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Abstract

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES

Objective

To investigate the hypothesis that cutaneous microvascular perfusion of the dorsum of the hand (in response to local heating) and distal phalanx (in response to occlusion) is impaired in patients with systemic sclerosis (SSc) compared with healthy controls.

Methods

Twenty-nine patients with SSc and 29 control subjects were recruited. Perfusion was monitored using novel dual-wavelength laser Doppler imaging, allowing measurement of both smaller (capillaries) and larger (thermoregulatory) vessels. Postacclimatization, a baseline dorsum scan (red or green wavelength) was performed. A heating pad was placed on the dorsum (total stimulus time 6 minutes at 34–40°C), and following removal of the pad, baseline wavelength scans were performed until perfusion returned to baseline values. This was then repeated for the second wavelength. The maximum perfusion increase due to heating (PEAK1) and area under the perfusion–time curve (AUC) were determined. In addition, scans (both wavelengths) of the index finger were performed prior to and during 2 minutes of suprasystolic occlusion, and the response upon occlusion release was monitored with single-point laser Doppler. The decrease in perfusion due to occlusion (from preocclusion baseline values) (%DECREASE) and the maximum increase (from baseline perfusion values under occlusion) in hyperemic perfusion upon removal of occlusion (PEAK/OCC) were calculated.

Results

PEAK1 and AUC values were not significantly different between patients and controls, as assessed with either wavelength. A significant difference between groups was found in the %DECREASE values with the green, but not the red, wavelength. A significant between-group difference was also found in PEAK/OCC values, using both wavelengths.

Conclusion

This study suggests that SSc has no effect on microvascular perfusion in the dorsum of the hand, and that the abnormal microvascular response is localized to the digits, affecting both smaller and larger vessels.

Abnormalities of both vascular structure and function are well recognized in systemic sclerosis (SSc). Abnormalities include endothelial impairment, lumen narrowing, and fibrosis of the microvasculature (1–3). The most characteristic clinical expression of this vascular dysfunction/damage is severe Raynaud's phenomenon (4). Recent investigations have indicated that this damage may be responsible for the abnormal production and regulation of vasodilators and vasoconstrictors such as cytokines, growth factors, and endothelin 1, and that these may in turn be the cause of increased collagen production and fibrosis (5, 6).

Laser Doppler imaging (LDI) and single-point laser Doppler flowmetry (LDF) are both sensitive methods of measuring cutaneous blood flow in patients with SSc and help to differentiate between primary (idiopathic) Raynaud's phenomenon and Raynaud's phenomenon secondary to SSc, as compared with healthy controls (7, 8). These techniques are particularly useful when monitoring a patient's response to dynamic stimuli that cause significant changes in blood flow, such as occlusion, heating, cooling, and iontophoresis (9–12). In addition, both methods can be used to monitor treatment response (13–16).

LDI and LDF utilize the Doppler effect, where changes in wavelength due to movement between the source (in this case, the red blood cells, from which light is back scattered) and observer are used to determine the speed of blood flow. The laser Doppler technique was first applied by Stern in 1975 to quantify changes in blood flow (17) and has since been used extensively. LDI involves perfusion mapping of areas, rather than examination of blood flow at a single point as with LDF; by taking into account blood flow in adjacent points within an image and performing some averaging techniques, LDI is more likely than LDF to give a clinically meaningful image of perfusion, since, due to the heterogeneity of cutaneous blood flow, LDF is subject to large fluctuations as a result of minute changes in the probe position (e.g., because of movement artefacts) (18). However, LDI scans take a finite time to acquire, and therefore cannot provide the continuous monitoring that is possible with LDF.

In both the LDI and the LDF method, blood flow is represented by flux (flow through an area) and is measured in arbitrary perfusion units, which are directly proportional to the product of the mean speed and concentration of red blood cells. In LDI, an image of relative blood flow is built up point by point as the laser scans over an area of skin, whereas in LDF, relative blood flow at a single site of interest is plotted over time. For our study, both techniques were noncontact, although some LDF systems used in previous studies require laser delivery via an optical fiber, and therefore require skin contact. Of the laser Doppler studies involving patients with SSc, very few have utilized LDI (most have used single-point monitoring with LDF).

