Hemodilution
 
Acute normovolemic hemodilution is intentional preoperative hemodilution induced by the isovolemic exchange of whole blood with colloid or crystalloid solutions to preserve autologous blood while maintaining normovolemia.  
Intraoperative hemodilution, blood salvage, and retransfusion are appropriate options when large blood loss is expected for a particular surgical procedure, and these are essential adjuncts when treating trauma and shock victims. Preoperative hemodilution however, is performed immediately before or after induction of anesthesia and is common among patients undergoing elective surgery.  
Hemodilution has been employed during orthopedic procedures, including total hip replacement; major general procedures, including liver resection; and, most extensively, cardiac surgery.  
If the diverted blood is maintained in continuous contact with the patient's circulation, this can be reinfused later without violating religious beliefs.
 
 Acute Normovolemic Hemodilution, A Review of Whole Blood Sequestration 
 
Procedure for Acute Normovolemic Hemodiluition

1 . Control of initial hematocrit (HTC), heart rate (HR), mean arterial pressure (MAP, central venous pressure (CVP.  
2. Venous or arterial bleeding into blood bags sitting on a  balance.  Exchange of blood for colloid to obtain autologous blood units 1 and 2.  Intravenous infusion of colloid on a volume-to-volume basis to maintain normovolemia  
3. Control of  HCT, HR, MAP, CVP  
4. Exchange of blood for colloid to obtain autologous blood units 3  and 4  
5. Storage of the numbered autologous blood units near the patient,  ready for retransfusion  
6. Surgical procedure  
7. Control of HCT, HR, MAP, CVP  
8. Retransfusion of  blood units in the order 4, 3, 2, 1
 
 
Contraindications to Acute Normovolemic Hemodilution
  •     Coronary heart disease
  •      Significant anemia
  •      Renal disease
  •      Severe hepatic disease
  •      Pulmonary emphysema
  •      Obstructive lung disease
  •      Severe hypertension
  •      Clotting deficiencies
 
"There is no standard nomenclature for degrees of hemodilution, but it is generally accepted that a hematocrit (HCT) level 28% is hemodilution, 10% is extreme hemodilution, and 15% is profound hemodilution. Degree of hemodilution varies according to the patient's preoperative condition, total body surface area, and duration of surgery. Moderate hemodilution is a safe procedure for patients > 60 years of age, although caution should be exercised with extreme hemodilution in the elderly. Extreme hemodilution is better tolerated in young patients, who have a better capacity to maintain constant circulating blood volume, although it is not suitable for routine use. Hemodilution is not recommended for patients with coronary artery disease, significant anemia. renal disease, severe hepatic disease, pulmonary emphysema, or obstructive lung disease.  
 During hemodilution. oxygen delivery (D02) is maintained primarily by increased cardiac output. Because blood is a thixotropic fluid, the effects of hemodilution on viscosity are determined in part by shear rates and the characteristics of pulsatile blood flow. During hemodilution, as blood viscosity decreases, resistance to flow of low  molecular-weight  fluids used for dilution decreases. Decreased fluid viscosity enhances cardiac output and does not jeopardize 02 flow. The maximum reduction in systemic vascular resistance occurs as the HCT level is lowered from 45% to 30%. Lower molecular-weight dextran colloid may improve the distribution of blood flow within peripheral microcirculation with enhanced efficiency of 02 exchange. Maximum 02 transport capacity occurs at a HCT level of  30%. Compensatory mechanisms that result from hemodilution include increased overall blood flow rates, increased 02 extraction, and reduced affinity of 02 to hemoglobin (Hb) that shifts the dissociation curve to the right. As long as the patient remains well oxygenated, increase in cardiac output compensates for the decrease in Hb concentration. The hemodynamic properties of blood are improved by reduced HCT values, and, provided the compensatory mechanisms are intact, tissue perfusion and oxygenation are not adversely affected. Even at levels of profound hemodilution, oxygen extraction increases and 02 consumption remains at near-normal levels. Preservation of D02 during hemodilution is predicated on the assumption that normovolemia be maintained -a caveat that cannot be overemphasized. Extremely low HCT levels (< 15%) are tolerable when hypovolemia is rigorously avoided. The degree of control exercised in an experimental setting cannot necessarily be achieved under operating room conditions. When moderate to profound hemodilution is coupled with any degree of  hypovolemia, decompensation occurs and cardiac output may fall precipitously. During hemodilution, blood is withdrawn pre- or intraoperatively, collected in standard bags, and stored as citrate phosphate dextrose (CPD) blood at room temperature. This process preserves the integrity of red blood cells and clotting factors and ensures the availability of safe, fresh, autologous blood for postoperative reinfusion. Neither special equipment nor personnel are required for ANH or retransfusion, and there are no delays for repeated phlebotomy. Operating time is often decreased. which ultimately results in reduced intraoperative blood loss. Other benefits of ANH include postoperative improvements in pulmonary, renal. and myocardial function".  
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Extract from "Alternatives to Allogenic Blood Use in Surgery: Acute Normovolemic Hemodiluition and Preoperative Autologous Donation"; D'Ambra MN and Kaplan DK, The American Journal of Surgery, Vol.170, N°6A (suppl.), December 1995