Laser Doppler Flowmeter Measurements Research Articles

Compact Laser Doppler Flowmeter (LDF) Fundus Camera for the Assessment of Retinal Blood Perfusion in Small Animals.

PurposeNoninvasive techniques for ocular blood perfusion assessment are of crucial importance for exploring microvascular alterations related to systemic and ocular diseases. However, few techniques adapted to rodents are available and most are invasive or not specifically focused on the optic nerve head (ONH), choroid or retinal circulation. Here we present the results obtained with a new rodent-adapted compact fundus camera based on laser Doppler flowmetry (LDF).MethodsA confocal miniature flowmeter was fixed to a specially designed 3D rotating mechanical arm and adjusted on a rodent stereotaxic table in order to accurately point the laser beam at the retinal region of interest. The linearity of the LDF measurements was assessed using a rotating Teflon wheel and a flow of microspheres in a glass capillary. In vivo reproducibility was assessed in Wistar rats with repeated measurements (inter-session and inter-day) of retinal arteries and ONH blood velocity in six and ten rats, respectively. These parameters were also recorded during an acute intraocular pressure increase to 150 mmHg and after heart arrest (n = 5 rats).ResultsThe perfusion measurements showed perfect linearity between LDF velocity and Teflon wheel or microsphere speed. Intraclass correlation coefficients for retinal arteries and ONH velocity (0.82 and 0.86, respectively) indicated strong inter-session repeatability and stability. Inter-day reproducibility was good (0.79 and 0.7, respectively). Upon ocular blood flow cessation, the retinal artery velocity signal substantially decreased, whereas the ONH signal did not significantly vary, suggesting that it could mostly be attributed to tissue light scattering.ConclusionWe have demonstrated that, while not adapted for ONH blood perfusion assessment, this device allows pertinent, stable and repeatable measurements of retinal blood perfusion in rats.

Confocal laser Doppler flowmeter measurements in a controlled flow environment in an isolated perfused eye

The aim of this study was to improve our ability to interpret and validate Heidelberg Retina Flowmeter (HRF) flow images by recording flow measurements from specific regions of the retinal vasculature by taking advantage of the ability to precisely regulate perfusion flow in an isolated eye preparation. The retinal vasculature in 16 isolated perfused pig eyes was perfused with a 50%/50% Krebs/RBC solution at known flow rates ranging from 0 to 300 μl min −1. At each flow rate, HRF images were obtained at a location approximately two disc diameters from the disc. After HRF image acquisition, the retinal vasculature was perfused with fluorescein isothiocyanate for fluorescence microscopy. Using the standard HRF software and a 10×10 pixel measurement window, flow rates were measured from a retinal artery, vein, arteriole, venule, and the retinal capillary bed and a capillary-free-zone. The relationship between HRF measured flow and perfusion flow in the different measurement locations was determined. At zero perfusion flow the measured HRF flow was consistently greater than zero (∼170 arbitrary units (AU)), and not significantly different at each measurement location except for the retinal vein, which had a significantly higher HRF flow value (∼230 AU). At higher perfusion flow rates the flow signal from the larger vascular elements (arteries and veins) increased rapidly thereafter to reach several thousand AU at a total perfusate flow of 50 μl min −1 and increased less rapidly at higher flow rates. In arterioles, the HRF flow was more linear over a broader range of perfusate flow rates but the peak flow signal was an order of magnitude smaller than that from the retinal artery. Both the linearity and magnitude of the flow signal in venules was less than that in arterioles. In capillary areas and in the capillary free zone, the HRF flow showed only a very weak relationship to perfusion flow when compared to the background noise. The choice of location for HRF flow analysis greatly influences the ability of the technique to measure changes in retinal blood flow. The major arteries and veins provide the strongest signal and greatest signal to noise ratio. However, the retinal arterioles produce an HRF signal that is more linear over a wider range of perfusate flow rates.

Feasibility of LDF Measurements of Optic Nerve Head Blood Flow in Children with Cerebral Malaria

The pathogenesis of cerebral malaria (CM), a significant cause of death in the tropics, is still not understood. Cerebral blood flow measurements would be important but are difficult under the conditions prevailing in CM clinics in the tropics. With the goal of using optic nerve head (ONH) blood flow ( F onh) instead of cerebral blood flow to help outcome prediction, we have tested the feasibility of performing F onh measurements in comatose CM children, using a portable ocular laser Doppler flowmeter (LDF). Measurements were performed in one eye of each of 13 children (2.7 ± 1.1 years) during a period of about 8 min. The Vel onh (the relative blood velocity), Vol onh (the relative blood volume), and F onh values were determined for each child from three to five measurements of 2 to 20 s. Average Vel onh, Vol onh, and F onh values were 0.49 ± 0.08 kHz, 2.4 ± 1.06 arbitrary units (a.u.), and 89 ± 16.8 a.u., respectively. The average coefficients of variation of the flow parameters based on all segments in each child were 13 ± 9% (range 1–29%) for Vel onh, 27 ± 13% (range 3–65%) for Vol onh, and 23 ± 12% (range 5–42%) for F onh. This study demonstrates the feasibility of LDF measurements in CM children. The large range of these coefficients of variations could be due to the presence of fluctuations of ONH blood flow on a short time scale in these neurologically unstable children.

