Acute Normovolemic Hemodilution in Cardiac Surgery

Transcription

1 TATM 2005;7(1):11-19 Acute Normovolemic Hemodilution in Cardiac Surgery S U M M A R Y PHILIPPE VAN DER LINDEN, MD, PhD, AND PAOLA SAKR, MD DEPARTMENT OF ANESTHESIOLOGY CHU BRUGMANN - HUDERF BRUSSELS, BELGIUM Acute normovolemic hemodilution (ANH), which was introduced into surgical practice in the 1970s, involves the removal of blood from a patient either immediately before or shortly after induction of anesthesia and the simultaneous replacement with an appropriate volume of plasma substitute to maintain normovolemia. It therefore aims at reducing allogeneic blood exposure through a reduction in the net red blood cell mass lost during surgery. It can be performed safely in patients with coronary and/or valvular disease undergoing cardiac surgery, providing adequate knowledge of physiological response and limits to normovolemic anemia. Experience in its practical aspects is also important. The efficacy of the technique essentially depends on the amount of Hemodilution Blood transfusion Cardiac surgery Coronary artery disease Valvular disease blood collected and on the amount of blood volume lost by the patients. It appears relatively modest in cardiac surgery, although well-designed studies are warranted. ANH has a place cardiac surgery but should rather be regarded as an integral part of a blood conservation strategy tailored to the patient s needs and adapted to specific surgical procedures. ( Page 11 )

2 Cardiac surgery remains a major consumer of blood components. 1,2 Increasing awareness of the adverse effects of allogeneic blood transfusion, but also the apparition of intermittent blood shortages have prompted both physicians and patients to search for alternatives to the use of donor blood. Acute normovolemic hemodilution (ANH) was introduced to surgical practice in the 1970s 3,4 to reduce requirements for allogeneic blood products. ANH entails the removal of blood from a patient either immediately before or shortly after induction of anesthesia and the simultaneous replacement with an appropriate volume of crystalloids and/or colloids to maintain normovolemia. 5,6 As a result, blood subsequently lost during surgery will contain proportionally less red blood cells by milliliters, thus reducing the loss of autologous erythrocytes. Potential benefits of ANH therefore include all the advantages associated with a reduction in allogeneic blood exposure, including a reduction of transfusion reactions from exposure to allogeneic blood antigens and a decreased exposure to blood-borne pathogens. When compared to other blood conservation techniques, ANH has several inherent advantages: it is inexpensive and easily available, it improves tissue oxygenation because of decreased blood viscosity, and it provides fresh autologous blood units for later retransfusion after control of surgical bleeding. However, the real efficacy of ANH in reducing allogeneic blood transfusions is discussed controversially in the literature. This article aims to describe the physiology, limits and clinical use of ANH with a focus on patients undergoing cardiac surgery. Physiological Compensatory Mechanisms The acute reduction of red blood cell concentration induced by hemodilution elicits intrinsic compensatory mechanisms which allow the maintenance of adequate oxygenation at the tissue level. 7,8 The development of these mechanisms is closely related to the improvement of whole blood fluidity achieved by hemodilution. The basic determinants of blood fluidity are the red cell concentration, the plasma viscosity, the cell-to-cell interactions and the prevailing shear rate (i.e., the mean linear flow velocity). The lower the shear rate, the more pronounced is the improvement in blood fluidity based on changes in hematocrit. 5 Increase in Cardiac Output At the systemic level, improvement in blood fluidity results in an increase of venous return and a reduction of left ventricular afterload. Enhancement of shear rate with subsequent release of nitric oxide also contributes to systemic vasodilatation, 9 while hemodilution-induced stimulation of aortic chemoreceptors increases the sympathetic activity of the heart resulting in increased myocardial performance. 10 All these phenomenons are responsible for an increase in cardiac output, mainly through a rise in stroke volume, but also to some extent through an increase in heart rate. Indeed, an increase in stroke volume and heart rate is observed in awake subjects undergoing ANH, whereas in anesthetized patients the increase in cardiac output is essentially related to an increase in stroke volume. 6,11 As a result, the rise in cardiac output is significantly smaller in anesthetized than in awake patients. 