• Acute normovolemic hemodilution (ANH) is a strategy that may be implemented to decrease the need for blood transfusions and their associated risks; it is a form of autologous blood transfusion. Other commonly utilized techniques include preoperative autologous donation and cell salvage.
• The process of ANH entails the controlled removal of whole blood from the patient prior to incision and blood loss and simultaneous replacement with an appropriate volume of crystalloid or colloid (non-red cell containing), in order to maintain normovolemia and avoid hypotension.
• The goal is to decrease the loss of red cell mass (as well as other blood cells and proteins) during surgical bleeding. ANH has the following advantages:
  • No risk of transfusion error
  • No risk of disease transmission
  • Simple
  • Inexpensive
  • Minimal use of collective resources
  • Stored in the operating room
  • Whole blood with all components is returned to the patient; minimal loss of coagulation factors and platelets secondary to limited storage time.
  • Does not require significant coordination with the patient and blood bank (unlike autologous blood donation)

• Transient intraoperative anemia is achieved by removing whole blood to a target hematocrit while concurrently administering crystalloid or colloid to maintain normovolemia. By decreasing the hematocrit, there is a reduced loss of erythrocyte mass during surgical bloodshed.
• Hemodynamics during dilutional anemia
  • Combination of decreased blood viscosity and local vasoregulatory factors. The endothelium senses changes in intraluminal blood flow, shear stress, and the chemical environment that result from hemodilution and changes in cardiac output; it appears to respond by releasing NO and causing vasodilation. Additionally, studies have suggested that hemodilution may decrease the blood's ability to scavenge and inactivate NO (results in increased levels). Autonomic nervous system-mediated vasodilation does not appear to play a significant role.
  • Cardiac output is increased (stroke volume) secondary to decreased viscosity (decreased afterload/tension of the left ventricle); the myocardium is capable of ejecting more volume.
  • Mean arterial pressure is usually maintained within the limits of normal, secondary to the compensatory increase in cardiac output.
  • Heart rate may increase with profound anemia.
• Critical red cell mass: A concept that describes the lower limit of hemoglobin that is capable of maintaining effective oxygen delivery. Below this threshold, ischemia and anaerobic metabolism/lactate production can result.
• Unlike techniques such as deliberate hypotension, ANH does not directly decrease surgical blood volume loss.

• Phlebotomy may be performed via a:
  • Large vein in the periphery; newly placed or pre-existing large-bore IV catheter or one-time stick
  • Central line
  • Arterial line

• Decreased blood oxygen carrying capacity occurs secondary to hemodilution. Studies have demonstrated that tissue oxygenation is usually not sacrificed if normovolemia is maintained.
• Cardiac ischemia: Usually presents with tachycardia and ECG changes as a result of decreased oxygen delivery
• Edema may be present postoperatively if large volumes of crystalloid and/or colloids are used to maintain normovolemia.

• Indications:
  • Spinal surgery
  • Prostatectomy
  • Hysterectomy
  • Hip arthroplasty
  • Major liver resections: The reported rate of blood transfusion is rarely <30% and patients normally require 1–2 units. One study demonstrated that the use of ANH did not result in adverse cardiac, renal, or neurologic outcomes. Benefit were particularly pronounced in patients who had significant blood loss that exceeded 800 mL.
• Contraindications:
  • Pre-existing anemia
  • Unstable angina
  • Coronary artery disease with significant stenosis or recent myocardial infarction in the last 6 months
  • High-grade aortic stenosis or carotid stenosis
  • Renal impairment
  • Bacteremia
• Invasive monitors may be needed to closely monitor the effects of anemia, including volume status and frequent blood draws in order to monitor oxygen delivery, confirm EBL, and electrolytes. The extent of monitoring should factor in comorbidities, the extent of hemodilution, and expected blood loss.
• Collection:
  • Phlebotomy: Whole blood is collected immediately prior to, or during, the initial stages of surgery (ideally prior to any blood loss).
  • Fluid replacement: May be done with either crystalloid or colloid to maintain normovolemia
  • Storage: Whole blood is collected into blood-collection bags with a predetermined anticoagulant (usually CPD) and kept in the operating room. Agitation of the bags throughout the case can decrease clotting.
• Considerations:
  • Patient becomes unstable during hemodilution: May be evidenced by unexplained tachycardia or ECG changes despite normal intravascular volume. Phlebotomy should be halted and blood returned to the patient in order to avoid any acute cardiac events.
  • Patient becomes unstable intraoperatively: The removed blood may need to be transfused to treat severe anemia. Should be administered prior to donated blood.
  • May be utilized with cell salvage techniques; however, should not be combined with deliberate hypotension.
• Determining the amount of blood to be collected:
  • Safe hematocrit: A standard, agreed-upon post-hemodilution value does not exist. Target levels usually range from 20% to 28% and are dependent upon myocardial oxygenation needs, as well as comorbidities (in particular cardiac and cerebral disease).
  • Red cell mass: Dependent on starting hematocrit and total blood volume. Healthy patients with normal and high hematocrits and men have a greater amount of red cells that can be collected (compared to those who are chronically ill, anemic, or women).
  • Anticipated surgical blood loss
• Efficacy of ANH is dependent upon:
  • Volume removed: Patients that can tolerate a large volume of blood removal will likely have a higher final hematocrit once surgical bleeding has stopped and the blood is returned. Whereas patients who are unable to donate as much blood will clearly not receive as much red blood cell mass when blood is returned.
  • Effectiveness of hemodilution: Inadequate volume repletion, initially or intraoperatively, can result in hemoconcentration and increased red blood cell mass.
  • Timing of autologous blood replacement: If the collected blood is returned after surgical bleeding is complete, the patient will have less absolute red cell losses.

• Estimated blood volume (EBV)
  • Male: 70 mL/kg
  • Female: 60 mL/kg
• Blood volume to be withdrawn = EBV × (Hct_initial – Hct_target)/Hct_average
• for a 70 kg man with a starting Hct of 40% and target Hct 28%: [(70 mL/kg × 70 kg) × (40 - 28)]/[(40 + 28)/2] = 1,729 mL
• for a 70 kg man with starting Hct of 35%: [(70 mL/kg × 70 kg) × (35 - 28)]/[(35 + 28)/2] = 1,089 mL
• Volume replacement with appropriate amounts of crystalloid (3 × volume withdrawn) or colloid (1 × volume withdrawn)

• Cell Salvage
• Autologous Blood Transfusion
• Anemia
• Myocardial Oxygen Supply
• Blood Oxygen Carrying Capacity

Teresa L. Moon , MD