Secondary transplantation following a period of isografting: experimental study in rats.

The authors describe a study in which groin flaps from 20 Wistar rats were transplanted to another group of 20 Wistar rats and, after various time intervals, the groin flaps were re-transplanted back to the original animals. The goal of the first transfers was to preserve the flap in the second group of animals (isopreservation). During the isopreservation period, the second rat (the preserver) was treated with steroids or FK506 for immunosuppression. Thirty-three free groin flap transfers were performed between 40 rats. If possible, the same flap was transferred twice between two animals, one as an isograft, and other as an autograft following an isopreservation period in 13 pairs of animals. (A second transfer was not possible in seven pairs of animals.) The period for isopreservation varied between 2 days and 1 week. The survival of the flap was observed by visual inspection, laser Doppler flowmeter measurements, and was correlated with serial histopathologic examinations of skin and vessel biopsy specimens, including the anastomosis site. The severity of histopathologic signs that might be related to developing rejection was increased by the preservation time, and was more noticeable after the second transfer. The authors showed that successful secondary transplantation of the rat groin flap with a 1-week follow-up could be achieved, following isopreservation of at least up to 5 days.

The influence of ocular pulsatility on scanning laser Doppler flowmetry

PURPOSE: To determine the effect of the cardiac cycle on scanning laser Doppler flowmeter measurements of retinal capillary blood flow in rhesus monkeys and humans.METHODS: Multiple scanning laser Doppler flowmetry images of rhesus monkey and human retinal capillary blood flow over a range of heart rates were obtained. Average flow values were determined for the 64 scan lines that compose the two-dimensional flow map. Cutaneous blood flow was measured simultaneously with a laser Doppler flowmeter. The temporal relationships between retinal capillary blood flow, peripheral arterial pulse, and cutaneous blood flow were determined. In addition, human retinal capillary blood flow in a 10 × 10-pixel area during different phases of the cardiac cycle was compared.RESULTS: Regular oscillations in human and rhesus monkey retinal capillary blood flow are evident as alternating bright and dark horizontal bands in scanning laser Doppler flowmetry images. These fluctuations are temporally correlated with cutaneous blood flow. Linear regression of actual vs predicted heart rate based on peaks in retinal capillary flow yielded r = 0.999 in a rhesus monkey and 0.938 in a human. Retinal capillary blood flow in a 10 × 10-pixel area fluctuated as much as 50% depending on the phase of the cardiac cycle.CONCLUSIONS: The alternating bright and dark banding pattern observed in scanning laser Doppler flowmetry scans of retinal capillary blood flow is related to the cardiac pulse. The errors introduced by pulse-related fluctuations in retinal capillary blood flow are significant and must be minimized or corrected for accurate and reproducible measurements of ocular hemodynamics.

Skin blood flow during vasoconstrictive and vasodilative stimuli in essential hypertension patients: a laser Doppler flowmetry study.

In order to compare the extent of the elicited vasoconstrictive and vasodilative response at the microcirculatory level in essential hypertension (EH), we measured the skin blood flow by means of a laser Doppler flowmeter (LDF). Thirty-four mild-to-moderate EH patients were enrolled. Twenty-two sex- and age-matched healthy subjects were selected as a control group. The LDF measurements were carried out with the probe over the fingertip of the distal phalanx at baseline conditions (Rest flow, RF), after an ischaemic test (post-ischaemic peak flow) and during an arithmetic stress test (AT). The flow was expressed in arbitrary units. The data were processed using the Perisoft computer program. The relative flows after the ischaemic test (Rel F1) and during the AT (Rel F2) were expressed as a percentage of the previous RF values (RF1 and RF2, respectively). During the AT, the lag time was calculated (in seconds). As compared to the control subjects, RF was significantly lower in the EH group (p < 0.01). During the AT, the EH patients showed a statistically lower mean Rel F2 decrease compared to the control subjects (p < 0.01). No statistically significant difference occurred in the Rel F1 and lag time. These data suggest that the vasoconstrictive capacity of the precapillary vessels is impaired in patients with hypertension.

Reactions to intradermally injected substance P and topically applied mustard oil in atopic dermatitis patients.