11 The lack of heart rate increase in anesthetized patients may be related to a depression of the autonomic system by anesthetics, although central vagal stimulation by drugs such as opioids may also contribute. Increase in Tissue Oxygen Extraction Capabilities At the microcirculatory level, improvement in blood fluidity results in an increased red blood cell velocity in the capillaries and an enhanced flow motion. 12,13 Both mechanisms allow for a better spatial and temporal distribution of red cell within the capillary network, leading to a more homogenous delivery of oxygen (O 2 ) to the tissues. A better matching of oxygen delivery to tissue oxygen demand also depends on a redistribution of blood flow to areas of high O 2 demand, like the brain and the myocardium, at the expense of other organs with lower metabolic needs such as the kidney, the gastrointestinal tract and the liver. 14 This regional blood flow redistribution is partly due to β-adrenergic stimulation, but does not seem to be altered in the presence of β-adrenergic blockade. 15 Tolerance to ANH As described above, maintenance of tissue oxygenation during ANH results from an increase in cardiac output and oxygen extraction. Several experimental and clinical studies have demonstrated the involvement of both mechanisms even in the early stage of ANH. 16 They allow systemic O 2 uptake to remain constant until the hemoglobin concentration falls to about 3-4 g/dl, at which point O 2 uptake becomes dependent on O 2 delivery. This so-called critical hemoglobin value has been demonstrated in several experimental studies. 17,18 Van Woerkens et al. studied a Jehovah s Witness patient who died from extreme hemodilution and they observed a critical hemoglobin concentration of 4 g/dl. 19 The efficacy of the mechanisms preserving tissue O 2 delivery when the O 2 carrying capacity of the blood is reduced depends primarily on the maintenance of an adequate circulating blood volume. Indeed, hypovolemia blunts the effects of decreased blood viscosity on venous return. 20 Although normovolemic conditions are difficult to define, replacement of blood and fluid losses by at least a volume of plasma substitute having the same expanding effect on the intravascular volume is required. Patient tolerance to ANH depends not only on the integrity of his compensatory mechanisms, but also on the level of tissue ( Page 12 )

3 O 2 demand. For a given cardiac output and O 2 extraction response, any increase in tissue O 2 demand will require a higher hemoglobin concentration. Any significant increase in tissue O 2 demand will therefore reduce the tolerance of the patient to ANH. ANH and the Cardiac Disease Patient In patients undergoing cardiac surgery, underlying coronary and/or valvular disease will decrease their tolerance to ANH. Maintenance of myocardial O 2 delivery during ANH depends essentially on the increase in coronary blood flow, as O 2 extraction is already nearly maximal at the level of the heart under resting conditions. 21 This is achieved by a reduction in coronary vascular resistance related to the decreased blood viscosity but also to specific coronary vasodilatation. Heart rate and possibly myocardial contractility have been shown to increase during hemodilution, 22,23 which results in an augmentation of myocardial O 2 demand. In healthy volunteers and awake patients undergoing major abdominal surgery, moderate to severe hemodilution is associated with a 10-20% increase in systemic O 2 consumption that was mainly related to this increased myocardial oxygen demand. 11,24,25 In dogs, myocardial O 2 consumption more than doubles when the hematocrit is reduced to about 10%. 26 In these conditions, coronary vasodilatation is nearly maximal. Below such a hematocrit, coronary blood flow can no longer match the increased myocardial O 2 demand, and ischemia develops, ultimately resulting in cardiac failure. This is in accordance with experimental data showing a decrease in systemic O 2 uptake at hematocrit values close to 10%. 17 ANH and Coronary Artery Disease As myocardial O 2 supply essentially depends on an increased coronary blood flow during ANH, the coronary reserve, i.e. the ratio between maximal coronary blood flow and resting coronary blood flow, is significantly reduced in these conditions. In intact dogs, Geha demonstrated a 50% reduction in the coronary flow reserve when the hematocrit is decreased from 43 to 20% by isovolemic hemodilution. 27 This indicates the vulnerability of the heart during ANH, especially if higher work demands on the myocardium should coexist. Among healthy conscious subjects undergoing acute isovolemic reduction of hemoglobin to 5-7 g/dl, Leung et al. 28 observed that those who developed transient reversible ST-segment depression on the Holter ECG monitoring were those who exhibited the higher maximal heart rate. This higher heart rate that developed during hemodilution may have contributed to the development of an imbalance between myocardial supply and demand, resulting in ECG evidence of myocardial ischemia. 