Spatial heterogeneity in normal skin perfusion recorded with laser Doppler imaging and flowmetry

Reliability of laser-Doppler flowmetry derived measurements of forearm and calf cutaneous vasodilation during gradual local heating in young adults

OBJECTIVES

Evaluate reliability of laser-Doppler flowmetry derived cutaneous vasodilation on the upper and lower limbs during gradual local heating.

METHODS

In twenty-eight young adults (21 (SD 3) years, 14 females), absolute cutaneous vascular conductance (CVC_abs) and CVC normalized to maximum vasodilation at 44 °C (%CVC_max) were assessed at two adjacent sites on each of the forearm and calf during gradual local skin heating (33-42 °C at 1 °C·5 min^-1) for two identical trials (∼1 week apart). Responses were assessed for baseline, the steady-state heating plateau at 42 °C and the span (i.e. plateau-baseline).

RESULTS

Between-day reliability was characterized as measurement consistency across trials. Within-day reliability was characterized as within-limb measurement consistency across adjacent skin sites. Between- and within-day absolute reliability (coefficient of variation) generally improved with heating, from poor (>25 %) at baseline to good (<10 %) for %CVC_max and moderate (10-25 %) for CVC_abs for plateau and span. However, relative reliability (intraclass correlation coefficient) was generally not acceptable (<0.70) for any condition. Responses were generally consistent for females and males and there were no major forearm and calf differences.

CONCLUSIONS

Consistency of CVC estimates improved during gradual local heating with negligible limb and sex differences, which are important considerations for experimental design and interpretation.

Multiple Laser Doppler Flowmetry Probes Increase the Reproducibility of Skin Blood Flow Measurements

Cutaneous microcirculatory perfusion is commonly measured using laser Doppler flowmetry (LDF) probes, which provide a continuous, non-invasive quantification of skin blood flow (SkBF). However, inhomogeneities in the skin’s microvasculature density contribute to a decrease in reproducibility whenever an LDF probe is removed and replaced, as is the case during pre- and post-intervention or between-day measurements. Therefore, this study aimed to determine whether increasing the total number of individual LDF probes in a localized area improves the reproducibility of the measurement. Seven laser Doppler probes were secured in a custom-made acrylic holder designed to attach to the skin’s surface easily. SkBF, local skin temperature (Tsk), and blood pressure (BP) were assessed in 11 participants (6 M, 5 F, 42 ± 15 years). SkBF and Tsk were measured from the dorsal forearm (arm trial) for 5 min. Next, the multi-laser device was moved to the lateral side of the calf (leg trial), and measurements were obtained for 5 min. Each arm and leg trial was cyclically repeated three times, and all trials were separated by intermissions lasting 10–15 min. The average SkBF and the cutaneous vascular conductance (CVC) from all possible LDF probe combinations were not statistically different across the three arm and leg trials. Two-way mixed-effects models with absolute agreement were used to compute the intraclass correlation coefficient (ICC) for CVC, and the minimum ICC increased with the addition of LDF probes. The ICC of the average CVC from seven LDF probes was 0.96 between the arm trials and 0.91 between the leg trials, which suggests that there is excellent reliability and little difference between trials following the removal and replacement of the device. Moreover, all individual ICC values from ≥3 LDF probe combinations were greater than 0.70 (i.e., good reliability). These data suggest that SkBF measurements with multiple laser Doppler probes in a custom-made holder have excellent reproducibility after replacing the probes within the same participant. Therefore, this application could provide more reproducible assessments between repeated measurements (e.g., before and after exercise or clinical procedures) where the LDF probes must be removed and replaced within the same location.

Influence of uncomplicated, controlled hypertension on local heat-induced vasodilation in non-glabrous skin across the body

OBJECTIVE

1) Examine pooled effects of hypertension on nitric oxide (NO)-dependent vasodilation during local heating across multiple non-glabrous skin regions, and 2) explore regional differences.

METHODS

Responses were compared between fourteen participants with uncomplicated hypertension controlled with medication (7 females, 61±6 years) and fourteen age-matched non-hypertensive controls (6 females; 60±5 years). Cutaneous vascular conductance, normalized to maximum vasodilation (%CVC_max) was assessed at the upper chest, abdomen, dorsal forearm, thigh, and lateral calf during local heating. Across all regions, local skin temperatures were simultaneously increased from 33-42°C (1°C·10·s^-1), and held until a stable heating plateau was achieved (~40 min), followed by continuous infusion of 20 mM of N(G)-Nitro-L-arginine methyl ester (L-NAME; ~40min) at all sites until a stable L-NAME plateau was achieved. The difference between heating and L-NAME plateaus was defined as the NO-contribution. Statistical equivalence for each heating phase was determined based on equivalence bounds of ±10%CVC_max for between-group differences.

RESULTS

Pooled (all-regions) %CVC_max responses were equivalent for baseline (two one-sided t-test; p<0.001), heating plateau (p=0.002), L-NAME plateau (p=0.028), and NO-contribution (p=0.003). For individual regions, responses were equivalent at baseline for the abdomen, thigh, and calf, the heating plateau for the thigh, and the L-NAME plateau for the calf (all p<0.05). Conversely, the calf heating plateau was lower in the hypertension group (t-test; p<0.05).

CONCLUSION

Local heat-induced cutaneous vasodilation was statistically equivalent between individuals with uncomplicated, controlled hypertension and non-hypertensive age-matched adults when pooled across multiple skin sites. Conversely, individual between-region comparisons were generally too variable to permit definitive conclusions.

Regional variation in nitric oxide-dependent cutaneous vasodilatation during local heating in young adults

NEW FINDINGS

What is the central question of this study? Are regional differences in nitric oxide (NO)-dependent cutaneous vasodilatation during local skin heating present in young adults? What is the main finding and its importance? NO-dependent cutaneous vasodilatation varied across the body. The abdomen demonstrated larger NO contributions, while the chest demonstrated smaller NO contributions, compared to other regions. This exploratory work is an important first step in characterizing regional heterogeneity of cutaneous microvascular control across the torso and limbs. Equally, it serves to generate hypotheses for future studies examining regional cutaneous microvascular control in ageing and disease.

ABSTRACT

Regional variations in cutaneous vasodilatation during local skin heating exist across the body. While nitric oxide (NO) is a well-known modulator of this response, the extent of regional differences in NO-dependent cutaneous vasodilatation during local skin heating remains uncertain. In 16 habitually active young adults (8 females; 25 ± 5 years), cutaneous vascular conductance, normalized to maximum vasodilatation (% CVC_max ), was assessed at the upper chest, abdomen, dorsal forearm, thigh and lateral calf during local skin heating. Across all regions, local skin temperatures were simultaneously increased from 33 to 42°C (1°C per 10 s), and held until a stable heating plateau was achieved (∼40 min). Next, with local skin temperature maintained at 42°C, 20 mM of N^G -nitro-l-arginine methyl ester (l-NAME) was continuously infused at each site until a stable l-NAME plateau was achieved (∼40 min). The difference between heating and l-NAME plateaus was identified as the NO contribution for each region. There was no evidence for region-specific responses at baseline (P = 0.561), the heating plateau (P = 0.351) or l-NAME plateau (P = 0.082), but there was for the NO contribution (P = 0.048). Overall, point estimates for between-region differences in the NO contribution varied across the body from 0 to 19% CVC_max . The greatest effects were observed for the abdomen, wherein the NO contribution was consistently greater than for the other regions (range: 9-19% CVC_max ). The chest was consistently lower than the other regions (range: 7-19% CVC_max ). The smallest effects were observed between limb regions (range: 0-2% CVC_max ). These findings advance our understanding of the mechanisms influencing regional variations in the cutaneous vasodilator response to local skin heating in young adults.

