and greater than viscosity of SS RBCs plus AA RBCs. The hypoxic challenge was detrimental for deformability and viscosity of SS RBCs alone or SS plus AA RBC suspension but not for SS plus IHP‐RBC suspension. CONCLUSION: Our results support the fact that IHP‐RBCs could be useful in SCA by decreasing RBC aggregation and blunting the adverse effects of hypoxia on RBC deformability and blood viscosity.     

Vancomycin-induced red cell aggregation

Vancomycin, an antibiotic similar in structure to ristocetin, is used to treat staphylococcal infections. However, vancomycin-induced hemagglutination complicated red cell (RBC) serologic testing in the blood bank. At concentrations greater than 3.0 mg per ml, vancomycin caused spontaneous macroscopic red cell (RBC) aggregation; concentrations of 2.0 and 2.5 mg per ml were associated with weakly positive aggregation with anti-IgG and polyspecific antiglobulin reagents negative with anti-complement; and concentrations less than 1.5 mg per ml had no apparent effect. Ficin-treated RBCs demonstrated negative reactions with the antiglobulin reagents. Vancomycin-induced aggregation was reversed partially with 0.2 M trisodium citrate, and supernatant transfer studies showed that normal RBCs retained a significantly (p less than 0.025) greater percentage of vancomycin than did ficin-treated RBCs. Vancomycin causes the aggregation of RBCs, which can be a source of confusion in the blood bank. The mechanism(s) through which vancomycin enhances aggregation may be related to its polycationic properties and to its direct protein binding to the RBC membrane, although other nonimmunologic mechanisms may be operative.     

Paroxysmal nocturnal hemoglobinuria and the transfusion of washed red cells. A myth revisited

Paroxysmal nocturnal hemoglobinuria (PNH) is an uncommon, acquired clonal stem cell disorder primarily affecting red cells that have an abnormal sensitivity to complement lysis. Since 1948, the use of saline-washed red cells (WRBCs) has been advocated to minimize hemolysis after transfusion to patients with PNH. Thirty-eight years of experience (1950 through 1987) with patients who had PNH were reviewed. Twenty-three patients with a positive Ham's test had been transfused with 556 blood components, including 431 RBC products: 94 units of whole blood, 208 units of packed RBCs, 80 units of white cell-poor RBCs, 38 units of WRBCs, 5 units of frozen RBCs, and 6 units of intraoperatively salvaged RBCs. Only one documented episode of posttransfusion hemolysis related to the underlying diagnosis of PNH was found, and it was associated with the transfusion of a unit of type O whole blood to an AB-positive individual. This unit contained ABO-incompatible plasma; this case was similar to one in an earlier report from which originated the recommendation for using WRBCs. The posttransfusion increment in hemoglobin concentration in patients receiving ABO-identical packed RBCs was comparable to that in patients receiving frozen or washed RBCs. These findings indicate that the use of WRBCs is unnecessary and that patients with PNH should be transfused with group-specific blood and blood products.     

Effects of aggregation of red cells and linear velocity gradients on the correlation-based method for quantitative IVUS blood flow at 20 MHz

Recent computer simulations suggest that the presence of aggregates of red blood cells (RBCs), at random angles and lengths, does not affect the measurements of blood flow transverse to the ultrasound (US) beam direction using a correlation-based method and an intravascular (IV) US array catheter. However, in case of aggregates of RBCs aligned with the flow, measurements of simulated blood velocity are affected. Blood velocity gradients were also shown not to influence the correlation-based method for blood velocity estimation. The objective of this study was to quantify the influence of aggregates of RBCs and blood velocity gradients on the correlation-based method during in vitro experiments. For this purpose, measurements were performed on washed RBCs (no aggregation), normal human blood, and two types of diseased blood in which a lower or a higher level of aggregation was present. The decorrelation pattern of a circular US transducer as a function of transverse blood flow was studied using a Couette system. Changing the shear rate of the Couette system modified the aggregation level of RBCs and the velocity gradient. With the exception of the results at low shear rates and abnormally high aggregation levels, agreements were found between the autoconvolution of the acoustical beam (reference curve) and the radiofrequency (RF) decorrelation patterns. For the high shear rate present in coronary arteries, the correlation-based method for blood flow estimation should not be influenced by these phenomena.     