28 In patients with coronary artery disease (CAD), coronary blood flow is limited by the atherosclerotic lesions. Experimental data in animals with extrinsically applied coronary stenosis have demonstrated a complete blunting of the coronary reserve. In these animals, cardiac failure developed at a significantly higher hematocrit than in controls. 29 The lowest tolerable hemoglobin concentration in CAD patients is not known and probably depends on several factors, including the severity of the disease. 30 Observational studies have shown an association between anemia (hematocrit < 30%) and increased mortality in patients with cardiovascular disease. 31,32 In this circumstances, however, there are no definitive data showing that blood transfusion either mitigates myocardial ischemia or improves survival. A recent post hoc analysis of patients with acute coronary syndrome who developed anemia during their hospitalization reported that blood transfusion in this setting was associated with higher mortality, 33 confirming the results observed in the TRICC trial. 34 In patients scheduled for CABG surgery, several studies demonstrated that moderate ANH (target hematocrit value 27-33%) is well tolerated, provided that determinant of systemic and myocardial oxygen demand are well controlled In these patients, cardiac medication, especially β-blockers, was maintained until the day of surgery and ANH was performed under anesthesia to avoid the increase in heart rate usually observed in awake patients in these circumstances. 11 However, any increase in myocardial O 2 demand in these conditions could be associated with the development of myocardial ischemia and cardiac dysfunction. Tonkovic et al. 38 recently assessed before surgery the hemodynamic response to moderate dobutamine doses in hemodiluted CAD patients scheduled for off-pump surgery. Patients were hemodiluted to a target hemoglobin concentration of either 9.5 to 10.5 g/dl (moderate ANH) or 7.5 to 8.5 g/dl (severe ANH). Dobutamine infusion at a rate of 5 µg/kg/min was associated with a significant increase in cardiac index in patients undergoing moderate ANH, but not in patients undergoing severe ANH. The dobutamine infusion was associated with an increase in heart rate and arterial pressure in both groups, but resulted in the more severe hemodiluted patients in the development of myocardial O 2 supply demand imbalance as evidenced by a decrease stroke volume index and left ventricular stroke work index. 39 The early postoperative period is certainly critical for hemodiluted CAD patients, because they have to face with an increased tissue metabolic demand. 40 However, CAD patients undergoing revascularization surgery differ from the previous studied populations in that coronary artery lesions would have been bypassed at the time they will recover from anesthesia. ANH and Valvular Disease Patients with valvular disease classically represent another population with decreased tolerance to ANH. However, patients with mitral regurgitation undergoing valvular surgery have been shown to tolerate moderate ANH (target hemoglobin of 10.3 ± 0.4 g/dl), although the decrease in O 2 carrying capacities ( Page 13 )

4 are essentially compensated by an increase in O 2 extraction. 41 The hemodynamic response to ANH was not affected by the patient s cardiac rhythm, i.e. whether it was sinus rhythm or atrial fibrillation. To date, ANH is also regarded as contraindicated in patients with significant aortic valve stenosis owing to decreased coronary vascular reserve with the hypertrophied myocardium and a presumed inability to increase stroke volume through the obstructed valve. However, Licker et al. recently showed that moderate ANH to a hemoglobin value of 9.1 g/dl can be safely performed in patients with critical aortic stenosis and preserved left ventricular function. 42 Although cardiac output increased in these patients, mainly through an enhanced venous return, the safety margin for ANH is reduced because dissipation of kinetic energy at the obstructed valve. ANH and Altered Cardiac Function Patients with decreased myocardial function will also exhibit a lower tolerance to ANH as they will be less capable to recruit their cardiac output when compared to those with an intact function. This has been shown by Yalavatti et al. in moderately anemic intensive care unit patients. 