How many kinds of blood volume monitoring are available and what is the different/if any technology behind them?

Clinical measurements can be generally classified into two categories. Bulk flow (i.e. macroscopic) measurements utilizing temperature change, density change, velocity and volume/time; and second, the ability to discern microscopic blood or biological constituents of the blood such as the hematocrit, red cell volume, oxygen saturation content of the erythrocyte, etc.

Bulk flow devices (i.e. Hemoscan, Fresenius BVM) measure averaged changes in parameters of the entire media - not individual components. Information about microscopic properties, such as hematocrit and blood volume, must be then inferred or extrapolated from the bulk flow information and require an invasive "calibration" sample. In fact, bulk flow devices must be calibrated (by invasive sample means) prior to each use, thus providing a reference for all subsequent measurements. In ALL cases, the potential for error is great due to the assumption that the bulk, averaged parameters remain constant. For example, it maybe assumed that if the density of the blood changes during dialysis, this density change is a result of changing hematocrit. The bulk flow or "relative" measurement technology (i.e. ultrasound or single wavelength) is not able to discern what is causing the density change - only that the density has changed. Constituents other than the red cell, such as fats, proteins, macromolecules, etc. could all be responsible in varying degrees for the density change. Changes of the mean red cell volume due to the presence of a foreign "inject-able" such as sodium concentration may also be responsible.

When it comes to BV changes, bulk flow or relative measure devices are able to estimate that the blood volume has fallen by some value, say 15%, but are unable to determine the actual (or absolute) value of a given parameter or its starting value; only that there has been a change. This could be roughly likened to holding a flashlight to the blood tube and inferring that the Hct is lower because the color of the blood is lighter because the net effect of the bulk flow on the light is to allow more light through to the observer. Hence, the measurement of the SUM of all microscopic constituents. However, this process cannot actually measure the individual constituents themselves.

In contrast, the CLM provides an absolute Hct, or an actual value. For example, the CLM is able to measure that a patient has a Hct of 36.2. This value could be confirmed by or correlated to a measurement made by a Coulter Counter, another absolute micro-measurement device.

CLM directly measures absolute measurement of the hematocrit independent of other blood parameters. Using wavelengths specific to the human erythrocyte, the CLM measures the scattering and absorption properties of the red cells as they pass through the disposable. A patented ratio-metric computation is then employed to determine the absolute Hct.

For Blood Volume monitorining the Crit-Line is the gold standard. Other devices (Hemoscan and Fresenius BVM) are being used outside the US, but do not have the additional abilities of the Crit-Line (i.e. Hct, Oxygen Saturation, Access Blood Flow, Recirculation).

Some of these devices do not graphically display BV change, instead they show one reading of BV change and ask that the clinican graph these data points on graph paper.

Additionally, the other types of BV monitors do not have the ability to print a hard copy of results.

[This message has been edited by Russell Lindberg (edited 04-24-2000).]