In this study, we describe the use of a novel dual-wavelength LDI system in the investigation of the microvascular abnormalities of SSc. We provide only a brief background describing this new system, since it has been described in detail elsewhere (19). The majority of LDI studies have been carried out at a single wavelength, which images blood flow in certain types of vessels depending on the penetration and absorption of the laser wavelength. Green, red, and infrared wavelengths have penetration depths of ∼0.3, 0.5, and 1.0 mm, respectively, in excised skin (20), and it has been shown, in in vivo studies with dual-wavelength LDF, that longer wavelengths allow for examination of deeper perfusion (21).

Within the skin, in the absence of melanin, it is hemoglobin that is the main visible light absorber, particularly of shorter wavelengths. In the skin, which is relatively avascular and transparent to light, green light is approximately twice as likely to be absorbed as red; however, in hemoglobin, green light is 20 times more likely to be absorbed (21,22). Light that enters the dermis (i.e., that is not absorbed by melanin in the epidermis) can be assumed to penetrate until it reaches a blood vessel, at which point the light is either scattered or absorbed. Upon scattering, the light will then either undergo further scattering, perhaps back toward the surface of the skin and a detector, or be absorbed. Green light that reaches the detector can be thought of as having come from smaller microvessels, such as capillaries, rather than a larger, thermoregulatory-type microvessel, in which it is more likely to be absorbed due to the higher chance of interaction with hemoglobin, although it may be scattered from the peripheral flow of a larger microvessel. Red laser light is less likely to be absorbed by hemoglobin, and it is more likely to be back scattered to the detector from larger microvessels (both from peripheral and from axial flow), having passed through the capillaries without significant interaction, although some of the signal will originate from the nutritive capillaries and other small microvessels encountered in the upper layers of the dermis. Therefore, it can be assumed that images obtained by green laser are mostly of small microvessels, whereas images obtained by red laser are of larger microvessels (19,23).

The advantage of a dual-wavelength system is the ability to image different components of the microcirculation. Dual-wavelength LDF and LDI therefore hold promise for application in SSc and are now being used to study other diseases in which microcirculatory abnormalities occur (22,24,25). Our novel red (633 nm) and green (532 nm) dual-wavelength system enables measurement of perfusion in both larger, thermoregulatory vessels and smaller, nutritive capillaries (19). The present study was undertaken to investigate the hypothesis that cutaneous microvascular perfusion in the dorsum of the hand and in the digits is impaired in patients with SSc, as quantified by the response to the dynamic stimuli of local heating and occlusion, respectively, when compared with a healthy control group.

PATIENTS AND METHODS

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES

Patients.

Twenty-nine patients with SSc (7 male, 22 female, median age 52 years [range 35–62 years], median number of years since onset of Raynaud's phenomenon 17 [range 2–46 years]) and 29 healthy controls (5 male, 24 female, median age 51 years [range 31–75 years]) were recruited into the study. Twenty-two patients had limited cutaneous SSc and 7 had diffuse cutaneous SSc as defined by LeRoy et al (26). Eleven patients had severe ischemia (defined as a history of admission for intravenous vasodilator therapy or history of surgical debridement and/or amputation). Thirteen patients were anticentromere antibody positive and 2 were antitopoisomerase antibody positive. The patient and control groups included 2 and 4 smokers, respectively; however, all subjects had been asked to refrain from caffeine consumption and smoking on the day of the study. Of the 14 patients receiving vasodilators, 4 were able to stop treatment for at least 2 days prior to the study. The study was approved by the Salford and Trafford Local Research Ethics Committee.

Equipment.

The dual-wavelength LDI system used for the study (a modified Moor LDI-vr, 633 nm [red, 3 mW] and 532 nm [green, 5 mW]; Moor Instruments, Axminster, UK) has been previously described (19). All images were obtained at scan speeds of 4 ms/pixel, were normalized for intensity (with electrical zero subtracted), and analyzed using system software; results from the Moor LDI version 3.08 images are displayed in arbitrary perfusion units.

