Three different measurement systems for recording relative blood volume are incorporated in currently available dialysis machines and in their feedback systems for automated blood volume–controlled ultrafiltration. In the system of the Gambro-Hospal group (Stockholm, Sweden), integrated in the machines Integra, AK 200, and Artis, an optoelectronic instrument measures the absorption of near-infrared monochromatic light (wavelength, 810 nm) transmitted through blood. The amount of absorption is directly related to the hemoglobin concentration. The removal of plasma volume leads to an increase in the relative hemoglobin concentration, thus causing an increased absorption of light.1 In the Nikkiso system (Tokyo, Japan), near-infrared monochromatic light (wavelength, 805 nm) is transmitted through blood likewise. The blood volume is deduced from the reflected light measured by a light-receiving element.2 The dialysis machines 4008 and 5008 (Fresenius Medical Care [FMC], Bad Homburg, Germany) perform blood volume monitoring with an ultrasonic method. Using this system blood volume is assessed by measuring the transit time of an ultrasonic pulse transmitted through blood. The speed of sound in whole blood depends on the total protein concentration (TPC), which is the sum of plasma proteins and hemoglobin. The relative blood volume is finally determined from changes in the TPC.3
On grounds of clinical evidence, and the findings by Mercadal et al.,4 suggesting a malfunction of the optical absorption method in the presence of sodium changes, we compared the AK 200 system with the ultrasonic method under different osmotic conditions in our previous study published in ASAIO Journal.5 We completed the study by additional examination of the optical reflection system by Nikkiso.
Six stable chronic hemodialysis patients5 underwent one dialysis session each with the AK 200, the FMC 5008, and the Nikkiso DBB-07 machine. The treatment conditions were identical in all sessions (described in detail elsewhere).5 To induce rapid osmotic changes, 10 ml of hypertonic saline (20% NaCl) was administered as a bolus injection into the venous blood line after 2 hours of treatment. The data recorded by the blood volume monitors of the three dialysis devices were compared.
One minute after the bolus injection, the monitor using the optical absorbance method (AK 200) showed a distinct −8.7% (±4.6%; range, −17.4% to −5.3%) decrease in blood volume, whereas the optical reflection device (Nikkiso DBB-07) displayed an increase of +11.1% (±3.1%; range, +7.2% to +15.8%). The ultrasonic monitor (FMC 5008) recorded a change of −0.8% (±0.9%; range, −1.8% to +0.5%) in blood volume 1 minute after the bolus injection (Figure 1). After 3 minutes, the ultrasonic and the optical reflection devices displayed the same results (+1.2%), whereas the absorbance monitor still showed significant differences compared with the other two methods after 20 minutes (p = 0.046).
Sudden osmotic influences alter the ratio of scattered and crossing light most likely because of changes of the red cell surface.6–8 Under the tested conditions, both optical systems show opposite and nonplausible courses of blood volume changes in the first minutes. The ultrasonic system seems to be less susceptible to osmotic changes.
A fourth system, the optical Crit-Line (FMC, Kaysville, UT) stand-alone device shows comparable results with that of the ultrasonic system under different experimental osmotic conditions.8 But this device is not included in a feedback system.
Devices used for relative blood volume measurement in hemodialysis should be insensitive to osmosis-induced changes in red blood cell volume,8 especially if they are included in sodium changing feedback-control systems. It is noteworthy that both examined optical systems work in feedback-control systems (Hemocontrol, Gambro; Haemo-Master, Nikkiso), which are partly based on the changes of dialysate conductivity. Therefore, the reliability of sodium changing feedback-control systems featuring these tested optical monitors has to be critically reviewed.
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