How is AIHD accomplished?
AIHD is accomplished early in the perioperative period, usually just after the induction of anesthesia. Blood is removed from the patient via a large-bore intravenous catheter and stored in standard blood bags containing anti-coagulant. An arterial catheter may be used for collecting autologous blood, but we have found this to be less satis-factory. Simultaneously, crystalloid, in a 3:1 ratio, or colloid (albumin, hydroxymethyl starch, or dextran) in a 1:1 ratio is infused through another large-bore intravenous catheter. The amount of blood to be removed can be calculated by any of the formulas used to calculate allowable blood loss. We typically use the following formula:
HctA = starting hematocrit
HctB = target hematocrit for hemodilution
EBV = estimated blood volume.
Typically, a target Hct in the mid to upper 20s (25–27%) is used; this allows for substantial hemodilution yet allows some margin of safety when blood loss begins to occur during surgery. In this instance, HctA = 40%, EBV = 70 kg
70 mL/kg = 4,900 mL, and we will choose a HctB = 27%. The formula then yields:
(40 − 27 × 4,900)/(40 + 27/2) = 1,900 mL
Thus, 3–4 units of the patient’s blood could be removed for later retransfusion.
As units of blood are removed, they are labeled and numbered consecutively. Blood is retrans-fused in reverse order of collection. The first unit removed is the least dilute and the richest in red cells, plasma factors, and platelets; therefore, it should be the last unit retransfused.
Blood removed during hemodilution may be stored in the operating room at room temperature for a maximum of 6 hours. Autologous blood remaining after surgery may be stored in a blood bank refrigerator for further use.
Invasive hemodynamic monitoring (arterial catheter, central venous catheter) is not mandatory during isovolemic hemodilution, but it facilitates serial Hct measurements and provides a guide to fluid replacement. Because CO rises in AIHD without an increase in heart rate, development of intraoperative tachycardia may indicate hypovolemia and the need for retransfusion. A urinary catheter to monitor urine output as a gauge of intravascular volume may be helpful. Also, replacing autologous blood with 3 times the volume of crystalloid initiates a diuresis.