Mathematical considerations in the practice of acute normovolemic hemodilution

Acute Hemodilution is Safe in Patients with Comorbid Illness Undergoing Partial Hepatectomy

Despite increased use of acute hemodilution (AH) to reduce perioperative blood transfusion in patients undergoing partial hepatectomy, there is a lack of data on safety in those with comorbid medical illness. We performed a retrospective review of 96 patients undergoing partial hepatectomy with AH and low central venous pressure (CVP) anesthesia. The cohort was compared with 63 patients undergoing partial hepatectomy using standard anesthetic management (SA) and low CVP anesthesia. All were American Society of Anesthesiologists physical status 3 or 4. 58 per cent were baseline hypertensive on medication. Hepatic resection was predominantly performed for metastatic colorectal (41%) and primary hepatic (32%) cancer. Forty per cent underwent major hepatectomy. The mean volume of blood removed for AH was 497 mL (range, 0 to 1 L). Most achieved low CVP (AH 90% vs SA 84%, P = 0.3). Blood loss was lower with AH (mean 480 mL vs 904 mL, P < 0.001). Blood transfusion rate was 74 per cent lower with AH ( P < 0.001). There was no difference in cardiac, respiratory, renal, or overall complications with AH compared with SA. Acute hemodilution is well tolerated by patients with comorbid illness undergoing partial hepatectomy, favoring ongoing use and further study.

Intraoperativer Volumenersatz, Transfusion und Behandlung von Gerinnungsstörungen

Sowohl der operative Eingriff als auch die Narkose führen zu Veränderungen des intravasalen Volumenstatus des Patienten. Ziele der perioperativen Flüssigkeits- bzw. Volumentherapie sind die Aufrechterhaltung des zirkulierenden intravasalen Volumen (Normovolämie), einer adäquaten Gewebeperfusion und -oxygenierung, des Gleichgewichts im Elektrolythaushalt sowie die Aufrechterhaltung der Normoglykämie.

Ranking the effectiveness of autologous blood conservation measures through validated modeling of independent clinical data

BACKGROUND

Future supply with allogeneic blood transfusions faces several crucial challenges that warrant thorough (re-)evaluation of existing alternatives. Meta-analyses have indicated a significant potential for autologous blood conservation (ABC) measures to reduce the need for allogeneic blood transfusions. The quality of currently available studies, however, prohibits definite conclusions. Under these circumstances, mathematical modeling offers unique opportunities to compare various ABC measures and to test the impact of different variables on efficacy and effectiveness.

STUDY DESIGN AND METHODS

We performed an extensive search for clinical ABC studies, focusing on acute normovolemic hemodilution (ANH), intraoperative blood salvage (IBS), or preoperative autologous deposit (PAD). Only ABC studies providing a minimum set of clinical variables were included. Using a clinically validated mathematical model, we then calculated maximal allowable blood loss (efficacy) and increase in red blood cell (RBC) mass (effectiveness) to rank the three techniques.

RESULTS

We identified 21 clinical ABC studies, including 3926 patients, as suitable for our model. Our model shows that IBS with high recovery rates is the most efficacious and effective ABC measure. PAD will reveal nearly similar efficacy and effectiveness, only if sufficient time for RBC regeneration has passed and if 4 PAD units or more are available. Our model further demonstrates that ANH as well as IBS with low recovery rates are the least efficacious and effective alternatives.

CONCLUSIONS

IBS appears to be the most efficacious and effective ABC measure. PAD can only reduce the need for allogeneic blood transfusions under certain circumstances. ANH does not appear to play an important role in ABC.

Intraoperativer Volumenersatz, Transfusion und Behandlung von Gerinnungsstörungen

Sowohl der operative Eingriff als auch die Narkose führen zu Veränderungen des intravasalen Volumenstatus des Patienten.

A mathematical model of cell salvage compared and combined with normovolemic hemodilution

BACKGROUND

Mathematical models have been used to describe the factors that affect cell salvage (CS) and normovolemic hemodilution (ANH). Here, the CS and ANH models were used to compare these two techniques alone or in combination with each other.

