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1.  Is there a simple formula that is clinically useful to estimate how much blood loss can occur before my patient's hematocrit decreases to a predetermined number? What is such a formula?
2.  Is there a formula that will allow the estimation of blood loss based on pre- and post-operative hemoglobin, taking into account the influence of any blood transfusion? What is such a formula?
公式系列
1.在确定红细胞比积允许的降低值（Hct允许值）情况下，临床如何计算允许失血量？
2.在明确术前、术后Hb值情况下，如何计算失血量？如果同时给与了输血时，又如何计算失血量（需要考虑输入的部分血液将随着出血而丢失）？

参考答案：

1.在确定红细胞比积允许的降低值（Hct允许值）情况下，临床如何计算允许失血量？

临床实践和研究中测量手术出血量困难，不精确且不标准。失血量可通过收集失血直接测量，通过吸引瓶、纱布等所含血量分析，或者通过血红蛋白浓度和红细胞比积间接估计。有学者提出了几种估计失血量的方法并使用了一些数学模型来证实[1-7]。假设可对围术期失血量进行粗略测量，我们就可以估计到患者达到某一红细胞比积所允许的失血量（ABL）。
简单且易记忆的公式如下：
ABL = Wt x BV/kg x ( Hct _preop - Hct_target ) x 3
ABL，允许失血量
Wt，患者公斤体重
Hct preop，Hct target单位为%
BV/kg，每公斤体重体表面积血液毫升数

年龄　　　　BV（ml/kg）
早产儿　　　　　95
小儿　　　　　　85
成年男性　　　　75
成年女性　　　　65

2.在明确术前、术后Hb值情况下，如何计算失血量？如果同时给与了输血时，又如何计算失血量（需要考虑输入的部分血液将随着出血而丢失）？

Flordal 认为失血是持续的，部分输血在随后的出血过程丢失。作者提出了下列公式：
①如果患者术前失血量为Ⅴ，Hb将根据下列方程降低：
Hb _postop = Hb_preop x e^-v/bv
②已知术前术后Hb，失血量可通过下式计算：
V = BV x ln _preop/Hb_postop
③然而，如果在测量Hb的同时进行输血，这必须被考虑。如果我们假设输血以与失血速度相当合理的速度进行，那么
V= BV x ln Hb _preop/(Hb_postop - n x Hb _trans x (1-e^-v/bv)/V)
这里：BV，血容量
V，术前失血量
n，输血单位数
Hb trans，每单位输血的红细胞Hb数量（大约55g）

上述方程备注：
A，这是假设术后BV=术前BV。计算机计算表明这种作用影响很小。
B，方程考虑到部分输入的血液在同时的出血中丢失。
C，该计算需要计算机的辅助。注意到V出现在方程的两边。作者表明计算被重复进行。

编者注：
这些方程可为临床和研究做一参考。然而，包括快速和/或进行性失血的动态临床状况常需要系列实验室分析以指导临床判断。

Is there a simple formula that is clinically useful to estimate how much blood loss can occur before my patient's hematocrit decreases to a predetermined number? What is such a formula?
The measurement of surgical blood loss during clinical procedures and for clinical studies is difficult, imprecise and is not standardized. Blood loss can be measured by direct measurement of collected blood, through analysis of blood content in suction bottles swabs, drains, etc., or indirectly by assessment of hemoglobin concentration and hematocrit. Several authors have offered means of estimating blood loss and have some have even employed mathematical modeling methods to address this issue (1-7). Assuming that we have some rough measurement of inta-operative blood loss, can we estimate a patients allowable blood loss (ABL) to reach a target hematocrit.
A very simple, easy to remember formula that has provided one of the editors (SBC) with excellent predictive value is:
ABL = Wt x BV/kg x ( Hct preop - Hct target) x 3
Where:
ABL = Allowable Blood Loss
Wt = patients wt in kg
Hct preop and Hct target are expressed in %
BV/kg = the number of milliliter of blood per kg of body mass
Age  Blood Volume (ml/kg)
  
Premature Infant  95
Infant  85
Male Adult  75
Female Adult  65
A few notes about the above formula
•  The Wt x BV/kg is a simple and useful estimate of total patient blood volume.
•  The "3" in the above formula is used as a simple alternative to dividing by the mean hematocrit, as found in more complex formulas. One will discover, that in most clinical situations, multiplying by 3 approximates dividing by the mean hematocrit.

Is there a formula that will allow the estimation of blood loss based on pre- and post-operative hemoglobin, taking into account the influence of any blood transfusion? What is such a formula?
Flordal  has taken into account that blood loss is a continuous process and that some of the transfused blood is lost in the continued bleeding. The author offers the following formulas:
1. If a patient has a peri-operative blood loss V, then the Hb will fall according to the formula:
Hb postop = Hb preop x e-v/bv

2. Therefore, if Hb has been measured before and after the operation, the blood loss may be calculated as:
V = BV x ln Hb preop/Hb postop

3. However, if blood has been transfused between Hb measurements, this has to be taken into account. If we assume that transfusions are given at a rate fairly proportional to the rate of blood loss, then
V= BV x ln Hb preop/(Hb postop - n x Hb trans x (1-e -v/vb)/V)
Where:
BV = blood volume.
V=perioperative blood loss.
n = number of units transfused.
Hb trans = amount of erythrocyte hemoglobin per transfusion unit (approximately 55g).
Some subtleties of the above formula (# 3) should be noted:
1.  It has been assumed that BV postop = BV preop. Computer calculations have shown that this effect is of minor importance.
2.  The formula accounts for the fact that some of the blood transfused is lost in bleeding.
3.  Computer assistance is needed for this calculation. Notice that V appears on both sides of the equation. The authors note that calculation has been performed by iteration.
Editors' note:
These formulas are provided for clinical and research considerations. However, dynamic clinical situations involving rapid and/or ongoing blood loss often require serial laboratory analysis to guide clinical judgment.
References:
1.  Singbartl K, et al. Hypervolemic hemodilution: An alternative to acute normovolemic Hemodilution? A mathematical analysis. J Surg Res. 1999;8:206-12.
2.  Weiskopf RB. Mathematical analysis of isovolemic hemodilution indicates that it can decrease the need for allogeneic blood transfusion. Transfusion. 1995 Jan;35(1):37-41.
3.  Kallos T, et al. Replacement for intraoperative blood loss. Anesthesiology. 1974;4:293-5.
4.  Kallos T, et al. Effects of hemorrhagic shock on pulmonary diffusion and capillary blood volume of the dog. J Trauma. 1973;13:218-22.
5.  Lorentz A, et al. Calculation of the allowable blood loss before transfusion with a programmable pocket calculator. Anaesthesist. 1987;36:306-12.
6.  Brecher ME, et al. A standardized method for calculating blood loss. Transfusion. 1997;37:1070-4.
7.  Nelson GH, et al. Method for calculating blood loss at vaginal delivery. South Med J. 1981;74:550-2.
8.  Flordal PA. Measurement of blood loss in clinical studies. Eur J Anaesthesiol Suppl. 1997;14:35-7.

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