Evaluation of an ultrasonic blood volume monitor

Hypotension complicates approximately 30% of all dialysis treatments. Although the genesis of hypotension is multifactorial, hypovolaemia is thought to play a major role as a direct result of decreased blood volume, particularly during ultrafiltration. The described blood volume monitor enables blood volume to be measured continuously by a non-invasive technique.

The blood volume monitor is based on the principle that the total protein concentration, the sum of haemoglobin and plasma proteins in the vascular space, changes during ultrafiltration. Changes of total protein concentration are determined from the velocity of sound waves in blood, measured using a cuvette in the extracorporeal circuit designed for this purpose. The precision of the blood volume monitor was evaluated in 180 dialysis treatments in 49 patients. The relative blood volume obtained by the monitor was compared with a standard reference method involving calculation of relative blood volume from serial measurements of haemoglobin.

A very good agreement between the two methods was achieved (SD = 1.70%, r > 0.96). The results showed no sensitivity to changes in serum sodium concentration (range 130-145 mmol). The 'noise' introduced in the blood volume signal was low (< or = 0.2%, sampling rate 10 s) allowing subtle blood volume changes to be detected with high resolution. In addition the device enabled the measurement of haematocrit (Hct) and haemoglobin (Hb) to be made since this is the largest blood component determining total protein concentration. A comparison with the centrifuge method revealed an accuracy of +/-2.9 Hct-%, and a comparison with the photometer an accuracy of +/-0.8 g Hb/dl.

In summary the blood volume monitor allows precise and reliable measurement of relative blood volume. It provides the instrumentation essential for feedback control of relative blood volume during dialysis.