Discharge Hematocrit Research Articles

A Micropipette Which Allows in Situ Perfusion of Arterioles and Capillaries

In order to investigate capillary physiology, a glass micropipette system was developed that allowed in situ perfusion of microvessels as well as rapid changes of perfusion solutions. Theta tube (WPI, Inc.; 1.5-mm o.d. glass stock capillary tubing which is divided into two hemicylindrical sides by a central glass septum) was pulled to a smaller diameter of approx 300-600 μm and inserted into the shank of a sharpened cannulating micropipette tip constructed from large-bore glass stock (1.6 mm i.d.). The resulting dead volume between the end of the Theta supply tube and the tip of the outer cannulating tip was approximately 90 nl. The perfusate was driven in a circuit from a pressurized feed reservoir down one side of the Theta supply tube pipette and back through the second side into a reservoir maintained at a lower pressure. The pressure gradient between the two reservoirs established a high-volume flow rate and subsequently a short perfusate transit time from the feed to the collection reservoir. The average pressure in the two reservoirs determined the pressure which drove the perfusate from the cannulating tip. At normal pressures and flows, the time required to change perfusion fluid composition at the pipette tip was less than 1 min, and discharge hematocrit of a red blood cell suspension was indistinguishable from the hematocrit measured in the feed reservoir. This pipette perfusion system should facilitate the investigation of a variety of physiological issues, including the causes of the normally low and variable capillary hematocrit and the effect of variations in the composition of suspending medium used to perfuse the capillaries and, further, would permit selected sites in the terminal vasculature to be perfused with materials which will label specific capillary networks in situ.

Multiparametric imaging of microregional circulation over the brain cortex by videoreflectometry.

One of the primary tasks of parenchymal circulation is to distribute blood flow as it flows through an organ among increasingly larger number and smaller volumes of tissue elements until the level of nutritive microcirculation is reached. It seems, the only adequate approach to characterize this process is to map the distribution of blood flow at the level of microcirculation over tissue microareas. It has to be recognized, however, that the term blood flow is rather inadequate to describe the passage of its two components, erythrocyte and plasma flow through the complex system of capillary networks of tissue microareas where the diameter of capillaries and the traversing erythrocytes are comparable. The methodology of multiparametric imaging of tissue microcirculation by computerized videoreflectometry provides the means of obtaining high resolution maps of local erythrocyte and plasma volumes, mean transit times, volume flows, local tube and discharge hematocrit, blood volume and flow in a repeatable manner. Current state of instrumentation and technology (Eke 1992) has made multiparametric assessment of cerebrocortical circulation for these parameters possible typically at a spatial resolution of 10000 sites/10 mm2 and a frequency of up to 1 measurement/minute. These features seem adequate to assess the complexity of tissue microcirculation in the brain cortex in the spatial and temporal domain alike.

The association of perioperative blood transfusion with colorectal cancer recurrence.

The relationship between blood transfusion, disease-free survival, and other potential prognostic factors was prospectively studied in 339 consecutive patients with colorectal cancer. Admission and discharge hematocrit, Dukes' stage, and blood loss were significantly related to both blood transfusion and disease-free survival. Using Cox proportional hazards model, however, the association of transfusion with disease-free survival was significant (p = 0.0196) after controlling for age, sex, blood loss, procedure, tumor differentiation, stage, admission hematocrit, duration of surgery, length of the specimen, and tumor size. Dukes' stage (p < 0.0001) and blood transfusion (p < 0.0001) were the only variables independently related to disease-free survival. Forty per cent (44) of the 110 patients who received transfusions developed cancer recurrence, compared with 22% (50) of the 229 patients who did not receive blood (p < 0.0001). Five-year disease-free survival of the transfused patients was 57%, compared with 77% for nontransfused patients. Patients who developed recurrence received an average of twice as much blood as patients without recurrence (1.26 versus 0.61 units, p = 0.0128). Perioperative blood transfusion is a significant independent prognostic factor for colorectal cancer.