Spatial heterogeneity of cutaneous blood flow respiratory-related oscillations quantified via laser speckle contrast imaging

LSCI technique provides experimental data which can be considered in the context of spatial blood flow coherency. Analysis of vascular tone oscillations gives additional information to ensure a better understanding of the mechanisms affecting microvascular physiology. The oscillations with different frequencies are due to different physiological mechanisms. The reasons for the generation of peripheral blood flow oscillations in the 0.14-0.6 Hz frequency band are as follows: cardio-respiratory interactions, pressure variations in the venous part of the circulatory system, and the effect of the sympathetic nervous system on the vascular tone. Earlier, we described the spatial heterogeneity of around 0.3 Hz oscillations and this motivated us to continue the research to find the conditions for the occurrence of spatial phase synchronization. For this purpose, a number of physiological tests (controlled respiration, breath holder, and venous occlusion tests) which influence the blood flow oscillations of 0.14-0.6 Hz were considered, an appropriate measurement system and the required data processing algorithms were developed. At spontaneous respiration, the oscillations with frequencies around 0.3 Hz were stochastic, whereas all the performed tests induced an increase in spatial coherence. The protocols and methods proposed here can help to clarify whether the heterogeneity of respiratory-related blood flow oscillations exists on the skin surface.

Thermoregulatory reflex control of cutaneous vasodilation in healthy aging

Reflex cutaneous vasodilation during heating is attenuated in healthy human aging secondary to blunted increases in efferent skin sympathetic nervous system activity (SSNA) and reductions in end-organ sensitivity. Whether age-related alterations in the mean body temperature ( T - _b) threshold for increasing SSNA and/or the sensitivity of responses are evident with aging have not been examined. We tested the hypotheses that the T_b threshold for SSNA and cutaneous vascular conductance (CVC) would be increased, but the sensitivity would be reduced, with aging. Reflex vasodilation was induced in 13 young (23 ± 3 y) and 13 older (67 ± 7 y) adults using a water-perfused suit to systematically increase mean skin and esophageal temperatures. SSNA (peroneal microneurography) and red cell flux (laser Doppler flowmetry) in the innervated dermatome were continuously measured. SSNA was normalized to baseline; CVC was normalized as a percentage of maximal CVC. Baseline T - _b was lower in older adults (36.0 ± 0.4°C vs 36.4 ± 0.3°C; p = 0.005). During passive heating, the ∆ T - _b thresholds for increasing SSNA and CVC were greater (1.3 ± 0.4°C vs 0.9 ± 0.3°C; p = 0.007 and 1.3 ± 0.4°C vs 0.8 ± 0.3°C; p = 0.002, respectively) in older adults. The slope of the relation between both SSNA (0.31 ± 0.23 vs 0.13 ± 0.10 V⋅s⋅°C ^-1; p = 0.01) and CVC (87.5 ± 50.1 vs 32.4 ± 18.1%max⋅°C^-1; p = 0.002) vs T - _b was lower in older adults. The relative T - _b threshold for activation of SSNA and the initiation of reflex cutaneous vasodilation is higher in older adults, and once activated, the sensitivity of both responses is diminished, supporting the concept that the efferent component of the thermoregulatory reflex arc is impaired in healthy aging. Abbreviations: CI: confidence interval; CVC: cutaneous vascular conductance; SSNA: skin sympathetic nervous system activity; T - _b: mean body temperature; T_es: esophageal temperature; T - _sk: mean skin temperature.

Phase unwrapping for Doppler spectral domain optical coherence tomography flow measurement

Doppler optical coherence tomography (OCT) offers additional flow velocity information, which extends the application of OCT. Phase wrapping is the inherent problem that limits measureable range of Doppler OCT. We propose a phase unwrapping method which is suitable for correcting phase in Doppler OCT images. Points (pixels) in flow region are divided into groups according to the radial distance. Points in the same group are supposed to have close velocity. Phase unwrapping algorithm begins at the boundary layer group and is performed sequentially toward the center. Using the proposed criterion, points in a group are separated into two categories, signal points and noise points. Wrapping rounds are determined for signal points phase unwrapping. Mean value of the corrected signal points replaces the noise points for noise reduction. The method is validated with capillary tube flow phantom and in vivo blood flow.

Historical reviews of the assessment of human cardiovascular function: interrogation and understanding of the control of skin blood flow

Several techniques exist for the determination of skin blood flow that have historically been used in the investigation of thermoregulatory control of skin blood flow, and more recently, in clinical assessments or as an index of global vascular function. Skin blood flow measurement techniques differ in their methodology and their strengths and limitations. To examine the historical development of techniques for assessing skin blood flow by describing the origin, basic principles, and important aspects of each procedure and to provide recommendations for best practise. Venous occlusion plethysmography was one of the earliest techniques to intermittently index a limb’s skin blood flow under conditions in which local muscle blood flow does not change. The introduction of laser Doppler flowmetry provided a method that continuously records an index of skin blood flow (red cell flux) (albeit from a relatively small skin area) that requires normalisation due to high site-to-site variability. The subsequent development of laser Doppler and laser speckle imaging techniques allows the mapping of skin blood flow from larger surface areas and the visualisation of capillary filling from the dermal plexus in two dimensions. The use of iontophoresis or intradermal microdialysis in conjunction with laser Doppler methods allows for the local delivery of pharmacological agents to interrogate the local and neural control of skin blood flow. The recent development of optical coherence tomography promises further advances in assessment of the skin circulation via three-dimensional imaging of the skin microvasculature for quantification of vessel diameter and vessel recruitment.

Chronic statin therapy is associated with enhanced cutaneous vascular responsiveness to sympathetic outflow during passive heat stress

KEY POINTS

Impairments in both central sympathetic and peripheral microvascular function contribute to blunted reflex cutaneous vasodilatation during heat stress in healthy older adults. Hypercholesterolaemia is associated with decrements in neurovascular function; however, little is known about the impact of hypercholesterolaemia on the integrated responses to heat stress. Further, whether chronic statin therapy alters skin sympathetic outflow or its relation to cutaneous vascular conductance during heat stress is unknown. We demonstrate that reflex cutaneous vasodilatation is impaired in older hypercholesterolaemic adults but not in formerly hypercholesterolaemic adults currently treated with a statin compared to age-matched controls. Additionally, chronic statin treatment-induced improvements in reflex vasodilatation are mediated, in part, by increases in end-organ responsiveness to efferent sympathetic outflow during whole-body heating. These data add to the growing body of literature substantiating the beneficial pleiotropic neurovascular effects of chronic statin treatment and provide further support for the use of statins to confer additional cardioprotective benefits in older adults.