STUDY DESIGN AND METHODS

Variables used for a hypothetical patient included an estimated blood volume of 5000 mL, a presurgery hematocrit (Hct) of 45 percent, and a transfusion trigger of 21 percent. The model accounts for both the effect of decreasing the Hct due to blood loss and the effect of increasing Hct due to the readministration of blood in an isovolemic patient. The efficacy of CS and ANH is defined to be the maximum allowable blood loss for a fixed blood volume and a fixed transfusion trigger.

RESULTS

Comparison of CS with ANH showed that 3 units of ANH was comparable to CS when CS recovery rates ranged from 19 to 24 percent. For a patient with a blood volume of 5000 mL and a starting Hct of 40 percent, 3 units of ANH would allow for 3972 mL of blood to be lost before crossing a 21-percent transfusion trigger, whereas CS with a 125-mL bowl would allow for 7611 mL.

CONCLUSION

When comparing ANH to CS, this mathematical model would suggest that CS has the potential to offer significantly greater red blood cell avoidance than does ANH; however, the combination of ANH with CS may offer allogeneic avoidance superior to either technique alone.

Effects of standardized acute normovolemic hemodilution on intraoperative allogeneic blood transfusion in patients undergoing major maxillofacial surgery

The aim of the present study was to demonstrate the practicality and efficacy of acute normovolemic hemodilution (ANH) to reduce allogeneic red blood cell (RBC) transfusion in patients undergoing elective surgery with anticipated high intraoperative blood loss (BL). 124 patients (age 48 +/- 18 years, ASA classes I-III) underwent major maxillofacial surgery in a university hospital (68% tumor surgery, 32% dysgnathia correction). After induction of general anesthesia, ANH was performed by standardized withdrawal of 900 ml (2 units) of whole blood and simultaneous infusion of 500 ml of hydroxyethyl starch solution (6% HES 130,000/0.4) and 1500 ml of crystalloidal solution. Intraoperative BL was fluid-compensated until physiologic parameters indicated the need for RBC transfusion. First, autologous ANH-blood was retransfused followed by, if necessary, allogeneic RBC. Total BL was referred to the patient's calculated blood volume (BV): fractional blood volume loss, BL(fract) = BL/BV. ANH took 16 +/- 2 min and was void of any adverse event. The costs for ANH was 24 per patient. 55 patients had a mean BL(fract) of 44 +/- 28% and required an intraoperative transfusion; 49/55 patients with an average BL(fract) of 37 +/- 14% were transfused with only autologous ANH-blood; 6/55 patients with a mean BL(fract) of 100 +/- 47% underwent additional transfusion with allogeneic RBC. Standardized, 2 unit, ANH is a practicable, safe and economic blood conservation technique that allowed for the complete avoidance of allogeneic RBC transfusion in 89% of patients undergoing maxillofacial surgery that required an intraoperative RBC transfusion.

Blood-transfusion requirements and blood salvage in donors undergoing right hepatectomy for living related liver transplantation

Living related liver donation for liver transplantation in adults including its risks is receiving increased attention. We present data from 44 liver donors focusing on transfusion requirements and avoidance of heterologous transfusion. The volume of blood transfused (both autologous from preoperative donation and heterologous) was assessed including that derived from intraoperative isovolemic hemodilution, cell-saver salvaged, and retransfused blood. Hemoglobin concentration and central venous pressure were measured at specified time points before and during surgery. Intraoperative blood loss was calculated and correlated to the duration of parenchymal transsection, liver volume resected, and central venous pressure. There were no specific anesthesia-evoked complications. In 4 donors, major bleeding (>2000 mL) occurred. Blood loss averaged 902 +/- 564 mL (SD), yielding a minimal mean hemoglobin concentration of 8.1 +/- 1.2 g/dL. One donor received 3 U of heterologous blood and 30 donors received autologous blood from their preoperative donation. An average of 592 +/- 112 mL of blood derived from perioperative acute isovolemic hemodilution was retransfused as was 421 +/- 333 mL of washed red cells from the cell-saving system. Avoidance of heterologous blood transfusion, application of blood-saving techniques, and efficient pain management are crucial for adult living liver donors. Transfusion of banked blood can be avoided in most patients when intraoperative cell salvage, preoperative autologous blood donation, and intraoperative hemodilution are combined.

Blood-transfusion requirements and blood salvage in donors undergoing right hepatectomy for living related liver transplantation

Living related liver donation for liver transplantation in adults including its risks is receiving increased attention. We present data from 44 liver donors focusing on transfusion requirements and avoidance of heterologous transfusion. The volume of blood transfused (both autologous from preoperative donation and heterologous) was assessed including that derived from intraoperative isovolemic hemodilution, cell-saver salvaged, and retransfused blood. Hemoglobin concentration and central venous pressure were measured at specified time points before and during surgery. Intraoperative blood loss was calculated and correlated to the duration of parenchymal transsection, liver volume resected, and central venous pressure. There were no specific anesthesia-evoked complications. In 4 donors, major bleeding (>2000 mL) occurred. Blood loss averaged 902 +/- 564 mL (SD), yielding a minimal mean hemoglobin concentration of 8.1 +/- 1.2 g/dL. One donor received 3 U of heterologous blood and 30 donors received autologous blood from their preoperative donation. An average of 592 +/- 112 mL of blood derived from perioperative acute isovolemic hemodilution was retransfused as was 421 +/- 333 mL of washed red cells from the cell-saving system. Avoidance of heterologous blood transfusion, application of blood-saving techniques, and efficient pain management are crucial for adult living liver donors. Transfusion of banked blood can be avoided in most patients when intraoperative cell salvage, preoperative autologous blood donation, and intraoperative hemodilution are combined.

New mathematical model for the correct prediction of the exchangeable blood volume during acute normovolemic hemodilution

BACKGROUND

The blood volume that has to be exchanged for crystalloids and/or colloids during acute normovolemic hemodilution (ANH) in order to reach a preset target hemoglobin concentration (hb) is usually predicted by the Bourke and Smith formula developed in 1974. This formula systematically overestimates the 'true' exchangeable blood volume (EBV), a fact that may potentially endanger patients because the target hb will be missed and the normovolemic anemia might turn out to be more severe than a priori intended. Our objective was to develop a more accurate mathematical model of hemodilution kinetics and to validate this new model in animals and in patients undergoing ANH.

METHODS

Twenty-two anesthetized beagle dogs and 18 patients under balanced anesthesia underwent isovolemic hemodilution with hydroxyethyl starch (HAES 6%, 200 000) to a target hb of 7 g dl-1 or 9 g dl-1, respectively. Exchangeable blood volume predicted by use of the different mathematical models was compared with the blood volume actually exchanged to meet the preset target hb.

RESULTS

Calculation of EBV by the Bourke and Smith formula (EBVB + S) systematically overestimated the volume actually exchanged (overestimation: dogs 15%, patients 20%), whereas our new iterative model predicted EBV (EBViterative) more reliably (overestimation: dogs 1%, patients 8%). In both cases EBVB + S differed significantly from the EBViterative.

CONCLUSION

Exchangeable blood volume is predicted more accurately by the new iterative model than by the Bourke and Smith formula. The iterative model leads to an improvement in patient safety and provides a physiologically adequate basis for future studies investigating the efficacy of ANH in reducing allogenic blood transfusions.

A mathematical model of cell salvage efficiency

Cell salvage (CS) is one of the modalities that can be used during surgery to decrease the use of allogeneic blood. Unlike acute normovolemic hemodilution, the efficiency of CS has not been mathematically modeled. In this article, we hypothesized that a mathematical model could predict the decline of hematocrit during CS. The model that was developed accounts for both the effect of decreasing the hematocrit because of blood loss and the effect of increasing hematocrit because of the readministration of washed blood in an isovolemic patient. The efficiency of CS is defined to be the maximum allowable blood loss (MABL) for a fixed blood volume and a fixed transfusion trigger. For demonstration purposes, variables used for a hypothetical patient included an estimated blood volume of 5000 mL, a presurgery hematocrit of 45%, and a transfusion trigger of 21%. The MABL in a typical case was 9600 mL, with a CS red cell recovery rate of 60%. Patient records from a convenience sample showed an average recovery rate of 57% with 20% variability. This mathematical model suggests that CS can be a highly effective blood conservation method when red blood cell collection is optimal.

IMPLICATIONS

In this study, a mathematical model of cell salvage was developed. The model was then matched against real clinical cases to gain an understanding of the variables that modify cell salvage efficiency. The model illustrates that cell salvage can be a highly effective method of avoiding blood transfusion.

Comparison of acute normovolemic hemodilution and preoperative autologous blood donation in clinical practice

STUDY OBJECTIVE

To compare, by model simulation, acute normovolemic hemodilution (ANH) and preoperative autologous blood donation (PABD) to predict their efficacy in current clinical practice. To discuss their similarities and offer guidelines based on expected operative blood loss.

DESIGN

Model simulation using data obtained from total hip arthroplasty procedures.

SETTING

University medical center.

PATIENTS

91 patients who participated in PABD undergoing single, primary, total hip replacement surgeries from January to December 1997.

INTERVENTIONS

A nonanemic (Hb baseline 14 g/dL), average-sized patient (estimated blood volume 5,000 mL) who donated two units by either PABD or ANH was assumed for model simulation. The Hb-final, as a function of 250-mL blood loss increments after retransfusion of two autologous units, was calculated for each technique and compared to a control (nondonor) to predict the effectiveness of PABD and ANH in conserving red cell mass. Data from hip arthroplasties over a 12-month period were used to determine the parameters for the model. Results were subjected to regression analysis and tested for parallelism of slopes, with p < 0.05 accepted to indicate a statistical difference.

MAIN RESULTS

The difference in Hb-final between PABD and ANH was not statistically different over a wide range of blood loss. When compared to control, there was no difference in Hb-final measurements in the estimated blood loss (EBL) range of less than 1,000 mL. ANH and PABD provide some benefit when EBL is greater than 1,500 mL in nonanemic, average-sized patients. As blood loss increases, the benefit of autologous collection becomes more clinically evident.

CONCLUSION

PABD exemplifies a "chronic" form of ANH in current clinical practice and offers little advantage over ANH as a blood conservation technique for high-blood-loss operations. When surgical blood loss is predictably mild to moderate (range 250 to 1,000 mL), neither blood conservation technique is necessary.

Hypervolemic hemodilution: an alternative to acute normovolemic hemodilution? A mathematical analysis

BACKGROUND

Hypervolemic hemodilution has been proposed as an alternative to normovolemic hemodilution to reduce homologous blood transfusions. So far, convincing data supporting this concept are unknown.

MATERIALS AND METHODS

We therefore present a mathematical model calculating the efficacy of hypervolemic, normovolemic, and "no" hemodilution. Hypervolemic hemodilution constituted volume expansion (20% of estimated blood volume) maintained throughout surgery. Normovolemic hemodilution contained isovolemic exchange of blood (40% of estimated blood volume) vs colloid as well as retransfusing blood plus colloid to maintain minimal acceptable hematocrit, e.g., transfusion trigger. To determine the efficacy of each technique maximal allowable blood loss and final postoperative hematocrit were calculated. Maximal allowable blood loss referred to the amount of blood lost during surgery after which homologous blood transfusion became necessary.

RESULTS

Recalculating published clinical data strongly validated the formulas used for our model. Hypervolemic hemodilution always revealed lowest maximal allowable blood losses. Normovolemic hemodilution constantly ensured highest maximal allowable blood losses. For blood losses <40% of blood volume, hypervolemic and normovolemic hemodilution provided almost identical final postoperative hematocrits. But in contrast to normovolemic hemodilution, hypervolemic hemodilution did not carry the risk of severe transient, retransfusion-induced hypervolemia. "No" hemodilution always gave lowest final postoperative hematocrits.

CONCLUSIONS

Thus, hypervolemic hemodilution cannot replace normovolemic hemodilution to reduce homologous transfusions, but for blood losses <40% of blood volume hypervolemic hemodilution appears to be superior.

Perioperative blood transfusions: indications and options

A reevaluation of the indications for and alternatives to transfusion of allogeneic blood was precipitated by transfusion-induced HIV. The transfusion trigger has shifted from an optimal hemoglobin level and hematocrit (10/30) to that level of hemoglobin necessary to meet the patient's tissue oxygen demands. This critical level can best be determined by physiologic measurements. A number of autologous blood options can reduce the patient's allogeneic blood needs. Pharmacologic measures to increase hemoglobin levels (erythropoietin) and to decrease blood loss at surgery are discussed as are the potential contributions of blood substitutes to transfusion support of the surgical patient.