Acute normovolemic (isovolemic) hemodilution (ANH), also referred to as intraoperative hemodilution, was introduced in the early 1970s . This blood conservation technique entails the removal of blood from a patient, either immediately before or shortly after induction of anesthesia, with maintenance of isovolemia using crystalloid and/or colloid replacement. The amount of blood removed varies between one and three units (450 to 500 mL constitutes one unit), although larger volumes may be withdrawn safely in certain circumstances (see below).
The blood withdrawn is anticoagulated and maintained at room temperature, in the operating room, for up to eight hours. It is reinfused into the patient as needed during, or after, the surgical procedure. ANH can be used as the sole blood conservation technique, or it can be combined with preoperative autologous donation (see "Preoperative autologous blood donation"), blood salvage (see "Intraoperative and postoperative blood salvage"), or both.
INDICATIONS AND CONTRAINDICATIONS
ANH should be considered for patients with good initial hematocrits who are expected to lose more than two units of blood (900 to 1000 mL) during surgery. This technique is better suited to healthy, young adults, but it has been successfully employed in small children and the elderly. Operative settings in which ANH is appropriate include vascular, orthopedic, and some general surgical procedures. In addition, some Jehovah's Witnesses will agree to ANH if the blood is maintained in a closed circuit continuous flow system . (See "The approach to the patient who refuses blood transfusion".)
ANH is contraindicated in the following settings:
- Cardiac disease, since the main compensatory mechanism for the induced anemia is an increase in the cardiac output. However, the decreased blood viscosity associated with the induced anemia may have cardioprotective effects in some cardiac surgical settings [3,4].
- Impaired renal function, since large amounts of the infused fluids will ultimately need to be excreted.
- Baseline hemoglobin <11 g/dL.
- Low concentrations of coagulant proteins, inadequate vascular access, and the absence of appropriate monitoring capability.