The societal unit cost of allogenic red blood cells and red blood cell transfusion in Canada

BACKGROUND: There is a dearth of information about the cost of allogenic red blood cells (RBCs) and RBC transfusion in Canada in the aftermath of the Canadian blood system reorganization and the introduction of various safety measures. The unit cost of allogenic RBCs and RBC transfusion in Canada in 1994 was estimated at 152.17 US dollars. The objective of this study was to determine the unit cost of allogenic RBC transfusion in Canada from a societal perspective. STUDY DESIGN AND METHODS: A cost-structure analysis using the cost information from 2001 through 2002 was used. Costs of blood collection, production, distribution, delivery (hospital transfusion service processing and patient administration), transfusion reaction management, and opportunity cost of donor's time were included in the analysis. Canadian Blood Services and Héma-Québec supplied the data for collection, production, and distribution stages. Delivery and transfusion reaction costs were collected from eight hospitals across six Canadian provinces. In-patient costs were assessed for the intensive care unit, emergency, general medicine ward, and operating room. RESULTS: The aggregate mean societal unit cost of RBCs transfused on an inpatient basis in 2002 was 264.81 US dollars (95% confidence interval [CI], 256.29 dollars-275.65 dollars). The mean cost of blood collection, production, and distribution was 202.74 US dollars (95% CI, 199.63 dollars-204.31 dollars), the mean opportunity cost of donor time was 18.21 US dollars (95% CI, 17.11 dollars-21.63 dollars), the mean cost of hospital transfusion service processing was 16.65 US dollars (95% CI, 13.50 dollars-19.79 dollars), of RBC transfusion was 26.92 US dollars (95% CI, 25.33 dollars-28.52 dollars), and of transfusion reaction management was 0.29 US dollars(95% CI, 0.22 dollars-0.36 dollars). There were substantial variations in hospital transfusion service processing and RBC transfusion costs across hospitals. CONCLUSION: The societal unit cost of RBC transfusion has doubled since 1994 to 1995. Further increases in unit costs would be expected as additional safety measures are introduced. This will have important financial implications for treating patient populations that require a high level of RBC transfusions.     

Transfusion of stored red blood cells adhere in the rat microvasculature

BACKGROUND: Ex vivo storage of red blood cells (RBCS) for transfusions is associated with a “storage lesion,” which decreases RBC deformability and increases RBC adhesiveness to vascular endothelium. This may impair microcirculatory flow with deleterious effects on oxygen delivery after transfusion. Previous studies have shown that human RBCs adhere to endothelial monolayers in vitro with prolonged storage and is reduced by prestorage leukoreduction (LR). The objective of this study was to determine whether duration of RBC storage and LR influence RBC adhesion in vivo in capillaries. STUDY DESIGN AND METHODS: Rat RBCs were collected and stored in CPDA‐1 under standard blood bank conditions. Three RBC products were compared: 1) fresh RBCs, less than 24 hours of storage (n = 6); 2) nonleukoreduced (NLR) RBCs stored for 7 days (n = 6); and 3) prestorage LR RBCs stored for 7 days (n = 6). RBCs were labeled with fluorescein isothiocyanate (FITC) 24 hours before transfusion and reinjected in an isovolemic manner into healthy rats. The FITC‐labeled RBCs were visualized in the extensor digitorum longus muscle using intravital video microscopy (20× magnification). The number of RBCs adherent in capillaries was counted 1 hour after transfusion in 10 random fields and the median values were compared with one‐way analysis of variance. RESULTS: Stored RBCs showed increased levels of adherence in capillaries compared to their fresh counterparts (p < 0.05). Prestorage LR decreased RBC adherence to levels equivalent to those of fresh RBCs (p < 0.05 for stored LR vs. stored NLR). CONCLUSION: Rat RBCs stored under conditions that closely mimicked clinical transfusion adhere in capillaries. The decreased RBC adherence with LR suggest a direct effect of white blood cells or their byproducts on RBC deformability and/or adhesiveness to microvascular endothelium. Further study will examine the mechanism of adherence and the impact it has on microcirculatory flow and oxygen delivery in the critically ill host.     

