What does laser Doppler Flowmetry measure?

 
Laser Doppler flowmetry is an established technique for the real-time measurement of microvascular red blood cell (or erythrocyte) perfusion in tissue. Perfusion is sometimes also referred to as microvascular blood flow or red blood cell flux.
 
Laser Doppler signals from the tissue are recorded in BPU (Blood Perfusion Units) which is a relative units scale defined using a carefully controlled motility standard comprising a suspension of latex spheres undergoing Brownian motion.
 
Laser Doppler Flowmetry (LDF) works by illuminating the tissue under observation with low power laser light from a probe containing optical fiber light guides. Laser light from one fiber is scattered within the tissue and some is scattered back to the probe. Another optical fiber collects the backscattered light from the tissue and returns it to the monitor.
 
Most of the light is scattered by tissue that is not moving but a small percentage of the returned light is scattered by moving red blood cells. The light returned to the monitor undergoes signal processing whereby the emitted and returned signals are compared to extract the Doppler shift related to moving red blood cells.
 
The LDF technique offers substantial advantages over other methods in the measurement of microvascular blood perfusion. Studies have shown that it is both highly sensitive and responsive to local blood perfusion and is also versatile and easy to use for continuous monitoring.
 
The method is potentially non-invasive (since the probe is not actually required to touch the surface of the tissue) and in no way harms or disturbs the normal physiological state of the microcirculation. Furthermore, the small dimensions of the probes have enabled it to be employed in experimental environments not readily accessible using other techniques.
 
Measurements obtained by LDF are intrinsically of a relative nature. Although such measurements are proportional to perfusion, the factor of proportionality will be different for different tissues.
 

 Blood Perfusion and the BPU

 
Our laser-Doppler blood perfusion monitors output a signal that is proportional to the red blood cell perfusion (or flux). This represents the transport of blood cells through microvasculature and is defined as:
 

Microvascular blood perfusion therefore, is the product of mean red blood cell velocity and mean red blood cell concentration in the volume of tissue under illumination from the probe.
 
Microvascular blood perfusion is displayed by our monitors in relative units called Blood Perfusion Units (BPU).
 
All of our blood perfusion devices have been calibrated with a constant known motility standard so that, for a given perfusion situation, all probes will read the same value of blood perfusion expressed in blood perfusion units (BPU).
 

 What volume of tissue is measured?

 
LDF defines a perfusion parameter from information contained in the optical spectrum of light remitted from the tissue. The actual measurement sampling volume or depth can only be determined by identifying precisely which blood vessels and erythrocytes have interacted with the remitted light, which in turn, is principally dependant on two parameters; namely the optical scattering and optical absorption coefficients of the tissue under observation.
 
Since both of these coefficients are entirely dependent on the site of observation and perfusion of the microvasculature at the time of measurement, it is impossible to determine the actual sampling volume/depth at any tissue site.
 
Generally speaking however, we have estimated that for well-perfused tissue such as muscle, the mean sampling depth for our probes is in the region 0.5-1.0 mm with a concomitant sampling volume in the region 0.3-0.5 mm3. For cutaneous measurements, the sampling depth is likely to be in the range 1.0 – 1.5 mm. These estimates have been obtained heuristically through many years of experience and are based on both in vitro observations and mathematical modeling of photon diffusion through 'imaginary tissues’ using Monte-Carlo techniques.
 

Unique features

 
Our range of dedicated laser-Doppler probes for blood perfusion measurements can be user-autoclaved up to 20 times. 
 
Both OxyFlo™ and OxyLab LDF™ instruments can be used to make up powerful multi-parameter monitoring systems when combined with our tissue pO2 monitors (OxyLite™ and OxyLab pO2™, respectively). Combined probes uniquely allow the simultaneous acquisition of pO2, blood perfusion and temperature from the same tissue micro-region.
 

The Instruments

 
OxyFlo™ - Simultaneous blood perfusion monitoring from up to 4 tissue sites
(for experimental applications only)
 
OxyLab LDF™ - Single-channel tissue blood perfusion monitoring
(for experimental applications only)