Protocol.

Design.

The protocol was designed to examine the change in perfusion due to simple, reproducible stimuli (local heating of the dorsum of the hand and occlusion of the index finger) in the 2 groups, and was adapted from our initial studies in healthy controls (19). The study was carried out in a temperature-controlled room at 23°C; all subjects were acclimatized for 20 minutes before the study began.

Local heating.

Following acclimatization, a baseline scan, ∼3 cm2 in area, was carried out on the dorsum of the dominant hand. The wavelength of the scan was randomized to red or green. A circular heating pad (PF3; Perimed, Järfälla, Sweden), 2 cm in diameter, was placed within the imaged area, avoiding any visible vessels. The position of the pad on the skin was marked with 3 small dots in order to locate the heated area once the pad was removed and to make detection of movement between frames more apparent. The pad temperature, initially 34°C, was increased to 40°C in increments of 2°C over 3 minutes, and then remained at 40°C for 3 minutes; the total stimulus time was 6 minutes. Upon pad removal, a series of scans was initiated using the same wavelength as that used for the baseline image; the single-scan duration was ∼40 seconds. Scans were continued until perfusion appeared to have returned to baseline values. The process was then repeated for imaging with the second wavelength. One set of scans using the green wavelength was not used in the analyses due to the presence of artefacts in the images.

Occlusion.

Prior to occlusion, the index finger (dominant hand) was imaged with both wavelengths (in a randomized order). The middle phalanx of the index finger was then occluded, suprasystolically, for 2 minutes at 200 mm Hg, using a pressure cuff. During the final minute of occlusion, scans of the index finger were repeated with both wavelengths, which, to avoid bias, were assigned randomly.

Hyperemia.

Immediately before removal of occlusion, the LDI was transferred into single-point measurement mode (i.e., LDF) and randomized to the red (n = 15) or green (n = 14) wavelength to allow continuous measurement over time (as already stated, this is not possible with LDI, in which the series of scans generated is not strictly continuous). The initial increase in perfusion, at the distal phalanx, following cuff release and consequent decrease in perfusion over time, as steady-state flow was restored, was observed for 5 minutes after the occlusion was removed.

Data analysis.

Local heating.

A circular region of interest (ROI), the size of the heating pad (as defined by the dots on the skin), was used for all subjects; when necessary, its position was altered between frames if movement occurred. The mean flux value of the ROI for each frame was recorded and plotted against time, allowing measurement of the maximum perfusion increase due to heating (expressed as the PEAK1 value) and area under the perfusion–time curve (AUC). The measurements were normalized by dividing by baseline values.

Occlusion.

The percentage decrease in perfusion due to occlusion was calculated by comparing images of the distal phalanx before (image 1, flux 1) and during (image 2, flux 2) occlusion. A rectangular ROI was defined for the distal phalanx, distal to the distal interphalangeal joint but proximal to the fingernail. The flux decrease was then calculated as a percentage of the preocclusion baseline values (%DECREASE), as follows:

  • equation image
Hyperemia.

For the single-point perfusion–time plots, the baseline perfusion value for the distal phalanx under occlusion (OCC), the peak hyperemic perfusion value upon cuff removal (PEAK), and the subsequent steady-state flow value (STEADY) were recorded, with results expressed as PEAK/OCC and STEADY/OCC values.

Statistical analysis.

A Wilcoxon signed-rank test for paired samples was used to compare all data (SPSS version 11.5; SPSS, Chicago, IL). Graphs were plotted with Excel 2000 (Microsoft, Seattle, WA).

RESULTS

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES

Local heating response.

