The measurement of blood volume--state-of-the-art

Abstract

The term blood volume (BV) measurement can be understood as the exact volumetric definition of both components of blood, the red cell volume (RCV) and the plasma volume (PV) using tracer dilution methods. The tracer used to measure the RCV must be bound to the erythrocytes and for the PV to plasma proteins, in order to label the distribution space of each carrier (i.e. erythrocytes and albumin molecules). To differentiate this there are indirect methods to estimate the BV, such as measurement of the diastolic pressure or transoesophageal echocardiography, which will not be discussed here. Alterations in the RCV and PV cannot be routinely measured, or at most only roughly estimated by means of the haematocrit (Hc) or haemoglobin (Hb) concentration which can lead to serious errors when large changes have occurred. At present measurements of the RCV and PV are not carried out in routine clinical practice. The introduction of nonradioactive tracers with a faster elimination now renders possible a relatively exact measurement of both volumes under certain clinical situations, albeit with a high technical outlay. The RCV is measured using the tracer sodium fluorescein (SoF) and the PV with the dye indocyanine green (ICG). The RCV measurement seems to be suitable for certain clinical situations, such as characterization of the preoperative 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 makes it more suitable for clinically stable situations. In contrast the PV estimation is based on the measurement of the ICG concentration in the arterial bloodstream after a bolus injection of the dye in the central veins and 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 changes in the PV can be monitored.