Blood volume measurement - state-of-the-art

The term blood volume (BV) measurement can be understood as the exact volum etric definition of both components of blood, the red cell volume (RCV) and the plasma volume (PV) using tracer dilution methods. The tracer used to m easure the RCV must be bound to the erythrocytes and for the PV to plasma p roteins, in order to label the distribution space of each carrier (i.e. ery throcytes and albumin molecules). To differentiate this there are indirect methods to estimate the BV, such as measurement of the diastolic pressure o r transoesophageal echocardiography, which will not be discussed here. Alte rations in the RCV and PV cannot be routinely measured, or at most only rou ghly estimated by means of the haematocrit (Hc) or haemoglobin (Hb) concent ration which can lead to serious errors when large changes have occurred. A t present measurements of the RCV and PV are not carried out in routine cli nical practice. The introduction of nonradioactive tracers with a faster el imination now renders possible a relatively exact measurement of both volum es under certain clinical situations, albeit with a high technical outlay. The RCV is measured using the tracer sodium fluorescein (SoF) and the PV wi th the dye indocyanine green (ICG). The RCV measurement seems to be suitabl e for certain clinical situations, such as characterization of the preopera tive condition of a patient or quantification of surgical blood loss after an operation, because it is less invasive and has a high precision. However the results of the RCV measurement can only be delivered after 1 h which m akes it more suitable for clinically stable situations. In contrast the PV estimation is based on the measurement of the ICG concentration in the arte rial bloodstream after a bolus injection of the dye in the central veins an d is used more in intensive care because of the invasivity. The results can be obtained 5 min after injection of the dye and therefore even rapid chan ges in the PV can be monitored.