Laser Doppler Flowmetry (LDF)

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Laser Doppler FlometryThe study of skin may be undertaken to provide primary knowledge about cutaneous tissue or to use the skin as a surrogate organ for the investigation of microcirculatory events. Essentially, there are no other easilly accessible microvascular beds in humans, making cutaneous flow studies of great importance. Measures of laser-Doppler flow (LDF) have recently allowed ready access to the superficial dermis and therefore have dominated studies of cutaneous microcirculation.

The surface area of dermal capillaries provides for approximately 30 M2 of endothelial coverage. Blood flow, usually averaging 10-20 ml/min/100 g, may vary from 1 to 200 ml/min/100 g, indicating a remarkable plasticity of regulatory control in this vascular bed. The skin therefore is highly vascular and very accessible. However, the key question confronting any study of surrogate tissues including laser Doppler studies of cutaneous microvasculature is: Does information obtained from cutaneous microcirculation correlate with changes in coronary, renal or muscle vascular beds, which are ultimately produce myocardial infarction, insulin resistance, hypertension, and progression of renal failure? There is positive evidence: Thus, it has been demonstrated that cutaneous microvascular dysfunction correlates with blood pressure and insulin resistance. Patients with cardiac allograft vasculopathy or with coronary three-vessel disease have been found to exhibit abnormalities of cutaneous microcirculation. Diabetic patients show cutaneous microangiopathy which is more pronounced in those with advanced retinopathy. Furthermore, cutaneous microcirculatory responses are blunted in patients with angiographically demonstrated coronary artery disease. These are all studies relating to changes in flow due to altered local microvascular factors which is primarily how we will employ such methods.  These data suggest that the investigation of cutaneous microcirculation using laser-Doppler flowmetry may adequately mirror the state of microcirculation in other vascular beds. 

 In experiments LDF can only be used to determine relative changes in blood flow in subjects. Typically, Doppler flows are  indexed to the maximum flow achievable with sodium nitroprusside microdialysis. 

We use a point source or an integrating probe employing continuous coherent laser light at a wavelength of 780 nm which is sampled at a similar sample rate of 200 sps for consistency with other signals interfaced through an A/D converter (DATAQ) using custom softwware. LDF is measured in arbitrary perfusion units (pfu). 

We have combined LDF measurements with thermal hyperemia and with reactive hyperemia measurements to provide physical stimuli to the system. Also, LDF may be combined with such techniques as local iontophoresis of biologically active chemicals and medications and the flow response assessed. More powerful methods include combining LDF with microdialysis techniques which, in addition to being more robust than iontophoresis in terms of locally delivering biochemicals, are also capable of sampling the interstitium for endogenous substances.  .

 

A laser Doppler Flowmeter is shown with the laser probe facing the camera. Representative laser-Doppler flow is shown below. The small high frequency" oscillations coincide with the heart beat. The superimposed low frequency flow rhythm is produced by changes in the opening and closing of the small arterioles (vasomotion) and is denoted "flowmotion"
 

Iontophoresis is an established method by which a small applied electrical field is used to facilitate the rate of penetration of drugs into the accessible tissues such as skin  When combined with laser flow measurements, iontophoresis has been used as a diagnostic tool in vascular diseases, and as a research tool to understand blood flow. On the one hand it has the disadvantage of local surface application i.e. it is applied by contact. On the other hand local application is a distinct advantage when studying skin since a small area can be tested without systemic consequences. Therefore, it protects research subjects from the effects of applied drugs while also circumventing systemic compensatory mechanisms.

Effective iontophoresis depends on proper solution composition and pH, current strength, ionic competition, drug concentration, molecular size, physiologic factors, and current mode application whether continuous or pulsed.  A current density can often but not always be found to avoid side effects including electrical discomfort and nonspecific electrical current neural activation .  

 

The figures demonstrates two iontophoretic arrangements (one on each leg) with positive (black) and negative (red) electrodes electrical contacts arising from a current source (the white box at lower left). The positive electrodes are attached to an electrode overlying a gauze saturated with the biochemical to be iontophoresed. Current in the 100's of microamp range is typically applied for seconds to minutes to deliver specific amounts of the test chemical. By altering the amount of charge delivered a dose response curve may be generated as shown below which demonstrates the response to acetylcholine.
 

 


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Peripheral Vascular Properties Measured by Strain Gauge Plethysmography (SPG)
Laser Doppler Flowmetry (LDF)
Indicator Techniques to Measure Blood Volume and Cardiac Output
Heart Rate and Blood Pressure
Near Infrared Spectroscopy
Microneurography and Muscle Sympathetic Nerve Activity (MSNA)
Transcranial Doppler Ultrasound (TCD)
Impedance Plethysmography (IPG)
Skeletal Muscle Pump