43 Acute administration of negative inotropic agents like anti-arrhythmic drugs or β-blocking agents will also reduce the cardiac output response to ANH. 25,44 In the postoperative period, tolerance of patients with altered myocardial function will closely depend on the potential recovery of their cardiac function and, again, on the level of metabolic demand. ANH and Hemostasis Although some plasma substitute used in the context of ANH may directly interfere with normal hemostasis, there is no evidence from the literature that ANH is associated with increased preoperative bleeding. In a first meta-analysis evaluating the efficacy of ANH in reducing perioperative allogeneic transfusion, Bryson et al. 50 reported that ANH had a small and insignificant effect on the volume of blood lost in the perioperative period (weighted mean difference: -117 ml; 95% CI, -292 to 58 ml). Similar results were found in the six studies evaluating ANH in cardiac surgery (weighted mean difference: -233 ml; 95% CI, -459 to -5 ml). 50 In a second meta-analysis published recently, Segal et al. 51 reported that the volume of intraoperative blood loss was similar in the ANH groups and in the usual care groups, but that total (intra- and postoperative) blood loss was significantly less in the ANH groups than in the usual care groups (weighted mean difference, -91 ml; 95% CI, -195 to -25 ml Q = 48; p < 0.001). The authors also noted that ANH was less effective in reducing perioperative blood loss in other surgeries compared to orthopedic or cardiac surgery. Efficacy Theoretical Aspects The basic concept behind ANH is that hemodiluted patients lose less autologous erythrocytes per milliliter of lost blood during surgery and after retransfusion of the patient s blood. 5 Consequently, ANH is most efficacious when a high volume can be collected prior to a surgical procedure associated with high blood loss. 6 Several authors have developed equations to calculate the efficacy of ANH as a function of surgical blood loss, initial hematocrit, target post-anh hematocrit, and hematocrit used as the transfusion trigger Presuming a usual surgical patient (preoperative hematocrit of 32-36%) and a transfusion decision based exclusively on a trigger hemoglobin of 6-7 g/dl, Weiskopf 55 calculated a minimal fractional blood loss of 50% to enable any saving of allogeneic red blood cells with ANH. Expressed as a fraction of the patient s blood volume, 55% to 77% of total blood volume must be lost during surgery in order to achieve savings of 180 ml of RBCs, representing one standard unit. In this theoretical model, however, fractional blood loss is overestimated because blood loss subsumes both the volume of blood removed during ANH and the volume of blood lost with the intraoperative hemorrhage. The calculation of fractional blood loss should be exclusively based on the intraoperative blood loss. In this case, the effective fractional blood loss will be found at about 25%. Other factors that limit the usefulness of these different models in clinical practice are related to the fact that some of them do not take into account postoperative blood loss or a higher postoperative transfusion trigger and that others Hemodilution could first affect hemostasis through the dilution of the plasmatic and cellular coagulation factors. Indeed, red blood cells have been shown to interfere with hemostasis, not only through a mechanical effect (they push away the platelets to the periphery of the vessels) but also through biological effects related to the release of intracellular adenosine diphosphate, the direct activation of platelets and the generation of thrombin. 45 The clinical consequences (i.e., importance of perioperative bleeding) of these interactions between the red blood cells and the hemostasis remain to be defined. A recent mathematical model even suggests that the decrease in coagulation factors and platelets may be more limiting than low hemoglobin values during advanced hemodilution. 46 Hemodilution could also affect hemostasis through the direct effects of plasma substitution fluids on the platelets and the coagulation mechanisms. 47 These effects are more marked with dextrans than with gelatins and albumin. For hydroxyethyl starches, these effects appear closely related to the intrinsic properties of the different starch solutions, such as a high in vitro molecular weight and a high degree of hydroxyethyl assume that once the transfusion trigger has been reached, substitution. 48,49 * This article was previously published available in autologous the Canadian Journal blood of will Anesthesia be transfused (June-July 2003 continuously Supplement). on ( Page 14 )