A typical perfusion–time curve representing the response to local heating of the dorsum is shown in Figure 1. Each point of the curve represents the mean flux value of the heated area within a sequential frame, with the first frame being the baseline scan obtained following acclimatization and before the heating pad was placed on the dorsum, and the second frame being the scan obtained immediately following pad removal. The perfusion curve has 2 peaks because of the different mechanisms of vasodilation (27); as can be seen in Figure 1, the second peak is smaller in magnitude. To determine whether any difference existed between the magnitudes of the 2 peaks in either group (thus representing differences in the 2 modes of vasodilation), both the first peak and the second peak were studied. The rhythmic oscillations seen in both the postheating and postocclusion perfusion curves are due to the myogenic activity of the smooth muscle cells of arteries and arterioles (28).

thumbnail image

Figure 1. Typical graph showing the percentage change from baseline over time following local heating of the dorsum (green laser wavelength). The initial increase in perfusion is due to local heating followed by a decrease with subsequent cooling after heat source removal. The first peak (PEAK1) is due to sensory nerve response, and the second peak (PEAK2) is mediated via nitric oxide.

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The data for PEAK1, PEAK2, and the AUC in the patient and control groups are shown in Table 1. No significant differences in these data were found between the patients with SSc and the controls, using scans with either wavelength (for patients versus controls, red wavelength PEAK1 P = 0.699, PEAK2 P = 0.689, and AUC P = 0.265; green wavelength PEAK1 P = 0.219, PEAK2 P = 0.665, and AUC P = 0.614).

Table 1. Heating and occlusion response in patients with systemic sclerosis (SSc) and healthy control subjects, using scans with the red and green wavelengths*
 Patients with SScHealthy controls
Red wavelength (n = 29)Green wavelength (n = 29)Red wavelength (n = 29)Green wavelength (n = 29)
  • *

    Values are the median (interquartile range) maximum perfusion increase from baseline due to heating (PEAK1) and second, smaller perfusion increase due to heating (PEAK2), the area under the perfusion–time curve (AUC), and the perfusion decrease due to occlusion (%DECREASE).

PEAK12.3 (1.6–2.9)1.2 (1.1–1.3)2.2 (1.7–2.7)1.3 (1.1–1.4)
PEAK21.6 (1.4–2.1)1.1 (1.0–1.2)1.5 (1.3–2.0)1.1 (1.0–1.2)
AUC60.6 (55.0–72.4)50.8 (50.1–51.9)56.7 (54.2–64.8)50.6 (50.2–51.9)
%DECREASE67 (29–83)9 (4–19)68 (47–76)3 (1–8)

Occlusion response.

The %DECREASE results are shown for both the patient and the control group in Table 1 and Figure 2. Although no significant difference in these values, representing decrease in blood flow, was found between the groups using scans with the red wavelength (P = 0.940), a significant difference was found using scans with the green wavelength (P = 0.015).

thumbnail image

Figure 2. Percentage decrease in perfusion of the distal phalanx following occlusion of the index finger compared with a baseline of no occlusion, for the red (R) and green (G) wavelengths in patients with systemic sclerosis (SSc) and healthy controls (HC). Data are presented as box plots, where the boxes represent the interquartile range, the lines within the boxes represent the median, and the lines outside the boxes represent the 10th and 90th percentiles. ∗ = P = 0.015 versus healthy controls.

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Responsive hyperemia.

A typical curve representing responsive hyperemia both during and after occlusion is shown in Figure 3 (for the red wavelength). The positions representing occluded perfusion at baseline, occlusion removal, peak hyperemia, and steady-state unoccluded blood flow are marked 1–4, respectively, in Figure 3. Values for the PEAK/OCC and STEADY/OCC are given in Table 2, and PEAK/OCC values are shown for both groups in Figure 4. Significant differences were found for both the red-wavelength and the green-wavelength PEAK/OCC values between groups (P = 0.015 and P = 0.013, respectively). However, no difference was found in the STEADY/OCC values between groups using either wavelength (P = 0.925 and P = 0.173, respectively).

