转自UpTo Date 网的综述：Acute normovolemic (isovolemic) hemodilution (ANH) Author Arthur J Silvergleid, MD INTRODUCTION — Acute normovolemic (isovolemic) hemodilution (ANH), also referred to as intraoperative hemodilution, was introduced in the early l970s . 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blood removed varies between one and three units (450 to 500 mL constitutes one unit), although larger volumes may be withdrawn safely in certain circumstances (see below).
The blood withdrawn is anticoagulated and maintained at room temperature, in the operating room, for up to eight hours.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 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 。
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 。
Impaired renal function, since large amounts of infused fluids need to be excreted.
Baseline hemoglobin <11 g/dL.
Low concentrations of coagulation proteins, inadequate vascular access, and the absence of appropriate monitoring capability.
TECHNICAL CONSIDERATIONS
Amount of blood to draw — The volume of blood that may be drawn is derived from the following formula:
V = EBV x ({Ho - Hf} ÷ Hav)
where V is the volume to be removed, EBV is the estimated blood volume, Ho is the initial hematocrit, Hf is the desired hematocrit, and Hav is the average hematocrit (average of Ho and Hf). As an example, in an 70 kg patient with an estimated blood volume of 4900 mL (rounded to 5.0 L), an initial hematocrit of 45 percent, and a desired hematocrit of 30 percent, the volume of blood to be drawn is two liters (four units):
V = 5.0 x ( {45 - 30} ÷ 37.5) = 2.0 liters
The actual amount blood removed will depend upon the patient's initial blood volume [5], tolerance of the procedure, and the anticipated blood needs. Although there is good evidence that healthy patients, appropriately monitored, can tolerate hematocrits as low as 20 percent, very few patients actually reach this level. Most anesthesiologists aim for a preoperative hematocrit between 25 and 30 percent.
ANH is generally initiated immediately before or after the induction of anesthesia, but before the commencement of surgery. However appealing it may seem to induce ANH a day or two prior to surgery, it is strictly an intraoperative procedure.
Diluents — Isovolemic fluid replacement can be accomplished either with colloid or crystalloid solutions. The volume infused depends upon the solution used. Crystalloids are distributed through the extracellular (intravascular plus interstitial) volume. As a result, the volume infused must exceed the volume of blood withdrawn by a factor of at least three. In comparison, colloids are restricted to the vascular space. Thus, the volume administered should approximate the volume of blood withdrawn.
The advantage of using crystalloid to replace the withdrawn blood is the ability to remove excess fluid by the administration of diuretics, prior to transfusion. In a typical patient, furosemide (0.15 to 1 mg/kg) is given five to 15 minutes before the transfusion of the patient's blood.
Given the significant fluid shifts and acute dilutional hypoalbuminemia accompanying ANH, marked peripheral edema is not uncommon. In comparison, pulmonary edema is rare in well-controlled procedures.
Equipment — Volume replacement must be simultaneous with blood withdrawal. As a result, the ideal set-up consists of two large-bore cannulae placed into a central (or large peripheral) vein or an artery, most frequently the radial artery. Blood is collected in standard blood bags containing anticoagulant. Also available are hemodilution kits that contain two blood bags, a Y-type connector set with a Luer lock adapter, and a blood recipient identification band. These kits provide a measure of convenience and, absent the phlebotomy needle, a measure of safety as well.
Cell separators, useful in intraoperative blood salvage, add an unnecessary level of complexity and are not recommended. A scale to determine the amount of blood removed is required, as is an agitator, unless gentle agitation of the blood bag during withdrawal can be accomplished by hand. An indwelling bladder catheter is also required, since the urine output serves as a guide to intravascular volume status.
Handling the blood — Each unit of blood should be labeled with the patient's name, hospital number, time of withdrawal, and sequential number (if more than one unit is removed). According to the AABB Standards, units collected by ANH can be stored at room temperature (in the operating room) for up to eight hours; if not used within that time, they can be stored at 1 to 6º C for up to 24 hours, provided that cold storage is begun within eight hours of initiating the collection . Blood removed from the operating room must contain the statement "For autologous use only" on the label.
Transfusion — The collected blood is transfused after major blood loss has ceased, or sooner if indicated. Estimation of blood loss and serial hematocrit determinations serve as a useful guide to transfusion requirements. The units are usually returned to the patient in the reverse order of collection. Thus, the first unit, which has the highest hematocrit and contains the most platelets and undiluted coagulation factors, is administered last. If, however, it is anticipated that all of the units might not be returned, the patient becomes hypervolemic in response to colloid administration, or there is an inadequate response to diuretics, it is prudent to change the sequence so that the first unit(s) do not become outdated.
The lowest safe hematocrit level varies from individual to individual. Except for special cases of extreme hemodilution, most anesthesiologists recommend beginning blood transfusion at a hematocrit of approximately 25 percent in order to have a degree of protection if excessive bleeding should occur.
CONCLUSIONS Potential benefits — The major presumed benefit of ANH and other blood conservation techniques is a decrease in exposure to allogeneic blood, thereby minimizing or avoiding the risk of transmission of transfusion-associated infectious disease. Other consequences of exposure to allogeneic blood are also avoided, including allergic reactions, allosensitization, and perhaps immunomodulation . ANH also has the following advantages:
It can be used in patients who, because of the emergency nature of the surgery or because they were ineligible to participate in a preoperative autologous blood donation (PAD) program, would otherwise not have an autologous transfusion option.
It is cost effective, since testing, storage, and crossmatching costs are not incurred.
It provides the only opportunity to transfuse patients with fresh, whole blood containing viable platelets and high levels of clotting factors.
It increases tissue perfusion as hemodilution decreases blood viscosity .
It may decrease the amount of red blood cells lost during surgery, since the blood lost has a lower hematocrit after hemodilution .
Potential drawbacks — Widespread utilization of ANH has been prevented by concerns about the possible need for increased anesthesia personnel as well as the requisite intensive monitoring of aggressively hemodiluted patients. Concern has also been expressed about the potential for increased blood loss, resulting from dilution of coagulation factors , although the opposite has been demonstrated in selected patients .
Even one of the major presumed benefits of ANH, the diminished loss of red cells, has been questioned by computer modeling. The savings in red cells was less than previously estimated when correction was made for the decreasing hematocrit in bleeding isovolemic patients and the associated need to begin transfusion at some minimal hematocrit level .
A meta-analysis of the English literature has concluded, based on strict inclusion criteria, that the efficacy of ANH is likely to be small, bleeding and allogeneic blood requirements are only modestly reduced, efficacy with regard to avoidance of allogeneic transfusion is unproven, and safety has not been addressed adequately . If the judgments rendered by the authors of this meta-analysis are valid, more definitive studies need to be conducted prior to recommending widespread use of ANH.
Additional comments on the benefits versus drawbacks of ANH have been in the literature for some time . A thoughtful editorial has assessed the hazards of over-interpreting mathematical modeling of blood loss, the need for (and feasibility of performing) additional randomized controlled trials, and other issues that will help bring the current status of ANH into clearer focus 。
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发表于 2009-11-13 20:38:17
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