Investigation of red cell size distribution histograms of normal male and female subjects

Size distribution histograms characterizing the red blood cell (RBC) populations of normal subjects are provided. Separate haemocytometric reference ranges were established for apparently healthy male and female adults by using an Ortho ELT 800 Hematology Analyzer, which is based on the principle of measuring light scattering intensity (LSI). Effects of sphering RBCs on some characteristics of size distribution histograms were also investigated. The absolute distribution width at half peak height (ADW0.5) of RBC size distribution histograms of sphered RBCs is shown to be approximately 40% lower than for unsphered RBCs. The mean RBC size distribution histogram determined for the group of female subjects was shown to be significantly shifted to the right in comparison with the mean size distribution histogram calculated for males, indicating a sex-related difference. The shift is concordant with a significantly higher fraction of microcytes in male subjects, as well as with lower mean corpuscular volume (MCV) values in males compared with females.     

Erythropoiesis and artificial blood substitution with a perfluorocarbon-polyol.

Rats were perfused with Fluosol-43, a perfluorotributylamine-pluronic F-68 emulsion, to produce hematocrits of about 6% and the mechanisms of blood cell replacement were studied. The packed RBC volume and number of RBCs per cubic millimeter increased gradually as the time after perfusion was completed until reaching the preperfusion level at about 1 week. The WBCs per cubic millimeter, which were not as severely depleted because of their vast storage compartment, increased to preperfusion levels by 24 hr and continued to increase through 7 days. Plasma did not show significant Ep activity until 48 to 72 hr after perfusion was concluded. Thus, although 90% of the RBCs were removed, elevated erythropoietin activity was not detected immediately because the perfluorocarbon compound replaced the capacity of the RBCs as an oxygen and carbon dioxide carrier.     

Band 3 and glycophorin are progressively aggregated in density-fractionated sickle and normal red blood cells. Evidence from rotational and lateral mobility studies.

Band 3 aggregation in the plane of the red blood cell (RBC) membrane is postulated to be important in the pathophysiology of hemolysis of dense sickle and normal RBCs. We used the fluorescence photobleaching recovery and polarized fluorescence depletion techniques to measure the lateral and rotational mobility of band 3, glycophorins, and phospholipid analogues in membranes of density-separated intact RBCs from seven patients with sickle cell disease and eight normal controls. The fractions of laterally mobile band 3 and glycophorin decreased progressively as sickle RBC density increased. Normal RBCs also showed a progressive decrease in band 3 fractional mobility with increasing buoyant density. Rapidly rotating, slowly rotating, and rotationally immobile forms of band 3 were observed in both sickle and normal RBC membranes. The fraction of rapidly rotating band 3 progressively decreased and the fraction of rotationally immobile band 3 progressively increased with increasing sickle RBC density. Changes in the fraction of rotationally immobile band 3 were not reversible upon hypotonic swelling of dense sickle RBCs, and normal RBCs osmotically shrunken in sucrose buffers failed to manifest band 3 immobilization at median cell hemoglobin concentration values characteristic of dense sickle RBCs. We conclude that dense sickle and normal RBCs acquire irreversible membrane abnormalities that cause transmembrane protein immobilization and band 3 aggregation. Band 3 aggregates could serve as cell surface sites of autologous antibody binding and thereby lead to removal of dense sickle and normal (senescent) RBCs from the circulation.     

Effect of 24-hour storage at 25°C on the in vitro storage characteristics of CPDA- 1 packed red cells

BACKGROUND: Packed red cells (RBCs) warmed above 10 degrees C are generally discarded. Few data exist on the degree of accelerated metabolism and increased hemolysis of packed RBCs allowed to warm. STUDY DESIGN AND METHODS: Twenty-four CPDA-1 packed RBC units were combined in 3-unit pools and subdivided into 2 test units and a control unit. One test unit from each pool was warmed to 25 degrees C for 24 hours on Day 6 and the other test unit was warmed on Day 20; control units were maintained at 1 to 6 degrees C. RBC and supernatant chemistries and RBC morphology were measured weekly (Days 0, 7, 14, 21, and 28) and on the day before warming (Days 6 and 20). RESULTS: Warming CPDA-1 packed RBCs accelerated the catabolism of glucose 10-fold and produced concentrations of glucose, lactate, and ATP after 25 days of storage that were equivalent to those in unwarmed units at 35 days. Supernatant sodium and potassium concentrations were corrected partially with warming. RBC morphology transiently normalized with warming and without increased hemolysis; no bacteria growth was detected. CONCLUSION: One day of 25 degrees C storage of CPDA-1 packed RBCs accelerates essential metabolite break-down equivalent to 10 days of storage at 1 to 6 degrees C. It does not appear to matter whether the packed RBCs are warmed on Day 6 or Day 20. This information may be useful in determining the acceptability of blood allowed to warm above 10 ° C.     

Particulate matter phenomenon: adverse event data and the effect of leukofiltration

BACKGROUND: In February 2002, a multiorganizational task force investigated blood center reports of unusual particulate matter (PM) visible in packed red blood cells (RBC). A cohort study assessed increase in adverse events (AEs) related to this phenomenon, as well as the effect of post-leukofiltration (LF) on PM. METHODS: Two blood centers assessed AEs in patients transfused with RBCs having visible PM (classified as normal by-products of RBC preparation), PM RBCs subsequently LF, or RBCs without visible PM, and the effect of LF on PM removal. RESULTS: There was no difference in AEs in patients transfused with PM RBCs with or without LF compared to patients transfused with RBCs without visible PM. Subsequently filtered PM RBCs had acceptable residual WBC counts and median platelet removal of 92%. CONCLUSION: Transfusion of RBCs with visible PM or RBCs subsequently LF does not appear to increase the risk of an AE. LF use on PM RBCs results in the PM removal without adversely affecting filter performance. The lack of evidence of an increase in AE reports does not mean that there is no effect, and there may be a baseline AE rate attributable to PM which has not been thoroughly researched.     

Neonatal mortality following transfusion of red cells with high plasma potassium levels

The death of a neonatal infant following cardiac surgery and the transfusion of packed red cells (RBCs) with high plasma potassium levels is reported. The patient had been diagnosed at 2 weeks of age as having multiple cardiac malformations. During cardiopulmonary bypass surgery, multiple units of packed RBCs less than 5 days old were transfused. In response to a "stat" order and after depletion of stock units prepared for neonatal usage, a 32-day-old unit of packed RBCs was issued for transfusion. After approximately 60 mL was rapidly transfused from this unit, the patient experienced cardiac arrest. Serum potassium concentration after transfusion and before death was 8.9 mmol per L. Plasma potassium concentration in the remainder of the transfused packed RBC unit was approximately 60 mmol per L. A model was created to calculate the posttransfusion plasma potassium concentration, and close correlation was found between the model and the observed potassium concentration, which assumes that the potassium load had not yet been distributed to the extravascular and intracellular fluid compartments. It is concluded that the transfusion of relatively large volumes of RBCs be limited to fresh packed RBCs or to packed RBCs that have been saline washed, to minimize the complications of electrolyte disturbances.     

Supernates from stored red blood cells inhibit platelet aggregation

BACKGROUND: Previously, we reported that red blood cells (RBCs) stored in AS‐5 accumulated proinflammatory substances during storage. We observed in those studies that supernates from nonleukoreduced (NLR) RBCs reduced mean anti‐CD41a‐fluorescein isothiocyanate (FITC) fluorescence on platelets (PLTs), indicative of decreased expression of glycoprotein (GP)IIb/IIIa on the PLT membrane. The objective of this study was to determine if supernates from stored RBCs impaired PLT aggregation as a consequence of reduction in GPIIb/IIIa expression. STUDY DESIGN AND METHODS: Leukoreduced (LR) and NLR RBC units were prepared in AS‐5 and stored at 1 to 6°C for 6 weeks. Supernates from RBC samples collected every 2 weeks were mixed with freshly collected type‐matched blood and incubated for 30 minutes at 37°C. PLTs in each incubated blood sample were evaluated for GPIIb/IIIa expression by flow cytometry and for aggregation response to collagen by whole blood aggregometry. RESULTS: Supernates from stored NLR RBCs reduced CD41a‐FITC fluorescence on PLTs by 15% to 31%. A reduction in fluorescence was induced by supernates of RBCs stored for 14 days and increased as storage time increased. Supernates from Day 42 NLR RBCs reduced the mean amplitude of PLT aggregation by 31% compared to Day 0 supernates and lengthened the time before onset of aggregation by 21%. In addition, amplitude correlated directly and lag time correlated inversely with CD41a‐FITC fluorescence in all samples. Supernates from prestorage LR RBCs did not affect PLT CD41a‐FITC fluorescence or aggregation response. CONCLUSIONS: Substances that decrease expression of GPIIb/IIIa and inhibit PLT aggregation accumulate in NLR RBCs. Accumulation of this material is prevented by leukoreduction.     

Measurements on the forward light scattering intensities of monodisperse latex particles and sphered or unsphered red blood cells

Characteristics with respect to light scattering intensity (LSI) were measured on latex spheres and on sphered and unsphered red blood cells (RBC). From the discrepancies in LSI signals from polymer spheres and RBCs it is obvious that latices cannot be used for absolute calibration of RBC sizes. Sphering of RBCs did not give rise to a closer resemblance. Monodisperse latex spheres produce extremely narrow size distribution histograms. Regarding these sharp peaks and the long-term stability, polymer spheres offer definite advantages if compared with blood controls to which preservatives were added. A shift from the RBC size distribution histogram, caused by gradually developing instrument drift, can be easily detected and corrected at an early stage.     

Concanavalin A-mediated binding and sphering of human red blood cells by homologous monocytes

Human red blood cells sensitized with concanavalin A became bound to homologous peripheral blood monocytes. Binding occured at a concentration of 10(5) molecules of tetrameric Con A per red blood cell (RBC) and increased with additional Con A. RBC binding began within 5 min and was maximal at 90 min. Phagocytosis of sensitized RBCs was minimal. RBC attachment was prevented by 0.01 M alpha-methyl-D- mannopyranoside, and, once the RBC-monocyte rosette was established, bound RBCs were largely removed with this specific saccharide inhibitor of Con A. RBCs attached to monocytes became spherocytic and osmotically fragile. The recognition of concanavalin A (Con A)-coated RBCs was not mediated through the monocyte IgG-Fc receptor. These studies demonstrate that, like IgG and C3b, Con A is capable of mediating the binding of human RBCs to human monocytes. Red cells so bound are damaged at the monocyte surface.     

Red cell aggregation and the echogenicity of whole blood.

To study the relationship between red cell aggregation and whole blood echogenicity, red cell aggregation was quantitated by a photometric method, whole blood echogenicity was quantitated by videodensitometry and sedimentation rate was quantitated by a modified Westergren method. Changes in red cell aggregation were produced by alterations in the hematocrit. The results showed that red cell aggregation increased in a linear fashion with increases in hematocrit. The sedimentation rate decreased in a linear manner with increases in hematocrit. Whole blood echogenicity showed a biphasic response, with an initial increase in echogenicity, peaking at hematocrits varying from 14-24% and decreasing thereafter. Over the physiologic range of hematocrits, an increase in the formation of red cell aggregates is associated with a decrease in the echogenicity of whole blood. Thus, red cell aggregates were not visible using our ultrasound equipment at physiologic hematocrits, and the echo contrast in blood under our experimental conditions at these hematocrits must represent either plasma spaces, platelet aggregates or possibly white cell aggregates. The association between spontaneous contrast and a propensity for thromboembolism imply that platelet aggregates are the most likely origin of in vivo echo contrast in flowing blood.