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Figure 3. An example of a typical hyperemic response flux curve obtained with the red laser wavelength (shown from just before and for ∼10 seconds after removal of occlusion), smoothed with a time constant of 3 seconds; flux, expressed in perfusion units, is shown on the left axis and time, expressed in seconds, is shown along the lower axis. Arrows mark the baseline flux value under occlusion (arrow 1), point of pressure release (arrow 2), the peak of hyperemia (arrow 3), and the final baseline value of flux (arrow 4).

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Table 2. Hyperemic response in patients with systemic sclerosis (SSc) and healthy control subjects, using scans with the red and green wavelengths*
 Patients with SScHealthy controls
Red wavelength (n = 15)Green wavelength (n = 14)Red wavelength (n = 15)Green wavelength (n = 14)
  • *

    Values are the median (interquartile range) maximum increase (from baseline perfusion values under occlusion) in hyperemic perfusion upon removal of occlusion (PEAK/OCC), and steady-state flow value upon removal of occlusion (STEADY/OCC).

  • P = 0.015 versus controls.

  • P = 0.013 versus controls.

PEAK/OCC8.9 (6.9–13.8)1.4 (1.4–1.7)18.7 (14.4–21.9)2.5 (2.0–6.7)
STEADY/OCC5.3 (2.6–7.9)1.2 (1.1–1.3)4.3 (2.9–5.2)1.1 (1.0–1.3)
thumbnail image

Figure 4. Ratio of peak flux to occluded flux (PEAK/OCC) for the red (R) and green (G) wavelengths in patients with systemic sclerosis (SSc) and healthy controls (HC). Data are presented as box plots, where the boxes represent the interquartile range, the lines within the boxes represent the median, and the lines outside the boxes represent the 10th and 90th percentiles. ∗ = P = 0.015 and ∗∗ = P = 0.013 versus controls, using the red and green wavelengths, respectively.

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DISCUSSION

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES

Our choice of dynamic tests was informed by the results of previous studies, which have shown that both local heating and occlusion of the digits are reproducible techniques that result in distinct, dynamic responses in blood flow while also offering a practical option in terms of time constraints.

As was observed in our previous study (in healthy control subjects), the maximum increase and AUC for the perfusion–time curve were both lower for data from scans with the green wavelength (19). This indicates that vessels imaged by the red laser experience a larger change in perfusion due to heating and take longer to return to baseline than the capillaries imaged by the green laser. The difference between the values of peak blood flow imaged by the 2 wavelengths is most likely due to the mechanism of perfusion control; arterioles dilate and therefore undergo an increase in both the speed and the volume of blood flow, whereas capillaries, which do not have smooth muscle to allow them to vasoconstrict and vasodilate, have precapillary sphincters to control flow. Consequently, the capillaries do not have the capacity to undergo the same increase in blood flow as that of larger vessels.

No difference was found between the healthy controls and SSc patients in the local heating data (AUC, PEAK1, and PEAK2 values) obtained with either wavelength. Both peaks were examined because they reflect different aspects of vasodilation. It has been observed in several studies that during sustained local heating, vasodilation is mediated by 2 time-dependent mechanisms (27, 29). During the first 5 minutes of heating, activation of local sensory nerves is responsible for the rapid increase in perfusion forming the main peak (PEAK1). The smaller, second peak (PEAK2) has been observed to occur when heating is continued beyond 10 minutes, since the heat is conducted to deeper levels of the skin. This second peak is dependent on nitric oxide (NO) production. Although the protocol for this study involved heating for only 6 minutes, it was apparent from the repeat scans that by 10 minutes after the start of heating, there was still a significant increase in blood flow within the heated area. It can be assumed that an increase in temperature, from baseline, accompanies this increase in perfusion. This is a consequence of the fact that although the skin's thermal relaxation time, for a pulsed heat source (such as a laser), is in the order of microseconds to seconds (30) (depending on the source and location of heating), the rate of temperature increase in the skin is too high for the blood to be able to convect heat away from the interaction site at the same rate, and therefore it accumulates until heating is ceased. Thus, the presence of PEAK2 is due to the fact that the skin is still experiencing an increase in temperature after the heating pad has been removed.