ABSTRACT

Attenuated reflex cutaneous vasodilatation in healthy human ageing is mediated by alterations in both central (sympathetic outflow) and peripheral (microvascular endothelial) function. Hypercholesterolaemia is associated with further impairments in neurovascular function. HMG-CoA reductase inhibitors (statins) improve cutaneous endothelium-dependent dilatation; however, whether statin therapy alters skin sympathetic nervous system activity (SSNA) or its relation to cutaneous vascular conductance (CVC) during passive heat stress is unknown. We hypothesized that (1) hypercholesterolaemic older adults would demonstrate blunted increases in both SSNA and CVC during passive heating and (2) chronic statin treatment would improve the response range and sensitivity of the SSNA:CVC relation. Reflex vasodilatation in response to a 1.0°C rise in oral temperature (T_or ; water perfused suit) was induced in 13 healthy normocholesterolaemic adults (62 ± 2 years; LDL = 113 ± 7 mg/dl), 10 hypercholesterolaemic adults (60 ± 1 years; LDL = 183 ± 2 mg/dl), and 10 previously hypercholesterolaemic adults (64 ± 1 years; LDL = 102 ± 2 mg/dl) treated with lipophilic statin (10-40 mg daily). SSNA (peroneal microneurography) and red cell flux (laser-Doppler flowmetry) in the innervated dermatome (dorsum of foot) were continuously measured. Reflex vasodilatation was blunted in hypercholesterolaemic adults, but not in statin-treated adults, compared to normocholesterolaemic adults (at ∆T_or  = 1.0°C: normal = 36 ± 1%CVC_max , high = 32 ± 1%CVC_max , statin = 38 ± 1%CVC_max ; P < 0.01). ∆SSNA was not different (at ∆T_or  = 1.0°C: normal: ∆ = 393 ± 96%, high: ∆ = 311 ± 120%, statin: ∆ = 256 ± 90%; P = 0.11). The slope of the SSNA:CVC relation was blunted in hypercholesterolaemic adults (0.02 ± 0.03%CVC_max /%_baseline ) compared to both normocholesterolaemic (0.09 ± 0.02%CVC_max /%_baseline ; P = 0.024) and statin-treated (0.12 ± 0.05%CVC_max /%_baseline ; P = 0.03) adults. Chronic statin treatment improves reflex cutaneous vasodilatation in formerly hypercholesterolaemic older adults by increasing end-organ responsiveness to sympathetic outflow during passive heat stress.

Optical monitoring of cerebral microcirculation in neurointensive care

Continuous optical monitoring of local cerebral microcirculation could benefit neurointensive care patients treated for subarachnoid hemorrhage (SAH). The aim of the study was to evaluate laser Doppler flowmetry (LDF) and diffuse reflectance spectroscopy (DRS) for long-term monitoring of brain microcirculation and oxygen saturation (SO₂) in the neurointensive care unit (NICU). A fiber optic probe was designed for intraparenchymal use and connected to LDF and DRS for assessment of the local blood flow (perfusion and tissue reflectance (TLI)) and SO₂ in the brain. The optically monitored parameters were compared with conventional NICU monitors and Xe-CT. The LDF signals were low with median and 25 to 75% interquartiles of perfusion = 70 (59 to 83) a.u. and TLI = 2.0 (1.0 to 2.4) a.u. and showed correlation with the NICU monitors in terms of heart rate. Median and interquartiles of SO₂ were 17.4 (15.7 to 19.8) %. The lack of correlation between local perfusion and cerebral perfusion pressure indicated intact cerebral autoregulation. The systems were capable of monitoring both local perfusion and SO₂ with stable signals in the NICU over 4 days. Further clinical studies are required to evaluate the optical systems’ potential for assessing the onset of secondary brain injury.

Near-infrared angiography for critical limb ischemia in a diabetic murine model

Peripheral arterial disease (PAD) is a highly prevalent disease process that afflicts more than 20% of individuals with diabetes. Progression of PAD in the setting of diabetes can lead to critical limb ischemia (CLI), which is associated with increased risk of wounds, gangrene, and limb loss. Prompt noninvasive evaluation of limbs affected by PAD progression and CLI is currently limited. Here, we evaluate the utility of a custom-designed multispectral imaging system for fluorescence-based near-infrared angiography and compare it to the existing gold standard of laser-scanning Doppler perfusion assessments. Due to its higher resolution and fluorescence sensitivity, near-infrared angiography demonstrates a greater capacity to characterize altered dynamic arterial perfusion in a clinically relevant diabetic murine model for CLI. Furthermore, we demonstrate that our imaging system can accurately track arterial perfusion recovery over time following induced ischemia, and reveal unique phenotypic differences in the setting of diabetes.

Blunted increases in skin sympathetic nerve activity are related to attenuated reflex vasodilation in aged human skin

Reflex cutaneous vasodilation in response to passive heating is attenuated in human aging. This diminished response is mediated, in part, by age-associated reductions in endothelial function; however, the contribution of altered skin sympathetic nervous system activity (SSNA) is unknown. We hypothesized that 1) healthy older adults would demonstrate blunted SSNA responses to increased core temperature compared with young adults and 2) the decreased SSNA response would be associated with attenuated cutaneous vasodilation. Reflex vasodilation was elicited in 13 young [23 ± 1 (SE) yr] and 13 older (67 ± 2 yr) adults using a water-perfused suit to elevate esophageal temperature by 1.0°C. SSNA (peroneal microneurography) and red cell flux (laser Doppler flowmetry) in the innervated dermatome (the dorsum of foot) were continuously measured. SSNA was normalized to, and expressed as, a percentage of baseline. Cutaneous vascular conductance (CVC) was calculated as flux/mean arterial pressure and expressed as a percentage of maximal CVC (local heating, 43°C). Reflex vasodilation was attenuated in older adults (P < 0.001). During heating, SSNA increased in both groups (P < 0.05); however, the response was significantly blunted in older adults (P = 0.01). The increase in SSNA during heating was linearly related to cutaneous vasodilation in both young (R² = 0.87 ± 0.02, P < 0.01) and older (R² = 0.76 ± 0.05, P < 0.01) adults; however, slope of the linear regression between ΔSSNA and ΔCVC was reduced in older compared with young (older: 0.05 ± 0.01 vs. young: 0.08 ± 0.01; P < 0.05). These data demonstrate that age-related impairments in reflex cutaneous vasodilation are mediated, in part, by blunted efferent SSNA during hyperthermia.

Exercise and Repeated Testing Improves Accuracy of Laser Doppler Assessment of Microvascular Function Following Shortened (1-minute) Blood Flow Occlusion

OBJECTIVES

To determine whether stability/accuracy of post-occlusive LDF following shortened, one-minute blood flow occlusion, increases in the post-exercise state or by averaging multiple measurements.

METHODS

Six healthy adults (3F) underwent LDF eight times at rest and following exercise, assessing post-occlusive (one-minute occlusion) reactive hyperemia in the cutaneous microcirculation of the forefinger. Measured variables included: pre- and post-occlusion steady-state perfusion (Plat1, Plat2), maximum post-occlusive perfusion (Max), PkT, and the ratio Max/Plat1.

RESULTS

Stability/accuracy of all variables improved performing measurements after exercise (p < 0.05 Plat 1, Plat 2, Max and Max/Plat1). PkT and Max/Plat 1 displayed the greatest accuracy at rest (26.6 ± 5.1% and 26.6 ± 4.4% average difference, %Diff, of single measurements from individual "true" means, respectively); for these variables, %Diff improved to 19.5 ± 5.3 and 17.6 ± 2.1, respectively, following exercise. Overall, averaging multiple measurements performed at rest also improved stability/accuracy in all variables. This improvement was comparable to that obtained with a single measurement following exercise.

CONCLUSIONS

A standardized exercise stimulus prior to testing significantly improves stability/accuracy of LDF following shortened, one-minute blood flow occlusion. Our results suggest the possibilities of broader applications of exercise to optimize measurements from a variety of skin perfusion methodologies.

Monitoring of partial and full venous outflow obstruction in a porcine flap model using laser speckle contrast imaging

BACKGROUND

In microsurgery, there is a demand for more reliable methods of post-operative monitoring of free flaps, especially with regard to tissue-threatening obstructions of the feeding arteries and draining veins. In this study, we evaluated laser speckle contrast imaging (LSCI) and laser Doppler flowmetry (LDF) to assess their possibilities to detect partial and full venous outflow obstruction, as well as full arterial occlusion, in a porcine flap model.

METHODS

Cranial gluteal artery perforator flaps (CGAPs) were raised, and arterial and venous blood flow to and from the flaps was monitored using ultrasonic flow probes. The venous flow was altered with an inflatable cuff to simulate partial and full (50% and 100%) venous obstruction, and arterial flow was completely obstructed using clamps. The flap microcirculation was monitored using LSCI and LDF.

RESULTS

Both LDF and the LSCI detected significant changes in flap perfusion. After partial (50%) venous occlusion, perfusion decreased from baseline, LSCI: 63.5 ± 12.9 PU (p = 0.01), LDF 31.3 ± 15.7 (p = 0.64). After 100% venous occlusion, a further decrease in perfusion was observed: LSCI 54.6 ± 14.2 PU (p < 0.001) and LDF 16.7 ± 12.8 PU (p < 0.001). After release of the venous cuff, LSCI detected a return of the perfusion to a level slightly, but not significantly, below the baseline level 70.1 ± 11.5 PU (p = 0.39), while the LDF signal returned to a level not significant from the baseline 36.1 ± 17.9 PU (p > 0.99). Perfusion during 100% arterial occlusion decreased significantly as measured with both methods, LSCI: 48.3 ± 7.7 (PU, p < 0.001) and LDF: 8.5 ± 4.0 PU (p < 0.001). During 50% and 100% venous occlusion, LSCI showed a 20% and 26% intersubject variability (CV%), respectively, compared to 50% and 77% for LDF.

CONCLUSIONS

LSCI offers sensitive and reproducible measurements of flap microcirculation and seems more reliable in detecting decreases in blood perfusion caused by venous obstruction. It also allows for perfusion measurements in a relatively large area of flap tissue. This may be useful in identifying areas of the flap with compromised microcirculation during and after surgery.

A laser Doppler system for monitoring cerebral microcirculation: implementation and evaluation during neurosurgery

The aim of this study was to adapt and evaluate laser Doppler perfusion monitoring (LDPM) together with custom-designed brain probes and software for continuous recording of cerebral microcirculation in patients undergoing neurosurgery. The LDPM system was used to record perfusion and backscattered light (TLI). These parameters were displayed together with the extracted heart rate (HR), pulsatility index (PI) and signal trends from adjustable time intervals. Technical evaluation was done on skin during thermal provocation. Clinical measurements were performed on ten patients undergoing brain tumour surgery. Data from 76 tissue sites were captured with a length varying between 10 s to 15 min. Statistical comparisons were done using Mann-Whitney tests. Grey and tumour tissue could be separated from white matter using the TLI signal (p < 0.05). The perfusion was significantly higher in grey and tumour tissue compared to white matter (p < 0.005). LDPM was successfully used as an intraoperative tool for monitoring local blood flow and additional parameters linked to cerebral microcirculation (perfusion, TLI, HR and PI) during tumour resection. The systems stability opens up for studies in the postoperative care of patients with, for example, traumatic brain injury or subarachnoid haemorrhage.

Assessment of microcirculation of the skin using Tissue Viability Imaging: A promising technique for detecting venous stasis in the skin

BACKGROUND

Venous occlusion in the skin is difficult to detect by existing measurement techniques. Our aim was to find out whether Tissue Viability Imaging (TiVi) was better at detecting venous occlusion by comparing it with results of laser Doppler flowmetry (LDF) during graded arterial and venous stasis in human forearm skin.

METHODS

Arterial and venous occlusions were simulated in 10 healthy volunteers by inflating a blood pressure cuff around the upper right arm. Changes in the concentration of red blood cells (RBC) were measured using TiVi, while skin perfusion and concentration of moving red blood cells (CMBC) were measured using static indices of LDF during exsanguination and subsequent arterial occlusion, postocclusive reactive hyperaemia, and graded increasing and decreasing venous stasis.

RESULTS

During arterial occlusion there was a significant reduction in the mean concentration of RBC from baseline, as well as in perfusion and CMBC (p<0.008). Venous occlusion resulted in a significant 28% increase in the concentration of RBC (p=0.002), but no significant change in perfusion (mean change -14%) while CMBC decreased significantly by 24% (p=0.02). With stepwise increasing occlusion pressures there was a significant rise in the TiVi index and reduction in perfusion (p=0.008), while the reverse was seen when venous flow was gradually restored.

CONCLUSION

The concentration of RBC measured with TiVi changes rapidly and consistently during both total and partial arterial and venous occlusions, while the changes in perfusion, measured by LDF, were less consistent. This suggests that TiVi could be a more useful, non-invasive clinical monitoring tool for detecting venous stasis in the skin than LDF.

Microvascular blood flow monitoring with laser speckle contrast imaging using the generalized differences algorithm

Laser speckle contrast imaging (LSCI) is a full-field optical technique to monitor microvascular blood flow with high spatial and temporal resolutions. It is used in many medical fields such as dermatology, vascular medicine, or neurosciences. However, LSCI leads to a large amount of data: image sampling frequency is often of several Hz and recordings usually last several minutes. Therefore, clinicians often perform regions of interest in which a spatial averaging of blood flow is performed and the result is followed with time. Unfortunately, this leads to a poor spatial resolution for the analyzed data. At the same time, a higher spatial resolution for the perfusion maps is wanted. To get over this dilemma we propose a new post-acquisition visual representation for LSCI perfusion data using the so-called generalized differences (GD) algorithm. From a stack of perfusion images, the procedure leads to a new single image with the same spatial resolution as the original images and this new image reflects perfusion changes. The algorithm is herein applied on simulated stacks of images and on experimental LSCI perfusion data acquired in three different situations with a commercialized laser speckle contrast imager. The results show that the GD algorithm provides a new way of visualizing LSCI perfusion data.

Linguistic Analysis of Laser Speckle Contrast Images Recorded at Rest and During Biological Zero: Comparison With Laser Doppler Flowmetry Data

Laser speckle contrast imaging (LSCI) is a newly commercialized imaging modality to monitor microvascular blood flow. Contrary to the well-known laser Doppler flowmetry (LDF), LSCI has the advantage of giving a full-field image of surface blood flow using simple instrumentation. However, laser speckle contrast images are not fully understood yet and their link with LDF signals still has to be studied. To quantify the similarity between LSCI and LDF symbolic sequences, we propose to use, for the first time, the index adapted from linguistic analysis and information theory proposed by Yang For this purpose, LSCI and LDF data were recorded simultaneously on the forearm of healthy subjects, at rest and during a vascular occlusion (biological zero). We show that there are different dynamical patterns for LSCI and LDF data, and the distances between these patterns differ through the space scales explored. Moreover, our results suggest that these different dynamical patterns could be linked to blood flow. The quantitative metric used herein therefore provides new information on LSCI and brings knowledge on links between LSCI and LDF.

A laser Doppler system for monitoring of intracerebral microcirculation

A two-channel standard laser Doppler perfusion monitor has been adapted for intracerebral measurements. Software developed in Labview makes it possible to present the microvascular perfusion, total light intensity (TLI), heart rate and trend curves in real-time during surgery. A custom-made optical probe was designed in order to enable easy fixation during brain surgery. The constructed brain probe was evaluated and compared to a standard probe. Both probes presented similar feasibility when used for the skin recordings. In addition, evaluation was done in one patient in relation to tumor resection. Stable perfusion and TLI signals were immediately recorded when the probe was positioned in cerebral tissue. Movement artifacts were clearly seen when the probe was moved to a new site. Recordings in cortex and tumor border showed higher perfusion and lower TLI compared to measurements in subcortical white matter. The calculated heart rate estimate agreed well with the noted value from the electrocardiographic patient monitoring system.

Cold hands, warm feet: sleep deprivation disrupts thermoregulation and its association with vigilance

STUDY OBJECTIVES

Vigilance is affected by induced and spontaneous skin temperature fluctuations. Whereas sleep deprivation strongly affects vigilance, no previous study examined in detail its effect on human skin temperature fluctuations and their association with vigilance.

DESIGN

In a repeated-measures constant routine design, skin temperatures were assessed continuously from 14 locations while performance was assessed using a reaction time task, including eyes-open video monitoring, performed five times a day for 2 days, after a normal sleep or sleep deprivation night.

SETTING

Participants were seated in a dimly lit, temperature-controlled laboratory.

PATIENTS OR PARTICIPANTS

Eight healthy young adults (five males, age 22.0 ± 1.8 yr (mean ± standard deviation)).

INTERVENTION

One night of sleep deprivation.

MEASUREMENTS AND RESULTS

Mixed-effect regression models were used to evaluate the effect of sleep deprivation on skin temperature gradients of the upper (ear-mastoid), middle (hand-arm), and lower (foot-leg) body, and on the association between fluctuations in performance and in temperature gradients. Sleep deprivation induced a marked dissociation of thermoregulatory skin temperature gradients, indicative of attenuated heat loss from the hands co-occurring with enhanced heat loss from the feet. Sleep deprivation moreover attenuated the association between fluctuations in performance and temperature gradients; the association was best preserved for the upper body gradient.

CONCLUSIONS

Sleep deprivation disrupts coordination of fluctuations in thermoregulatory skin temperature gradients. The dissociation of middle and lower body temperature gradients may therefore be evaluated as a marker for sleep debt, and the upper body gradient as a possible aid in vigilance assessment when sleep debt is unknown. Importantly, our findings suggest that sleep deprivation affects the coordination between skin blood flow fluctuations and the baroreceptor-mediated cardiovascular regulation that prevents venous pooling of blood in the lower limbs when there is the orthostatic challenge of an upright posture.

Effects of nitric oxide synthase inhibition on cutaneous vasodilation in response to acupuncture stimulation in humans

OBJECTIVES

The aim of the present study was to elucidate the mechanism of cutaneous vasodilation following acupuncture stimulation by investigating the roles of nitric oxide (NO) and axon reflex vasodilation.

METHODS

The subjects were 17 healthy male volunteers. The role of NO was investigated by administering N(G)-nitro-l-arginine methyl ester hydrochloride (L-NAME, 20 mM), an NO synthase inhibitor or Ringer's solution (control site), via intradermal microdialysis (protocol 1; n=7). The role of axon reflex vasodilation by local sensory neurones was investigated by comparing vasodilation at sites treated with 'eutectic mixture of local anaesthetics' (EMLA) cream (2.5% lidocaine and 2.5% prilocaine) with untreated sites (control site) (protocol 2; n=10). After 5 min of baseline recording, acupuncture was applied to PC4 and a control site in proximity to PC4 for 10 min and scanning was performed for 60 min after acupuncture stimulation. Skin blood flow (SkBF) was evaluated by laser Doppler perfusion imaging. Cutaneous vascular conductance (CVC) was calculated from the ratio of SkBF to mean arterial blood pressure.

RESULTS

In the first protocol, sites administered L-NAME showed significant reductions in CVC responses following acupuncture stimulation compared to control sites (administered Ringer's solution) (p<0.05). In the second protocol, changes in CVC responses after acupuncture stimulation did not differ significantly between treated sites with EMLA cream and untreated sites (p>0.05).

CONCLUSIONS

These data suggest that cutaneous vasodilation in response to acupuncture stimulation may not occur through an axon reflex as previously reported. Rather, NO mechanisms appear to contribute to the vasodilator response.

Inhibition of nitric oxide synthase attenuates cutaneous vasodilation during warm moxibustion-like thermal stimulation in humans

OBJECTIVES

This study investigated if nitric oxide (NO) and/or prostaglandin (PG) are responsible for cutaneous vasodilation during warm moxibustion-like thermal stimulation (WMTS).

DESIGN

For two protocols, two microdialysis membranes were placed in the medial forearm skin. In the first protocol (n=8), the sites were randomly assigned and perfused with N(G)-nitro-l-arginine methyl ester hydrochloride (l-NAME), an NO synthase inhibitor or Ringer's solution (control site). Similarly, two microdialysis membranes were placed in the medial forearm skin in the second protocol (n=6). One site was perfused with ketorolac (Keto), the cyclo-oxygenase (COX) pathway inhibitor, and the other site was perfused with Ringer's solution (control site). In both protocols, cutaneous vasodilation was induced using WMTS with an electronic warm moxibustion treatment appliance. After 10 minutes of baseline recording, WMTS was applied to the forearm skin for 20 minutes and recovery was monitored over a period of 20 minutes. Skin blood flow (SkBF) at each site was measured using laser-Doppler flowmetry. Cutaneous vascular conductance (CVC) was calculated as laser-Doppler flux/mean arterial blood pressure (BP).

SETTINGS/LOCATION

The study was conducted in a laboratory at the Kansai University of Health Sciences.

SUBJECTS

The subjects were 14 healthy male volunteers.

INTERVENTIONS

WMTS was applied to the medial forearm skin using an electronic warm moxibustion treatment appliance.

OUTCOME MEASURES

SkBF, skin temperature (Tsk), core body temperature (Tc), heart rate (HR), and BP were outcome measures.

RESULTS

In the first protocol, peak CVC values during WMTS at the site perfused with l-NAME were significantly decreased, compared to those at the control site (p<0.05). In the second protocol, peak CVC values during WMTS did not differ between the control site and the Keto site (p>0.05).

CONCLUSIONS

These data demonstrate that NO is involved in the mechanism of cutaneous vasodilation induced by WMTS. Furthermore, increases in CVC despite inhibition of the COX pathway suggest that PG does not contribute to cutaneous vasodilation during WMTS.

Axon reflex-related hyperemia induced by short local heating is reproducible

OBJECTIVES

The axon reflex (AR) flare is induced by antidromic activation of afferent C-fibers during nociceptive stimulation. This response has been suggested to be modulated by sympathetic activity and basal level of nitric oxide. In previously used protocols of local thermal hyperemia (LTH), AR flare has been used in combination with maximal vasodilatation to study the integrated endothelial function. The aim of this study was to investigate the intra-session reproducibility of short heating-induced AR flare, the specific neural-mediated portion of LTH, and to compare the reproducibility between different forms of data expression.

METHODS

Short-heating LTH was assessed using single-point laser Doppler flowmetry (LDF) on bilateral volar surface of the forearm in 10 men and 10 women. The blood flux measurement included a non-heating process for 5 min, followed by a quick heating process from 33°C to 42°C for 5 min. The test was repeated 45 min later at the same recording sites with fixed holders. Baseline and heating blood flux were recorded and expressed as different forms of data. Reproducibility was assessed using coefficient of variation (CV) and intra-class correlation coefficient (ICC) statistics.