Skin reactions and itch or burning pain sensations following intradermal injection of the neuropeptide substance P and topical application of the substance P releasing agent mustard oil were studied in 20 atopic dermatitis patients and 20 healthy controls. Changes in skin blood flow were measured with a Laser Doppler flowmeter. Areas of wheal and flare reactions were evaluated planimetrically. Simultaneous with Laser Doppler flowmeter measurements, subjective itch and burning pain ratings were verbally reported on a category partitioning scale at 10-second intervals. Substance P evoked dose-dependent wheal, flare, and itch reactions in both patients and controls. However, substance P doses of 10(-9) -10(-11) mol elicited smaller flares in patients than in the controls whereas the wheal sizes were similar in both groups. Substance P-induced itch ratings were lower in patients at a dose of 10(-10) mol, and the onset of itching was delayed at all substance P levels applied. Mustard oil elicited similar neurogenic inflammatory reactions in both groups, although pain sensations were significantly delayed in atopic dermatitis patients at two mustard oil concentrations, which is further indication of a desensitization of afferent nerve endings contributing to the neurogenic inflammatory reactions in the skin of these patients.

Postoperative monitoring of allogeneic limb transplantation in rats.

Allogeneic limb transplantation with and without immunosuppression has been successfully reported. We used the experimental model described by Fritz and colleagues to investigate the value of different factors in the rejection phase. Twenty rats of inbred strains sustained an allogeneic limb transplantation without immunosuppression. In 6 rats an isologous limb transplantation was performed as a control group. Factors investigated in the transplanted limb were glucose, lactic acid, and blood gases of the peripheral blood, as well as laser Doppler flowmeter (LDF) recordings and histology. All factors were compared with the normal contralateral limb of the rat. All rats were killed between days 2 to 14 following operation. In the allogeneic limb transplantation model in the rat, clinical and histological examination as well as LDF measurements (p less than 0.0001) are good factors for rejection. In contrast, peripheral blood gases, glucose, and lactate were not useful as factors for rejection. In this model, however, LDF values cannot predict an initial onset of the rejection before clinical signs are evident, presumably because of the impossibility of continuous measurements.

Noninvasive measurement of the human peripheral circulation: relationship between laser Doppler flowmeter and photoplethysmograph signals from the finger.

Under certain conditions laser Doppler flowmeter (LDF) signals obtained from the finger pulp may appear very similar to those obtained by use of a direct current (dc) photoplethysmograph (PPG). A combined LDF/PPG system was used in conjunction with a circumference strain gauge as an index of volume change to identify the conditions in which the correlation between these signals was good. Simultaneous LDF and dc PPG measurements were made on 10 normal volunteers by using arterial occlusion and on 7 normal subjects by using the Valsalva maneuver at different elevations of the forearm and hand with respect to the midsternum. By altering the elevation of the upper limb the influence of venous filling on each of the signals during these maneuvers was observed. Since the dc PPG signal always appeared similar to the volume change indicated by the circumference strain gauge, it is concluded that the dc PPG signal is related to blood volume change if allowance is made for the effects of blood oxygenation. In circumstances of low venous filling, however, blood volume changes correlate well with blood flow changes, producing the correlation between the dc PPG and LDF traces. The dc PPG signal may be used as a means of monitoring changes in blood flow in the finger only when venous filling is low and the return remains unrestricted. Thus, in investigations using this method, the relative position of the limb with respect to the heart should always be indicated. The LDF method appears to be a reliable indicator of blood flow changes in the microcirculation irrespective of the degree of venous filling.

Angiotensin II-induced changes in cochlear blood flow and blood pressure in normotensive and spontaneously hypertensive rats

Previous investigations in our laboratory have measured significant increases in the circulating levels of the potent vasoconstric-tive hormone, angiotensin II (AII; 26 and 64 pg/100 μl plasma, normal and noise exposed, respectively), during and following noise exposure in the alert rat (Wright et al., 1981). In the present study, these levels were approximated through intra-arterial infusion in the anesthetized spontaneously hypertensive rat (SHR) and normotensive Wistar-Kyoto (WKY) rat. Laser Doppler flowmeter measurements of cochlear blood flow (CBF) indicated that despite equivalent All-induced elevations in systemic blood pressure, CBF in the SHR did not increase to the levels measured in the WKY. Pretreatment with the specific angiotensin receptor antagonist sarile, (Sar 1,Ile 8-AII), reduced All-induced elevations in systemic blood pressure in members of both strains, but did not change the overall pattern of CBF. These results indicate that SHRs may have a compromised cochlear circulation that is refractory to increases in systemic blood pressure.

Laser Doppler flowmeter measurements of skin perfusion changes associated with arterial and venous compromise in the cutaneous island flap

Laser Doppler flowmeter measurements of skin perfusion changes associated with arterial and venous compromise in the cutaneous island flap

Laser Doppler flowmeter measurements of skin perfusion changes associated with arterial and venous compromise in the cutaneous island flap.

Laser Doppler flowmetry (LDF) is a noninvasive, continuous measure of tissue surface microcirculatory red blood cell (RBC) flux. LDF was calibrated against varying RBC volume fractions and flow rates using extracorporeal perfusion of saphenous island flaps of dogs. The RBC flux was found to be linear with LDF signal. The difference in vascular geometry of the skin appeared to play a minor role in generating the LDF signal compared to changes in the RBC flux. In vivo experiments of varying arterial perfusion pressure, flap venous pressure, and total blood flow of the cutaneous island flap had a linear relationship with LDF signal.