The second peak (PEAK2) was of interest due to the recent hypothesis that NO levels are elevated or reduced in patients with SSc when compared with healthy controls; the direction of change in NO levels is most likely dependent on the duration and severity of disease (31–34). However, no statistically significant differences in PEAK2 values were found between patients and controls. This could be due to the fact that subjects were not maximally vasodilated due to curtailed heating, and therefore the effect could not be differentiated.

Since there were no significant differences between the patient and control groups in the peaks of the perfusion curve or the AUC, it can be concluded that microvasculature function as assessed at the dorsum of the hand is not affected by the SSc disease process (in contrast to the fingers). This is consistent with the findings of several other studies in which the dorsum of the hand or other more proximal sites were examined with blood flow monitoring, and no difference in blood flow was observed between groups (35–37). Albrecht et al (35) used local heating on the dorsum (proximal to the digits) and found no difference in the flux values following heating in SSc patients compared with controls. Geirsson et al (36) investigated the effect of local heating in 5 locations of the body, comprising the forehead and sites on the wrists and ankles bilaterally. No difference was found between the control group and patients with SSc. They concluded that the function of the precapillary sphincters and the smallest vessels in the SSc patients was preserved. Walmsley and Goodfield (37) used LDF to study the increase in perfusion during dorsal foot warming in patients with SSc; although the baseline perfusion before heating was significantly higher in the control group, and patients were found to have a lower threshold for vasodilation, no significant difference in blood flow was found at the increased temperature (36°C) in patients with SSc compared with control subjects. In contrast to these studies, Wollersheim et al (38) found that blood flow in the distal digits (assessed with LDF) during whole-hand heating was significantly reduced in patients with SSc compared with controls. Therefore, the results from these previous studies are consistent with our findings.

By occluding the finger rather than performing brachial occlusion, it is possible, in addition to measuring hyperemic response, to easily image the area being occluded with well-defined boundaries (19). The decrease in blood flow was more pronounced, both in patients and in controls, in the vessels imaged by the red wavelength than in those imaged by the green wavelength. As discussed above, this finding is most likely explained by the fact that capillaries do not have the capacity to dilate and constrict in the same way as larger, thermoregulatory vessels, because their mechanism for increasing or decreasing blood flow is reliant on sphincters and one-way valves. The residual Doppler signal under occlusion is the biologic zero and is attributable to several factors, including collateral circulation through bone, movement of fluid within the interstitial space, Brownian motion of red blood cells, and residual vasomotion (39).

No significant difference in blood flow during occlusion was found between patients and controls on images with the red wavelength, whereas a difference was found on the green-wavelength images. SSc affects primarily the microvasculature (1), and it is therefore a fair assumption that the smaller vessels are more sensitive to challenges that cause disturbances to microvascular flow and compromise it further. This result indicates that the smaller vessels are more impaired than the larger ones, which corresponds with the observations in a study carried out by Kristensen (40), who examined vascular resistance in the digits of patients with SSc, using the Xe washout technique. In response to occlusion, he found that vessels in the subdermis, i.e., larger vessels, were more reactive than those in the dermis, i.e., small vessels, indicating that small vessels were more impaired than larger vessels. In addition, he found that vessels in the hand were more reactive than those in the fingers, further supporting the suggestion of the digit vessels being more impaired than the dorsal vessels. Stucker et al (41) found, with digit subtraction angiography, that 90% of patients with SSc had arterial narrowing in the fingers, 31% had narrowing of the ulnar artery, and none had narrowing of the radial artery. These findings, together with our own, suggest that in SSc there are abnormalities of the digit microvasculature and also of the larger vessels. It is therefore likely that both large- and small-vessel disease is responsible for digit ischemia in SSc.