5 a milliliter-for-milliliter basis, which does not correct for the effective RBC mass substituted. Results From the Literature The efficacy of ANH as a blood conservation technique remains controversial. Three meta-analysis have systematically reviewed the literature to determine whether ANH was effective in reducing the likelihood of exposure to allogeneic blood in the perioperative period. 50,51,56 They concluded that the efficacy of ANH is relatively modest and closely depends on the use or not of protocols to guide transfusion practice. They all emphasized the fact that proper analysis of the results was hampered by the relatively poor quality of the studies and the marked heterogeneity between trials, partly explained by study factors (patient collectives, target hematocrit values, transfusion triggers, ANH technique, etc.). Specific results for cardiac surgery were only presented in two of them. In the first one, 50 ANH reduced allogeneic blood exposure (OR, 0.51; 95% CI, ; results from 6 studies including 266 patients) and the amount of units transfused (weight mean difference, units; 95% CI, to -0.31; results from 6 studies including 365 patients). In the other one, the pooled relative risk of exposure to allogeneic blood transfusion was also reduced with ANH (OR, 0.77; 95% CI, ; results form 10 trials). In both subanalyses, the use of a protocol to guide perioperative transfusion was not reported. The major pitfall of these meta-analyses, however, is that they all included studies not specifically designed to assess the efficacy of ANH. In fact, only eight prospective randomized studies have really evaluated the capability of ANH to reduce allogeneic blood exposure in patients undergoing cardiac surgery (Table 1) From these eight studies, results about the efficacy of ANH remain inconclusive. Most of the studies are older (transfusion medicine has rapidly evolved over the last years), used quite different ANH techniques, included a small number of patients, did not use protocols to guide blood transfusion and performed only low-volume ANH. More recent studies have been published, with negative results. 65,66 However, in these studies, low-volume ANH was added to a well-defined blood conservation approach aiming at decreasing perioperative blood losses by the use of cell saving techniques and the administration of aprotinin or tranexamic acid. As a result, the hematocrit (or the hemoglobin concentration) before leaving the operating room was similar in the control and ANH patients. As postoperative blood losses were comparable, the net erythrocyte loss was similar in both groups. It is therefore not surprising that the addition of limited ANH to such a blood conservation approach did not result in a decreased allogeneic Table 1. Studies Evaluating the Effect of ANH on the Likelihood of Allogeneic Blood Exposure and on the Number of Units Transfused First author, year Surgery (ANH / control) Baseline Hct (%) Trigger Hct (%) Blood collected (ml) Plasma substitute Dietrich, CABG 25 / ml/kg HES Allogeneic blood exposure Units transfused Hallowell, Cardiac 25 / NA 1252 ± 84 Ringer s lactate, 5% albumin or PPF NA Herregods, CABG 39 / NA 750 ± 245 Gelatin Kahraman, CABG 14 / ml/kg Kaplan, Cardiac 60 / 20 NA NA ± 750 Lillehaasen, Tempe, Aortic valve Valve repair 15 / NA 855 ± / Triulzi, Cardiac 18 / 28 NA NA 924 ± 130 Abbreviations: HES = hydroxyethyl starch; NA = not available Crystalloids + gelatin Plasma & Ringer s lactate Ringer s lactate Ringer s lactate Crystalloids + colloids ( Page 15 ) NA NA

6 blood requirement. As described above, the efficacy of ANH depends on the amount of blood collected, the hematocrit value used as the transfusion trigger, and the volume of surgical blood loss. Demonstrating the efficacy of ANH as a blood conservation technique requires protocols that either avoid the concomitant use of other techniques decreasing perioperative blood loss or compare these techniques with ANH. 67 Figure 1. Technique of Intentional Hemodilution Practical Aspects Intentional hemodilution can be performed just before or shortly after the onset of anesthesia. Performing hemodilution after induction of anesthesia has gained wider acceptance, since hemodynamic stability of the patient is ensured and there is usually enough time to collect the predicted blood volume before significant surgical blood loss occurs. 5 In skilled hands the procedure does not require an extension of the period of anesthesia prior to the onset of surgery. ANH could also be performed just before going on bypass, collecting heparinized blood from the venous cannula and reinfusing colloids or crystalloids through the arterial cannula. 68,69 Calculating Blood Collection Different formulas and nomograms have been proposed to determine the volume of blood that should be withdrawn to reach the target hematocrit or hemoglobin concentration. 70,71 The simplified formula proposed by Gross is one of the most often used (Figure 1). 