RESULTS

The reproducibility of peak cutaneous vascular conductance (CVC) (CV=16.02-17.31%, ICC=0.77-0.78), peak CVC change (CV=14.30-18.12%, ICC=0.80-0.86), and the 4 min area-under-the-curve (CV=18.37-18.70%, ICC=0.60-0.78) was acceptable. The time to peak flux of each recording site ranged from 90 to 209 s and all the peak fluxes have been achieved before 4 min of heating.

CONCLUSIONS

Single-point LDF is a reproducible technique of assessing AR flare on volar surface of the forearm when the heating period is reduced to 5 min and the recording sites are fixed. Using this new protocol, short-heating LTH has a potential to be used to evaluate the effects of acute physical or chemical interventions between two short-heating LTH tests to further explore the pathophysiological meaning of heating-induced AR flare.

Polarized light spectroscopy for measurement of the microvascular response to local heating at multiple skin sites

OBJECTIVE

To evaluate whether TiVi, a technique based on polarized light, could measure the change in RBC concentration during local heating in healthy volunteers.

METHODS

Using a custom-made transparent heater, forearm skin was heated to 42 °C for 40 minutes while the change in RBC concentration was measured with TiVi. The perfusion response during local heating was measured at the same time with Laser Doppler flowmetry.

RESULTS

Mean RBC concentration increased (91 ± 34 vs. 51 ± 34 A.U. at baseline, p < 0.001). The spatial heterogeneity of the RBC concentration in the measured skin areas was 26 ± 6.4% at baseline, and 23 ± 4.6% after 40 minutes of heating. The mean RBC concentrations in two skin sites were highly correlated (0.98 at baseline and 0.96 after 40 minutes of heating). The change in RBC concentration was less than the change in perfusion, measured with LDF. Unlike with LDF, a neurally mediated peak was not observed with TiVi in most of the test subjects.

CONCLUSIONS

TiVi is a valuable technique for measuring the microvascular response to local heating in the skin, and offers a high reproducibility for simultaneous measurements at different skin sites, provided carefully controlled experiments are ensured.

Laser Doppler imaging for early detection of hemorrhage

BACKGROUND

Laser Doppler Imaging (LDI) is a noninvasive means to measure blood flow through the superficial skin capillary plexus using flux units. Our objective was to determine the ability of LDI of the skin to detect and quantify rapid, severe hemorrhage.

METHODS

Five Yucatan mini-pigs (25-35 kg) underwent controlled hemorrhage of 25 mL/kg blood for 20 minutes. Median flux of a 10 cm × 10 cm area of the lower abdomen was measured at 2-minute intervals from initiation of hemorrhage to resuscitation with concurrent measurement of heart rate (HR), systolic blood pressure (SBP), and mean arterial pressure (MAP).

RESULTS

Average time to a change of 5 U in flux following start of hemorrhage was 2.4 minutes. This was significantly faster than time to change in HR (19.2 minutes, p < 0.05) and showed a trend toward more rapid identification of hemorrhage relative to changes in SBP (3.2 minutes, p = 0.157) and MAP (3.6 minutes, p = 0.083). Flux changes occurred at smaller % total blood volume lost than HR (3.94% vs. 28.8%, p < 0.05) and trended toward smaller volume identification than SBP (4.88%, p = 0.180) and MAP (5.36%, p = 0.102). Average correlation (ρ) of blood volume lost to flux was -0.974; HR, 0.346; SBP, -0.978; and MAP, -0.975. A change of 5 flux units was significantly more sensitive for hemorrhage than a change of 5 beats per minute in HR or 5 mm Hg in SBP or MAP (0.596 vs. 0.169, 0.438, and 0.287 respectively, all p < 0.05).

CONCLUSION

LDI is a sensitive, specific, and early means to detect and quantify severe hemorrhage.

Comparison of laser speckle contrast imaging with laser Doppler for assessing microvascular function

OBJECTIVE

To compare the inter-day reproducibility of post-occlusive reactive hyperaemia (PORH) and sympathetic vasomotor reflexes assessed by single-point laser Doppler flowmetry (SP-LDF), integrating-probe LDF (IP-LDF) and laser speckle contrast imaging (LSCI), and the spatial variability of PORH assessed by IP-LDF and LSCI. We also evaluated the relationship between IP-LDF and LSCI perfusion values across a broad range of skin blood flows.

METHODS

Eighteen healthy adults (50% male, age 27 ± 4 years) participated in this study. Using SP-LDF, IP-LDF and LSCI, indices of skin blood flow were measured on the forearm during PORH (1-, 5- and 10-min occlusions) and on the finger pad during inspiratory gasp and cold pressor tests. These tests were repeated 3-7 days later. Data were converted to cutaneous vascular conductance (CVC; laser Doppler flow/mean arterial pressure) and expressed as absolute and relative changes from pre-stimulus CVC (ΔCVC(ABS) and ΔCVC(REL), respectively), as well as normalised to peak CVC for the PORH tests. Reproducibility was expressed as within-subjects coefficients of variation (CV, in %) and intraclass correlation coefficients.

RESULTS

The reproducibility of PORH on the forearm was poorer when assessed with SP-LDF and IP-LDF compared to LSCI (e.g., CV for 5-min PORH ΔCVC(ABS)=35%, 27% and 19%, respectively), with no superior method of data expression. In contrast, the reproducibility of the inspiratory gasp and cold pressor test responses on the finger pad were better with SP-LDF and IP-LDF compared to LSCI (e.g., CV for inspiratory gasp ΔCVC(REL)=13%, 7% and 19%, respectively). The spatial variability of PORH responses was poorer with IP-LDF compared to LSCI (e.g., CV ranging 11-35% versus 3-16%, respectively). The association between simultaneous LSCI and IP-LDF perfusion values was non-linear.

CONCLUSION

The reproducibility of cutaneous PORH was better when assessed with LSCI compared to SP-LDF and IP-LDF; probably due to measuring larger skin areas (lower inter-site variability). However, when measuring sympathetic vasomotor reflexes on the finger pad, reproducibility was better with SP-LDF and IP-LDF, perhaps due to the high sensitivity of LSCI to changes in skin blood flow at low levels.

Postexercise reduction in lung diffusion capacity is not attenuated by skin cooling

Pulmonary diffusion capacity for carbon monoxide (DL(CO)) is reduced by approximately 10% 1-6 h after maximal exercise. The mechanisms may be interstitial alveolar oedema and reduced pulmonary capillary blood volume. It was hypothesized that thermal stress following exercise contributes to the reduction in DL(CO), and that skin cooling would attenuate the postexercise reduction in DL(CO). Cutaneous vascular conductance (CVC), mean surface temperature (MST), rectal temperature and DL(CO) were measured before and 90 min after maximal incremental cycle exercise. Thereafter, the subjects were exposed to cold air without eliciting shivering one day and another day served as control. The measurements were repeated 120 min after exercise. Twelve healthy subjects (six male) aged 20-27 years were studied. DL(CO) was reduced by 7.1% (SD = 6.3%, P = 0.003) and 7.6% (SD = 5.3%, P<0.001) 90 and 120 min after exercise in the control experiment. It was reduced by 5.6% (SD = 5.5%, P = 0.014) 90 min after exercise and remained reduced by 6.1% (SD = 6.1%, P = 0.012) after cooling despite a significant reduction in CVC and in MST from 31.9 (SD = 0.6) degrees C to 27.4 (SD = 1.9) degrees C. We conclude that the postexercise reduction in DL(CO) is present when thermal status is restored after exercise, and that it is not influenced by further skin surface cooling.

Laser Doppler scanning study of axillary skin before and after liposuction curettage in patients with focal hyperhidrosis

BACKGROUND

Minimally invasive surgeries are frequently used in patients suffering from focal axillary hyperhidrosis (FAH). Sweat glands are removed surgically and the axillary skin is thinned out, with skin necrosis being a possible complication due to reduced microcirculation. Although of considerable interest, studies evaluating pre- and postoperative skin perfusion are unavailable.

OBJECTIVE

To evaluate the blood flow of axillary skin in patients with severe focal axillary hyperhidrosis before and after liposuction curettage (LC).

MATERIAL AND METHODS

The blood flow in the axillary skin of 11 patients was measured by laser Doppler perfusion imaging before surgery and on days 1, 7 and 28 after LC with a rasping cannula. Skin perfusion was measured in arbitrary units (AU) with measuring points in the axillary center (AC), the operated skin 2 cm from the center (2C) and the surrounding healthy skin (HS).

RESULTS

No significant differences of preoperative skin perfusion were found (AC: 0.39 +/- 0.08 AU/2C: 0.38 +/- 0.07 AU/HS: 0.39 +/- 0.07 AU; p > 0.05). On the first and seventh postoperative days, AC (0.2 +/- 0.04 AU/0.27 +/- 0.81 AU) and 2C (0.2 +/- 0.03 AU/0.28 +/- 0.06 AU) area were significantly less perfused, whereas the HS showed higher perfusion values (0.59 +/- 0.1 AU/0.53 +/- 0.09 AU). Twenty-eight days after LC the 2C (0.36 +/- 0.07 AU) and HS (0.4 +/- 0.06 AU) skin revealed no significant differences compared to preoperative skin perfusion (p > 0.05). The AC perfusion was still slightly reduced (0.37 +/- 0.09 AU) without significant difference compared to preoperative findings.

CONCLUSION

LC reduces the axillary skin blood flow with the axillary center being the least perfused area. However, in our collective, no correlation to possible side effects was observed.

Laser Doppler imaging of skin blood flow for assessing peripheral vascular impairment in hand-arm vibration syndrome

The objective of this study was to evaluate the usefulness of laser Doppler imaging (LDPI) of the skin blood flow for assessing peripheral vascular impairment in the hand-arm vibration syndrome (HAVS). The subjects were 46 male patients with HAVS, aged 50 to 69 yr, and 31 healthy male volunteers of similar age as controls. A cold provocation test was carried out by immersing a subject's hand on his more severely affected side into cold water at a temperature of 10 degrees C for 10 min. Repeated image scanning of skin blood flow of the index, middle, and ring fingers was performed every 2 min before, during, and after the cold water immersion using a PMI-II laser Doppler perfusion imager. The mean blood perfusion values in the distal phalanx area of the fingers were calculated on each image. The patients suffering from vibration-induced white finger (VWF, n=20) demonstrated significantly lower skin blood perfusion at each interval of the test as compared with those without VWF (n=26) and the controls (p<0.01, ANOVA). The blood perfusions in the HAVS patients were associated with the severity of the symptoms as classified by the Stockholm Workshop scale for vascular staging. When a subject was considered to be positive if any of the tested fingers showing a decreased blood perfusion and/or a delayed recovery pattern, the sensitivity was 80.0%, and the specificity was 84.6% and 93.5% for patients without VWF and the controls, respectively. These results suggest that the LDPI technique could provide detailed and accurate information that may help detect the existence of impaired vascular regulation to cold exposure in the fingers of workers exposed to hand-transmitted vibration.

Hyperoxia decreases cutaneous blood flow in high-perfusion areas

The mechanism by which hyperoxia decreases blood flow is still not understood. Hyperoxemia-induced vasoconstriction is known to occur in many organs, including brain and retina, skeletal muscle, and myocardium. Whether this also occurs in skin is unknown. This study was conducted in healthy volunteers exposed intermittently to 100% oxygen (F(I)O(2) 1.0). Perfusion of forearm skin was measured by laser Doppler imaging (LDI). In series 1, it was measured in 7 subjects before, during, and after 15 min of oxygen breathing. In series 2, flow was measured, also during air and O(2) breathing, after perfusion was raised by (a) sympathetic blockade (induced by a topically applied local anesthetic) (n=9) and by (b) current-induced vasodilation (n=8). In normal unperturbed skin, there was no significant change with hyperoxia. When basal perfusion was raised by topical anesthesia or by current, there was also no change in mean perfusion overall with hyperoxia. However, areas with the highest perfusion (upper decile) showed a significant perfusion decrement with hyperoxia (-30% and -20%, respectively; p<0.001). Vasoconstriction with hyperoxia has been demonstrated in human skin. The fact that it is observed only when flow is increased above basal levels and then only in high-flow vessels suggests that cutaneous blood flow control is primarily regulated by variables other than oxygen.

Cutaneous blood flow and sweat rate responses to exogenous administration of acetylcholine and methacholine

The aim of this study was to evaluate cutaneous vasodilation and sweating responses to exogenous administration of acetylcholine (ACh) and methacholine (MCh), which have different sensitivities to endogenous cholinesterase. Four intradermal microdialysis probes were placed in dorsal forearm skin: two sites were perfused with ACh (1 x 10(-7)-1 M) and the other two with the same molar concentrations of MCh. Sweat rate (SR) and cutaneous blood flow were simultaneously assessed directly over each microdialysis membrane. Dose-response curves were constructed, and the effective concentration of the drug resulting in 50% of the maximal response (EC(50)) was identified. For SR and cutaneous vascular conductance (CVC), there were no significant differences in EC(50) between sites receiving the same drug: -1.52 +/- 0.18 and -1.19 +/- 0.09 log-molar concentration of ACh at distal and proximal sites, respectively, and -2.35 +/- 0.24 and -2.42 +/- 0.23 log-molar concentration of MCh at distal and proximal sites, respectively, for SR (P > 0.05) and -3.87 +/- 0.32 and -3.97 +/- 0.27 log-molar concentration of ACh at distal and proximal sites, respectively, and -4.78 +/- 0.17 and -4.46 +/- 0.16 log-molar concentration of MCh at distal and proximal sites, respectively, for CVC (P > 0.05). However, the EC(50) for CVC and SR was significantly lower at the MCh than at the ACh sites. A second procedure was performed to confirm that differences in responses between ACh and MCh could be attributed to different cholinesterase sensitivities. Similarly, four microdialysis membranes were placed in dorsal forearm skin: two sites were perfused with ACh and other two with MCh. However, one of each of the ACh and MCh sites was also perfused with 10 microM neostigmine (an acetylcholinesterase inhibitor). Neostigmine at the ACh site induced a leftward shift (i.e., lower EC(50)) of the SR and CVC dose-response curves compared with the site treated with ACh alone, resulting in no difference in the EC(50) for SR and CVC between the ACh + neostigmine and the MCh site. These results suggest that elevations in SR and CVC occur earlier with MCh than with ACh treatment because of differences in cholinesterase susceptibility between these drugs.

Evaluation of antidecubitus mattresses

Pressure sores are a current problem in hospitals and care of the elderly, leading to protracted hospital stays and a high care burden. The trauma for the patients is severe, and the cost of pressure sore prevention and treatment, is considerable. Antidecubitus mattresses are used for prevention and in treatment, but they also contribute to the cost of treating pressure sores. The problem highlighted in the review is that the mattresses' effectiveness in preventing and treating pressure sores has not been sufficiently evaluated. When antidecubitus mattresses are evaluated, it is often only with regard to aspects of the interface pressure and the mattresses' ability to redistribute the pressure. The review points out the important observation that, to be able to evaluate the efficacy of the antidecubitus mattress, the mattress's effect on tissue viability needs to be studied. The parameters that ought to be considered when evaluating a support surface are: interface pressure, pressure and blood flow distribution, temperature and humidity in the skin-support surface interface. The authors propose that the effect on tissue viability of external loading can be assessed by simultaneous measurement of the interface pressure and tissue perfusion.