Our observation of a significant decrease in PEAK/OCC values in the SSc patients compared with the controls, using both wavelengths, indicates that SSc affects both small vessels such as nutritive capillaries and larger vessels such as those in the thermoregulatory circulation. This is consistent with the findings of other studies that also identified a reduction in hyperemic response. Using LDF, both Albrecht et al (35) and Wigley et al (9) found a significantly decreased response to postocclusive hyperemia in patients with brachial and digit occlusion. Wigley et al also performed occlusion following administration of a vasodilator; no significant improvement was observed, and thus they concluded that structural defects limit microvascular flow, causing dysfunction. Wollersheim et al (42), also using LDF to measure the occlusion response in the fingers, found a lower baseline and lower peak response, but concluded that since the percentage changes in both patients and controls were not significantly different, the peak increase was directly related to the baseline value of flow. Similarly, Goodfield et al (43), using digit arterial occlusion, found that the decreased response in patients occurred when resting blood flow was low, and that the absence of a hyperemic response could be alleviated by prewarming the patients' hands, after which responses compared well with those in controls; they concluded that this was attributable to abnormal vessel wall reactivity. Kristensen (40) concluded that decreased response to occlusion and abnormal hemodynamics were due to a local fault in the vasculature.

We found that 1) with local heating of the dorsum (monitored using LDI), there was no difference between groups in either the small (e.g., capillaries) or large (e.g., thermoregulatory) microvessels, 2) before and during occlusion (monitored using LDI), there was a significant difference between groups in the small microvessels, and 3) with regard to the hyperemic response on release of occlusion (monitored with LDF at the fingertip), there was a difference between groups in both the smaller and the larger microvessels.

It should be recognized that with dual-wavelength LDI, certain assumptions are made regarding the types of vessels that are imaged with each wavelength. However, these assumptions are based on spectral and penetration depth data from many scientific studies in the field of laser photomedicine, as discussed above. Tulevski et al and Ubbink et al (44, 45), whose studies compared near-infrared and green-wavelength LDF to capillary microscopy, obtained significantly different results between the 2 wavelengths, suggesting that the green wavelength measures more superficial flow. Although they concluded that the green wavelength did not exclusively examine superficial capillary perfusion, it should be noted that our previous study (19) showed that the type of vessel imaged was dependent on the power of the laser used; therefore, the situation is complex.

We believe these results warrant further studies to examine the microvascular responsiveness in patients with SSc. These studies could include simultaneous heating (using standard stimuli) at several sites (including the finger, dorsum, and forearm) and should assess correlation of the results with the degree of skin thickening. Although in our study we found no association between the degree of skin thickening of the dorsum of the hand (as assessed by the modified Rodnan skin score [46]) and hyperemic heating response (results not shown), examination of a larger number of different sites would be required to investigate this more fully. Avascular skin is relatively transparent to light, since there are no highly absorbing molecules present, even if thickened. Therefore, the effect of absorption or scattering in blood is much more relevant than thickness or fibrosis of skin in monitoring blood flow response. Thus, if future studies reveal any association between the degree of scleroderma and perfusion, this could be attributable to the clinical implications of fibrosis on blood flow responses, as opposed to increased optical absorption by the skin.

In summary, no differences in the dorsal heating response were found between groups, whereas differences were found in the finger occlusion response, suggesting that microvascular impairment in SSc may be selective to the digits. The additional observation that the postocclusive hyperemic response was decreased using both the green and the red wavelengths in SSc patients (even though, under occlusion, differences were observed only on green laser wavelength images) indicates that larger vessels are also involved, but to a lesser degree.

As discussed above, several other heating and occlusion laser Doppler studies have demonstrated differences between patient and control groups in the digits but not in the dorsum of the hand. Iontophoresis studies examining endothelial-dependent and -independent responses to vasodilation similarly have provided evidence to support this theory. In one study, forearm iontophoresis produced no differences between the control and patient groups (47), whereas similar studies with digit iontophoresis showed impaired vasodilation in the patient group (12, 48, 49). This selective microvascular impairment is likely to contribute to the severe Raynaud's phenomenon that is characteristic of the SSc disease process. Therapies should therefore be targeted specifically to the digit vasculature.

Acknowledgements

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES

We thank Rodney Gush for helpful discussions.

REFERENCES

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES