Flordal, P. A.
Department of Surgery, Danderyd Hospital, Danderyd, Sweden
Direct measurement of blood loss has poor reliability and poor validity. A formula is proposed for how blood loss may be calculated from pre- and post-operative haemoglobin, taking the influence of any blood transfusions into account.
The measurement of surgical blood loss in clinical studies is difficult. Although it is the main end-point in many studies, it is measured with poor precision and there is no standardized technique. This paper will discuss various pitfalls of several measuring techniques and make a proposal for a new technique for assessment of surgical blood loss.
There are two ways of measuring blood loss: one is by direct measurement of collected blood (swabs, suction bottles, drainage bags, etc.), and the other is by indirect measurement. The latter reflects blood loss by assessment of haemoglobin concentration, haematocrit or the need for blood transfusions.
Direct measurement of blood loss
The amount of blood collected in swabs is often estimated by visual assessment. However, this has poor reliability and poor validity. Weighing the swabs is an improvement, but there is always some evaporation and dilution. The swabs may be dry or partially wet before use and may also have absorbed irrigation fluids or exudate. Swabs can be washed to extract the blood, but this is complicated.
The volume or weight of blood from suction device bottles can be used to assess blood loss. This can be inaccurate as often only small volumes are collected. Blood may remain in the tubes leading to the suction bottles and there is also a problem with dilution (irrigation fluids and exudate), which may influence the accuracy of the method.
The drainage of blood from the operation site may be incomplete for different reasons. Compartments of blood may form in the operation area where only some compartments are drained. Moreover, drainage tubes may clot. Large volumes may remain in the operation site with no external evidence. The function of the drainage system may also depend on the treatment, e.g. if haemostatic drugs are being studied, drainage may be better with one treatment than with another.
Summary of direct measurements
Direct measurement of blood loss does not consider haematomas or haemolysis, which are important causes of blood loss and are as important to the patients as overt blood loss. Further, direct measurements of blood loss are inaccurate and systematic errors may occur. In addition, dilution/drainage function may depend on treatment. It is also difficult to assess the proportion of whole blood in collected fluid. Direct measurement of blood loss is invaluable in the clinical situation, although it is inadequate for research purposes.
Indirect measurement of blood loss
Haemoglobin (Hb) drop is the usual means of measuring blood loss indirectly. It has direct clinical relevance to the risk of myocardial infarction. This method cannot be used when there are transfusions. Sometimes, haematocrit (Hct) is used instead of Hb. Either may be chosen, because there is an almost constant relationship between Hb and Hct: Hct (%)=Hb (g L-1/3.4.
The second indirect measurement of blood loss is the transfusion volume, which has a direct clinical relevance. However, the transfusion policy, compliance with transfusion policy and use of different blood preparations may vary from centre to centre.
Summary of indirect measurements
Hb and transfusion volume have a direct relationship with clinical risks, e.g. acute myocardial infarction and viral transmission. These measurements are accurate. Haematomas and haemolysis are implicitly taken into account. The main disadvantage is that Hb drop measurements and transfusion volume do not strictly determine 'blood loss', but are a reflection of blood loss. Also, both quantities need to be stated if blood transfusions are used. Hb and transfusion volume obviously affect each other, but the relationship is complicated.
Relative blood loss
Relative blood loss is proposed to be defined as blood loss in relation to the patient's estimated blood volume (BV) :EQUATION 1. This provides an assessment of blood loss, where differences between men and women, weight differences and age differences are eliminated . In addition, there will be a reduction of variance leading to an increased statistical power. Furthermore, relative blood loss has a direct relationship with Hb drop.
Calculated blood loss
If a patient has a peri-operative blood loss V, then the Hb will fall according to the formula:EQUATION 2. Therefore, if Hb has been measured before and after the operation, the blood loss may be calculated as:EQUATION 3. If blood has been transfused between Hb measurements, this has to be taken into account. If transfusions are given at a rate proportional to the rate of blood loss, then: EQUATION 4. Here, n=number of units transfused and Hbtransf=amount of erythrocyte haemoglobin per transfusion unit (typically 55 g).
It has been assumed that BVpostop=BVpreop. In hip arthroplasty this is true on the third post-operative day . Blood volume and fluid balance variations in the peri-operative period could also disturb the calculation, but computer simulations have shown this to be of minor importance.
The formula accounts for the fact that some of the blood transfused is lost in bleeding. Blood loss calculated in this way allows a 'weighted sum' of Hb drop and transfusions to form one single measure.
Because V, the calculated blood loss, also appears on the right side of the equation, the calculation has to be done by iteration. A first guess, say, V=500 (ml), is used to evaluate the right-hand side. The result is used as a new better guess for V and the right side is evaluated again. This procedure is repeated until the right and left sides are equal. Usually about five iterations are needed to obtain identity down to the last interger digit (1 ml).
Computer assistance is needed for the calculations. They are easily carried out using Excel or a statistical package. Table 1 gives an example of application of the concepts of relative blood loss and calculated blood loss on data from patients undergoing hip arthroplasty . In this example, both Hb drop and transfusion requirements were about 20% smaller in the desmopressin treatment group, but this was not statistically significant. However, taking both into account in calculating blood loss reveals statistical significance.
Formulae for calculation of total blood loss from transfusion data and pre- and post-operative Hb or Hct have previously been suggested by other authors [5-9]. However, they ignore the fact that blood loss is a continuous process and that some of the blood that has been transfused is lost in continued bleeding.
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Proceedings of an Advisory Board Meeting, held at Hotel Scribe, Paris, France; 31 May, 1996