71 Whatever the formula used, isovolemia as a starting point is essential for correct calculation for the allowable collected blood volume. Clinical experience indicates that it is not easy to achieve precisely the desired target hemoglobin concentration or hematocrit by blinded adherence to the hemodilution nomogram. It is therefore recommended to determine on-site hemoglobin or hematocrit intermittently by means of a portable measuring system. 72 Recently, a new mathematical model has been developed that seems to predict more accurately the exchangeable blood volume. 73 This algorithm, which can be obtained from the authors, could enhance patient safety and provide a correct physiological basis for further studies addressing the efficacy of ANH. 73 Technique Depending on the patient s characteristics, up to liters of blood are withdrawn from a venous or an arterial line into standard collection bags containing anticoagulant, usually CPD (citrate-phosphate-dextrose). In cardiac surgery patients, two important points have to be taken into account: the presence of the underlying cardiac disease that reduces the tolerance of the patient to severe hemodilution and the importance of the priming used in the cardiopulmonary bypass (CPB) that will further dilute the patient. The hematocrit (or hemoglobin concentration) to be maintained during the CPB will depend on several factors (i.e., the pump flow, the temperature). During moderate hypothermic (26-28 C) CPB, maintenance of an hematocrit of 20% has been recommended, while performing CPB at higher temperature while require to monitor mixed venous O 2 saturation to adapt pump flow and/or hematocrit. 74 Using such an approach has been shown to significantly reduce allogeneic blood exposure in patients undergoing cardiac surgery under CPB. 75 Although several retrospective studies reported that severe hemodilution (hematocrit below 20-22%) could be associated with perioperative organ dysfunction and increased postoperative morbidity, randomized clinical trials are needed to determine whether this is a cause-and-effect relationship or simply an association. These observations, however, stressed the importance of adapting the monitoring of systemic O 2 delivery as a function of the degree of hemodilution. As cardiac disease patients exhibited a decreased tolerance to acute anemia, large-volume ANH in such patients requires a two-steps procedure: moderate hemodilution (hematocrit 27-30%) will be performed just after induction of anesthesia, which will be completed just before the beginning of the CPB. In these conditions, replacing the blood collected through the venous cannula by the prime fluid administered through the arterial line will improve the efficiency of the technique by decreasing the total amount of fluid administered. The bags containing the autologous blood are numbered sequentially, labeled and stored at room temperature in the operating room to preserve platelet function. Aseptic technique is of utmost importance during the procedure. The blood bags usually contain anticoagulant for ml of autologous blood. In case significantly less blood is collected into the bags, hemostasis of the patient may be altered during retransfusion due to the high concentration of the anticoagulant in the stored blood. 5 On the other hand, collecting more than ml per bag could result in massive blood clotting in the bag, due to ( Page 16 )

7 insufficient anticoagulant. Colloids and/or crystalloids can be used to maintain normovolemia, the volume to be administered depending on the physicochemical characteristics of the substitute that is used (Figure 1). From time to time, the bags should be gently agitated to ensure adequate mixing. Before and after CPB, normovolemia and normothermia must be maintained. Normovolemia is essential for optimal cardiac output response, and normothermia is crucial to minimize surgical blood loss. Autologous blood will be retransfused once the transfusion trigger (hematocrit or hemoglobin concentration) has been reached. Depending on the patient characteristics, the clinical situation, the ongoing blood loss and/or the likelihood of achieving surgical hemostasis, this trigger may vary somewhat between 20 and 30%. Autologous blood will retransfused in the inverse order of collection, the first unit of blood collected, which is the richest in red blood cells, platelets and coagulation factors, being transfused preferably at the end of the operation or when surgical bleeding has been controlled. Blood collected just before going on bypass contains heparin, and will be ideally retransfused during protamine administration. Any autologous blood kept at room temperature must be retransfused within 6 hours from harvesting. 6 Monitoring During intentional hemodilution, physiologic compensatory mechanisms play a major role in maintaining tissue oxygenation. 