Identifying elective orthopedic surgical patients transfused with amounts of blood in excess of need: the transfusion trigger revisited.

The discharge hematocrit has been analyzed as a clinical indicator of the transfusion trigger by which to identify patients undergoing elective orthopedic surgery who were transfused with blood in excess of need. The volume of red cells lost by each patient during surgical hospitalization was compared to the volume of red cells transfused. Three clinical indicator levels were considered. Red cell losses of 10, 20, and 30 percent of each patient's baseline red cell volume at admission were considered to be appropriate before subsequent blood transfusion replacement, representing generous, intermediate, or strict clinical indicator levels, respectively. With Level I as a generous clinical indicator, 110 (25%) of 525 patients were transfused in excess of blood needs; by Level II (intermediate) and Level III (strict) criteria, 221 (42%) and 314 (60%) of 525 patients, respectively, were transfused in excess of blood needs. Significant differences were found for transfused patients analyzed by gender (26% of women vs. 13% of men; Level I, p less than 0.001) and preoperative autologous blood donation (25% of autologous blood donors vs. 11% of those who did not donate autologous blood; Level I, p less than 0.001). It can be concluded that the discharge hematocrit and amount of blood lost during hospitalization can be used as clinical indicators with which to identify patients receiving transfusions in excess of needs in the elective surgical setting. With this method, it was found that the transfusion trigger is different for women and for men as well as for autologous blood donors and those who did not donate autologous blood undergoing elective orthopedic surgery [corrected].

Instrumentation and technology for multiparametric mapping of intraparenchymal circulation in the brain cortex.

The intraparenchymal distribution pattern of erythrocytes and plasma should certainly interact with the oxygen supply and energy balance in tissue microareas. To enhance our knowledge of how this interaction takes place in the brain cortex, a non-destructive videophotometric method has been developed. The current state of this method has evolved through a number of intermediate steps 1, 2, 3, 4, 5 representing an interplay between the actual level of understanding of how microcirculation can be assessed by intraparenchymal indicator-dilution and the level of instrumentation and technology available to do quantitative spatial imaging as the means of detecting intraparenchymal indicator dilution from capillary beds of the intact brain cortex. The latter has always been a limiting factor in achieving the ultimate goal of obtaining high resolution maps of local erythrocyte and plasma volumes, mean transit times, volume flows, local tube and discharge hematocrit, blood volume and flow in a repeatable manner. The purpose of this paper is to present the current state of instrumentation and technology that has made multiparametric assessment of cerebrocortical circulation for these parameters possible at a spatial resolution of 10000 sites/10 mm2.

Perioperative Blood Transfusion Adversely Affects Prognosis of Patients with Stage I Non-Small-Cell Lung Cancer

It has been speculated that blood transfusion might adversely affect prognosis in cancer patients by immunosuppression. To avoid the confounding affect of advanced disease, we tested this hypothesis in 117 patients with stage I non-small-cell lung cancer. Mean and median follow-up were 49.7 months and 47 months, respectively. Patients who died during the postoperative period were not included. Perioperative transfusion was defined as administration of whole blood or packed cells within 30 days of operation. The overall cumulative 5-year disease-free survival rate was 67%. In patients with transfusion, it was 53% and in patients without transfusion it was 81% (p=0.0055). A multivariate analysis was performed that included patient age, race, sex, cell type, extent of operation (pneumonectomy versus lobectomy/segmentectomy), operative blood loss, admission hematocrit, discharge hematocrit, and the presence or absence of perioperative transfusion. The only variable that significantly correlated with 5-year disease-free survival was the presence or absence of perioperative transfusion (p=0.0278), and this effect was not related to the number of transfusions. Retrospective analysis of long-term results of patients surviving curative operation for stage I lung cancer shows that any perioperative transfusion significantly worsens the patient's prognosis and suggests very strongly that this association is due to an adverse effect of the transfusion rather than the transfusion serving as a marker for another risk factor.

A three-dimensional analysis of plasma skimming at microvascular bifurcations

This paper analyzes an important underlying mechanism for the discharge hematocrit reduction observed in microvessels, which refers to the plasma skimming from the cell-free layer near the parent tube wall in the presence of a side branch. The three-dimensional theory recently developed by the authors (Yan et al., 1991, J. Fluid Mech., in press) for treating the simple shear flow past a side branch tube in a plane wall with suction is first summarized and then extended to treat T bifurcations from parent vessels with an upstream Poiseuille flow. For unequal vessel bifurcations, a fundamental new dimensionless group, Q = 1 8 ( q ̃ b q ̃ p )( R ̃ p R ̃ b ) 3 , is derived whose value determines the shape of the upstream capture tube of the plasma phase, when the partitioning q ̃ b q ̃ p of the flow into the side branch and the ratio R ̃ p R ̃ b of the radii of the parent and side branch vessels are varied. Closed form expressions are then presented for the three-dimensional fluid capture tube shape upstream of the bifurcation which are valid when Q > 1 or Q < 0.2. Based on this theory and its modification for an upstream Poiseuille velocity profile, the separating surface shape, the critical minimum fractional flux for incipient cell capture, and the discharge hematocrit defect and its dependence on the flow rate are predicted. It is shown, furthermore, that for flows typical of the microcirculation, a single dimensionless number, P = 3πQ( R ̃ b \\ ̃ gg 2 ) , with \\ ̃ gg being the cell-free layer thickness, can be defined whose value determines the discharge hematocrit defect that arises from plasma skimming. The minimum critical flow rate for any red cells to enter the side branch is then given by the criterion P = 1. Although this theory does not account for the cell screening effect arising from the hydrodynamic interaction between the cells and the tube walls, it leads to predictions which exhibit the same trends as the experimental observations and is able to explain the results of several seemingly contradictory microvascular experiments that have puzzled investigators in recent years.

A semi-empirical model of apparent blood viscosity as a function of vessel diameter and discharge hematocrit

A semi-empirical model is developed to describe the dependence of apparent viscosity of blood on vessel diameter (2.7 to 500 microns) and vessel discharge hematocrit (5% to 60%). The blood flow is modeled as a cell-rich core and a cell-free marginal layer in the larger vessels and an axial-train in the smaller vessels. Laminar (Poiseuille) flow is assumed in all cases. An equation is derived in which apparent viscosity is a function of vessel diameter, core viscosity, and width of marginal layer. This is then complemented by empirical equations in which core viscosity varies exponentially with discharge hematocrit while the width of marginal layer varies linearly with discharge hematocrit. The model correlates well with several sets of experimental data and behaves according to the Fahraeus-Lindqvist effect. Predicted apparent viscosity tends to the expected finite value for large vessel diameters. Dependence of apparent viscosity on vessel diameter is realistically smooth in the whole diameter range.

Perioperative blood transfusion adversely affects prognosis of patients with stage I non-small-cell lung cancer

It has been speculated that blood transfusion might adversely affect prognosis in cancer patients by immunosuppression. To avoid the confounding affect of advanced disease, we tested this hypothesis in 117 patients with stage I non-small-cell lung cancer. Mean and median follow-up were 49.7 months and 47 months, respectively. Patients who died during the postoperative period were not included. Perioperative transfusion was defined as administration of whole blood or packed cells within 30 days of operation. The overall cumulative 5-year disease-free survival rate was 67%. In patients with transfusion, it was 53% and in patients without transfusion it was 81% (p = 0.0055). A multivariate analysis was performed that included patient age, race, sex, cell type, extent of operation (pneumonectomy versus lobectomy/segmentectomy), operative blood loss, admission hematocrit, discharge hematocrit, and the presence or absence of perioperative transfusion. The only variable that significantly correlated with 5-year disease-free survival was the presence or absence of perioperative transfusion (p = 0.0278), and this effect was not related to the number of transfusions. Retrospective analysis of long-term results of patients surviving curative operation for stage I lung cancer shows that any perioperative transfusion significantly worsens the patient's prognosis and suggests very strongly that this association is due to an adverse effect of the transfusion rather than the transfusion serving as a marker for another risk factor.

Red Blood Cell Motion and Hematocrit Distribution in a Deforming Capillary

Based on the relation between the red blood cell velocity and the blood velocity of the Fahraeus effect, we analyze the motion of a red blood cell in a deforming capillary to investigate how the ventilatory induced deformation affects the transit of the red blood cell through the pulmonary capillary and changes the hematocrit discharged from the capillary. An analytical solution is also obtained for the case of an infinitesimal deformation. The numerical and analytic solutions demonstrate that the variation in discharge hematocrit is proportional to the change of pulmonary capillary blood volume between the time that the red cell enters and the time that it exits the capillary. We also find that this hematocrit variation could be regarded, in terms of transport, to originate from the mid-section of the capillary.

Intraoperative Autotransfusion in Vascular Surgery

To determine the impact of intraoperative autotransfusion on vascular surgical care, data related to 304 major vascular surgical operations performed over a 42-month period were retrospectively analyzed. Procedures included abdominal aortic aneurysmectomy (N = 152), aortobilateral femoral bypass (N = 60), thoracoabdominal aortic aneurysmectomy (N = 20), and other vascular procedures (N = 68). Fifty percent of the transfusion requirement was met by autotransfusion for the average patient. The per patient average volumes (liters) autotransfused were as follows: elective abdominal aortic aneurysmectomy, 0.87 L and nonelective, 1.45 L; elective aortobilateral femoral bypass, 0.63 L; elective thoracoabdominal aortic aneurysmectomy, 2.47 L, and nonelective, 2.15 L; and elective other, 0.53 L and nonelective, 1.30 L. Results of immediate postoperative and hospital discharge hemoglobin, hematocrit, and coagulation studies (prothrombin time, partial thromboplastin time, and platelets) did not differ from results of preoperative studies in any group. Neither mortality nor morbidity was related to intraoperative autotransfusion. These data suggest that intraoperative autotransfusion is a safe blood replacement method during major vascular surgical operations.

A two-fluid model for hematocrit distribution in microvascular networks.

A new theoretical network model for evaluating discharge hematocrits, explicitly based on plasma skimming at branches, is introduced. The particular network geometry chosen simulates bat wing microvasculature. Blood in vessels is approximated to be a two fluid with red cell suspension as the core and a plasma layer surrounding it. The plasma layer width depends on hematocrit, which leads to nonlinear hydrodynamic equations solved by iteration. Discharge hematocrit distributions are calculated by a computer for five generations of vessels. Dispersion of hematocrit values was found to be correlated to plasma skimming at branches. Contrary to previous suggestions, plasma skimming did not result in lowered mean hematocrit towards the capillaries. Network structure was found to be an important factor affecting the hematocrits. Mean discharge hematocrit remained steady against changes in vessel dimensions, capillary resistances, red cell concentration in plasma layer, and shape of the separation surface defining the streamlines entering the side branch. This high stable mean hematocrit is based on symmetry of the model network. Enhanced asymmetry tended to lower the hematocrit.

The Fahraeus effect in sheet flows with a gap thickness of 4 to 36 microns.

We measured the Fahraeus effect of blood flowing in a sheet flow model formed with two glass slides. The number of red blood cells in the sheet flow was counted to determine a sheet hematocrit Hs and the discharge hematocrit Hd was measured from blood collection. For a Hd in the range of 3 to 30 percent, we find that Hs/Hd is about .83 for a gap of 4.1 microns. When the discharge hematocrit is 30 percent, the ratio decreases to .66 as the gap approaches 7 microns and then increases as the gap becomes thicker. The results indicate that the hematocrit ratio for a gap thicker than 4.1 microns is an increasing function of the discharge hematocrit. The value of Hs/Hd found for the sheet flow models and its dependence on Hd are comparable to those of circular tubes when their diameter equals the gap thickness.

Computer simulation of cerebral microhemodynamics.

Microvascular network hemodynamics was simulated by computer in an anatomically reconstructed cerebral microvascular network. A video microscope system was used for three-dimensional mapping of the vessel network in the rat brain cortex. The complete topology, length and mean diameter of the microvessels were determined. The distribution of blood flow and red cell flux in the network was calculated based on vessel resistance estimated from geometrical data and a rheological model of blood. This model described apparent relative blood viscosity as a function of vessel diameter and local discharge hematocrit. The calculations predicted highly heterogeneous cell flux distribution at any feed hematocrit between 10 and 40 percent. The frequency distribution of microvessel hematocrit was bimodal and included values exceeding the feed hematocrit value. A probabilistic simulation of cell transit resulted in transit time distributions which agree with experimental findings. The most probable transit time and capillary path length and 4s and 300 microns, respectively.

Autologous Blood Donation for Elective Surgery Effect on Physician Transfusion Behavior

The hospital records of 69 autologous blood donors undergoing elective surgical procedures and those of matched controls were reviewed in an effort to assess whether autologous blood donation altered physician transfusion behavior. Physicians tolerated significantly lower admission (0.372 vs 0.437 [37.2% vs 43.7%]), nadir (0.304 vs 0.334 [30.4% vs 33.4%]), and discharge (0.326 vs 0.358 [32.6% vs 35.8%]) hematocrits in patients with autologous blood predeposits compared with matched controls. Additionally, 44 patients undergoing orthopedic procedures stored insufficient blood to prevent homologous blood exposure: ten of 17 patients could have avoided homologous blood if 4 U of autologous blood had been stored. Twenty-five patients undergoing nonorthopedic procedures stored an insufficient volume of blood to satisfy an unexpected transfusion need but were given transfusions of more than needed amounts of their own blood when transfusion was not clinically indicated. Future studies designed to maximize procurement of autologous blood are needed; a salutary effect on physician transfusion behavior would then be coupled with an autologous blood inventory that could eliminate homologous blood exposure in these patients. (JAMA1987;258:3135-3137)

Microhemodynamics of blood flow in narrow glass capillaries of 9 to 20 micrometers; the Fahraeus effect.

Velocities of the red blood cell (RBC) and the suspending medium in glass capillaries of 9 to 20 micron were measured under microscopic observation. The effects of physical factors such as driving pressure, capillary diameter, hematocrits and RBC deformability on flow velocities were studied using freshly drawn blood of the rat resuspended in phosphate buffered saline solution in the hematocrit range between 5 and 12.5%. These RBC suspensions were made to flow through the test glass capillaries under known negative driving pressures. Ratios of capillary hematocrit to feed hematocrit taken as measures of the Fahraeus effect showed almost constant value of about 0.74. While, ratios of capillary hematocrit to discharge hematocrit showed a characteristic dependence on capillary diameter, showing minimal values at about 13 micron in capillary diameter. The same hematocrit ratios were found to be well correlated with values of wall shear rates estimated from the relative RBC velocities.

Haematocrit distribution in rabbit tenuissimus muscle.

Low values of mean capillary haematocrit have been reported in many tissues including skeletal muscle. The present study was undertaken to analyse haematocrit distribution in the transverse and terminal arterioles, capillaries and venules of the rabbit tenuissimus muscle preparation. Tube haematocrit, i.e. the volume fraction of red cells, in muscle capillaries (n = 85) was found to be 39% of systematic haematocrit Hsys. In part, this haematocrit reduction is due to the Fahraeus effect. Corresponding capillary discharge haematocrit HD was 56% of Hsys. Tenuissimus muscle capillaries are fed by terminal arterioles originating from transverse arterioles. The latter extend into and supply adjacent connective tissue septa in addition to the muscle tissue proper. In transverse arterioles leaving the muscle to enter the connective tissue, HD was found to be 127% of Hsys (n = 18), and in collecting venules at the muscle edge HD was 129% of Hsys (n = 18). These findings indicate that the connective tissue microcirculation represents a functional red-cell shunt in resting tenuissimus muscle. Since only about 20% of the inflow to the preparation passes through the connective tissue, this shunting is not sufficient to satisfy conservation of red-cell mass. In addition, it is likely that the observed low capillary haematocrit is in part due to a positive correlation between blood-flow velocity and HD in capillaries originating from individual terminal arterioles. This phenomenon is called the network Fahraeus effect.

Autologous Blood Donation for Elective Surgery

The hospital records of 69 autologous blood donors undergoing elective surgical procedures and those of matched controls were reviewed in an effort to assess whether autologous blood donation altered physician transfusion behavior. Physicians tolerated significantly lower admission (0.372 vs 0.437 [37.2% vs 43.7%]), nadir (0.304 vs 0.334 [30.4% vs 33.4%]), and discharge (0.326 vs 0.358 [32.6% vs 35.8%]) hematocrits in patients with autologous blood predeposits compared with matched controls. Additionally, 44 patients undergoing orthopedic procedures stored insufficient blood to prevent homologous blood exposure; ten of 17 patients could have avoided homologous blood if 4 U of autologous blood had been stored. Twenty-five patients undergoing nonorthopedic procedures stored an insufficient volume of blood to satisfy an unexpected transfusion need but were given transfusions of more than needed amounts of their own blood when transfusion was not clinically indicated. Future studies designed to maximize procurement of autologous blood are needed; a salutary effect on physician transfusion behavior would then be coupled with an autologous blood inventory that could eliminate homologous blood exposure in these patients.

Microvessel hematocrit: measurement and implications for capillary oxygen transport.

Microvascular preparations of the hamster cheek pouch and cremaster muscle were used to establish the relationship among three measures of red cell distribution: 1) the systemic hematocrit defined as the volume percentage of red cells sampled from a toe clip; 2) the tube hematocrit defined as the instantaneous volume fraction of red cells in a microvessel segment of measured length and diameter; and 3) the discharge hematocrit defined as the volume percentage of red cells that flowed into micropipettes inserted into microvessels. The results show the tube hematocrit is low and highly variable in the presence of a nearly constant systemic hematocrit. The discharge hematocrit, in contrast, consistently approximated systemic values in blood taken from arterioles or venules with diameters of 6-98 micron. Indeed, the mean ratio of the discharge to the systemic hematocrit did not differ from unity (P greater than 0.25). The observed similarity between the discharge and systemic hematocrit indicates that red cells are distributed uniformly across capillary networks. Differences between the absolute value and variance of the tube hematocrit compared with the discharge hematocrit imply that the anatomical volume of a microvessel can differ from the volume available to cells and plasma within a microvessel. Thus moment-to-moment variation in the tube hematocrit or capillary red cell spacing may be indicative of a change in the effective capillary flow cross section and not solely on the inflow hematocrit.

Perioperative transfusions associated with colorectal cancer surgery: clinical judgment versus the hematocrit.

AbstractWe prospectively studied 123 patients with colorectal cancer in order to identify determinants of perioperative blood transfusions and unnecessary transfusions. Transfusions were considered unnecessary if the preoperative hematocrit reading exceeded 36% or the discharge hematocrit level exceeded 33%. Age, sex, admission hematocrit reading, operative procedure, specimen length, duration of surgery, estimated blood loss, tumor size, tumor differentiation, nodal status, Dukes' staging, and attending surgeon were evaluated in relation to perioperative blood transfusion using stepwise logistic regression. Fifty‐one (41%) of the 123 patients in the study were transfused and 35 patients (28%) received at least 1 unnecessary unit of blood. Advanced age, low admission hematocrit reading, high estimated operative blood loss, prolonged procedures, and lengthy specimens were significantly related to the administration of blood. Excessive intraoperative transfusions and the practice of administering blood in pairs of units accounted for the unnecessary transfusions. This study indicates that factors that influence clinical judgment and thereby the decision to transfuse a patient are not accurate indicators of when blood should be given. Greater use of pre‐transfusion hematocrit testing, especially in the operating room and before transfusing a second unit of blood, will dramatically reduce blood usage in elective colorectal cancer surgery.