7 It is therefore important to adequately monitor the cardiorespiratory parameters. 5,79 Depending on the patient s health status (preexisting disease, preoperative treatment, etc.), the type and the duration of the surgical procedure and the degree of hemodilution, the extent of monitoring will vary (Table 2). Developing a Blood Conservation Strategy Acute normovolemic hemodilution should be considered as one of the components of the overall strategy aimed at reducing patients exposure to allogeneic blood products. This blood conservation approach has the advantage of being focused on the patient, as opposed to those related to a surgical or a medical procedure. 80 As a rule, combining techniques decreases allogeneic blood exposure. However, the effects of combining different techniques are not as predictable as it may seem at first glance. The efficacy of combining different techniques should also take into account the costs of these alternatives. 81 Last but not least, to be really effective, a blood conservation approach also requires the adoption of a standardized multidisciplinary blood transfusion policy. 82 Any actor who decides to transfuse a patient at a higher transfusion trigger than the others will ruin the efforts of the whole team. Developing a blood conservation strategy starts with the establishment of a reliable system of data collection, both at the surgical team and the hospital levels. The Table 2. Hemodynamic Monitoring in the Perioperative Period 1. Hemodilution to a hematocrit of 25% a. Continuous monitoring i. ECG monitor (lead II and V5) ii. On-line ST-segment analysis iii. Invasive arterial pressure iv. Pulse oximetry v. Urine output b. Intermittent monitoring i. Central venous pressure ii. Arterial blood gas analysis iii. Hemoglobin or hematocrit 2. Hemodilution to a hematocrit of 20% a. Additional monitoring i. Central venous pressure ii. Venous blood gas analysis iii. Arterial lactate b. Facultative i. Non invasive cardiac output measurements ii. Pulmonary artery catheter iii. Continuous cardiac output measurement (pulmonary or arterial catheter) iv. Continuous mixed venous O 2 saturation (SvO 2 ) measurement v. Transesophageal echocardiography Adapted from Ref. 5, 79. choice of the techniques being applied will primarily depend on the type of situation one faces. Identifying patients risks for transfusion should alter patient management perioperatively to decrease their transfusion rate and make more efficient use of blood resources. 83 Any addition to a set of existing blood conservation methods needs careful assessment to avoid useless or even counter-productive efforts. As a good example, Casati et al. recently evaluated the effects of moderate ANH added to a comprehensive blood-sparing protocol in off-pump coronary surgery. 84 They reported a significant reduction in the likelihood of allogeneic blood exposure and a decrease in the total number of allogeneic blood transfused in the group of patients undergoing ANH. Because the interests of patients and actors change over time, the developed blood conservation strategy must be continuously adapted to the needs of specific surgical populations. 80 Conclusions ANH entails the removal of blood from a patient either immediately before or shortly after induction of anesthesia ( Page 17 )

8 and the simultaneous replacement with crystalloids or colloids to maintain normovolemia. It is a relative simple, cheap and effective tool to avoid or to reduce allogeneic blood transfusion. The efficacy of ANH depends on the preoperative hematocrit (initial value), the target hematocrit (to which hemodilution is performed), and the preset intra- and postoperative transfusion trigger. Based on the available literature, it seems relatively modest in cardiac surgery, although this needs to be confirmed with well-designed studies. Knowledge of the physiologic compensatory mechanisms that occur during ANH and their limits are essential for the safe use of this technique. In addition, the anesthesiologist must be familiar with its practical aspects. ANH has a place in cardiac surgery but must be regarded as an integral part of blood conservation strategy tailored to the patient s needs and adapted to specific surgical procedures. 1. Stover EP, Siegel LC, Parks R, et al. 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Anesth Analg 2004;98: is published quarterly by LMS Group, 75, rue Guy Môquet Malakoff, FRANCE S.A.R.L. au capital de 7622 Phone: Fax: Printed in Singapore by COSPrinters Pte Ltd, No. 9 Kian Crescent, Singapore Numéro ISSN: All rights reserved Copyright LMS Group Advertising: For details on media opportunities within this journal, please contact the advertising sales department (phone: ) Dépôt légal 4 ème trimestre ( Page 19 )