Abnormal microvascular response is localized to the digits in patients with systemic sclerosis

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 degrees 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.

Hyperspectral imaging: a new approach to the diagnosis of hemorrhagic shock

BACKGROUND

Skin color changes and mottling are frequently described signs of hemorrhagic shock (HEM). Based on this, we developed a noninvasive, noncontact hyperspectral imaging system (HSI), which quantifies and depicts the surface tissue saturation of oxygen (SHSIO2) for each pixel in a region of interest (ROI). Our purpose was to assess HSI in a porcine HEM model. We hypothesized that HEM would cause decreases in SHSIO2 of the skin.

METHODS

The HyperMed HSI system employs a spectral separator to vary the wavelength of light admitted to a digital camera. During image acquisition, a "hypercube" of images, each at a separate wavelength, is generated (at 5-nm intervals, from 500 to 600 nm). Then, the visible light spectrum for each pixel in the hypercube is compared by linear regression to standard spectra for oxyhemoglobin (OxyHb) and deoxyhemoglobin (DeoxyHb). The resulting fit coefficients for OxyHb and DeoxyHb are used to calculate SHSIO2 values for each pixel in the ROI. The mean values for OxyHb, DeoxyHb, and SHSIO2 across the ROI are calculated. Grayscale SHSIO2 pictures of the ROI are also generated, in which the brightness of each pixel is proportional to its value. Seventeen pigs, 36.4 +/- 0.11 kg, underwent standard preparation, and were maintained on ketamine and isoflurane. Normothermia was maintained (37 degrees C to 39 degrees C). The hemorrhage group (HEM, n = 9) underwent three blood withdrawals, each 10 mL/kg, with 15 minutes between withdrawals. After the third withdrawal, animals were resuscitated with lactated Ringer's and then shed blood. The control group (CTRL, n = 8) received intravenous fluids at 100 mL/h. HSI images were obtained of the inner hindlimb throughout.

RESULTS

All HEM animals showed linear decreases in both mean SHSIO2 and OxyHb values with blood loss, which were reversed by resuscitation. These changes were evident on the grayscale SHSIO2 pictures, but not to the naked eye, and paralleled those of invasively obtained arterial base excess and mixed venous oxygen saturation.

CONCLUSIONS

HSI is a promising noninvasive and noncontact tool for quantifying changes in skin oxygenation during HEM and resuscitation.

Effect of an 8-week resistance training program on cutaneous perfusion in type 2 diabetes

A positive association has previously been demonstrated between chronic aerobic exercise and prior maximal exercise and enhanced dorsal foot skin perfusion in physically active individuals with type 2 diabetes. The current study examined whether an 8-week resistance training program would also positively affect cutaneous perfusion in type 2 diabetic individuals. Ten individuals with type 2 diabetes and nine similar nondiabetic controls participated in 8 weeks of moderate-intensity resistance training. Prior to training, dorsal foot cutaneous perfusion was measured noninvasively by continuous laser Doppler assessment at baseline and during localized heating to 44 degrees C. These measurements were repeated exactly 48-72 h following 8 weeks of resistance training performed 3 days per week. Interstitial nitric oxide (NO) levels were measured concurrently in the contralateral foot dorsum. Neither subject group experienced significant increases in dorsal foot perfusion responsiveness during local heating to 44 degrees C following moderate resistance training, nor did the training significantly enhance baseline skin perfusion. Interstitial NO levels were not significantly different under any condition. At baseline, groups differed only on fasting serum glucose and overall glycemic control. In conclusion, the responsiveness of cutaneous perfusion in response to heating to 44 degrees C is not significantly enhanced by 8 weeks of moderate resistance training in diabetic individuals or their matched controls, independent of interstitial NO levels.

Characterization of a “Blanch-Blush” Mechano-Response in Palmar Skin

Palmar finger skin reacts to extension under mechanical load -- blanching over proximal (intercrease skin, ICS) and middle phalanges, while blushing in crease skin (CS), which we have called the Blanch-Blush Reaction (BBR). The idea that the BBR is a result of surface capillary blood flow changes that relate to predictable deformation of aligned collagen matrices under applied uniaxial loads was tested. Nondestructive techniques, digital image analysis (DIA), laser Doppler scanning, and elastic scatter spectroscopy (ESS) were used to measure color and blood flow changes in healthy fingers when at rest and extended. Skin strain increased directly with applied load and DIA identified blanching (loss of redness) in the ICS, reflected by a decrease in hemoglobin (by ESS). Laser Doppler flowmetry identified an increase in blood flow in the CS zone on extension, with a minor increase in blood flow in the ICS zone, apparently due to diversion of flow to deeper vessels, when monitored by this technique. These changes correlated with the BBR, owing to altered capillary flow in the ICS and CS. The histology of orientation of collagen fibers and vessels in the two zones was consistent with this mechanism. This study demonstrates the interdependence between matrix orientation, applied load, and flow. It represents an elegant demonstration of collagenous tissue function through an everyday tissue reaction, which has not been described previously.

Change in cutaneous perfusion following 10 weeks of aerobic training in Type 2 diabetes

A small, but positive, association between aerobic training status or prior maximal exercise and enhanced dorsal foot skin perfusion in active individuals with Type 2 diabetes has been shown. This study, therefore, was designed to examine whether an aerobic training intervention would positively affect cutaneous perfusion in sedentary Type 2 diabetic individuals as well. Nine sedentary participants with Type 2 diabetes (DS) and 10 obese nondiabetic controls (CS) were studied. Prior to the initiation of aerobic training, dorsal foot cutaneous perfusion was measured noninvasively by continuous laser Doppler assessment at baseline and during localized heating to 44 degrees C. These measurements were repeated 48-72 h following 10 weeks of moderate aerobic training performed 3 days per week. Interstitial nitric oxide (NO) levels were measured concurrently in the contralateral foot dorsum. Aerobic training did not significantly enhance baseline skin perfusion, nor were interstitial NO levels different under any condition. At baseline, groups differed only in glycated hemoglobin (HbA1c), fasting serum glucose, HDL-cholesterol, and insulin resistance. At rest, cutaneous perfusion during local heating to 44 degrees C was significantly lower in DS before training, but did not differ significantly from CS afterward. Neither group, however, experienced significant increases in dorsal foot perfusion during local responsiveness to heating to 44 degrees C following 10 weeks of moderate aerobic training, despite slightly lower perfusion in DS before training; these findings were independent of interstitial NO levels. Thus, moderate aerobic training for only 10 weeks does not appear to reverse the impairment in cutaneous perfusion of the foot dorsum in response local heating in a Type 2 diabetic population.

Bootstrap resampling method to estimate confidence intervals of activation-induced CBF changes using laser Doppler imaging

Laser Doppler imaging (LDI) signal and noise characteristics can vary significantly depending upon the underlying vascular caliber. Further, noise characteristics are not constant over time (non-stationary) and can vary during resting and activated conditions in a typical experiment. Since only a limited number of images can be acquired in a single run, concatenation of data from similar experimental trials becomes necessary which can induce further variation in temporal noise due to instrumental response. In conventional statistical analysis methods such as cross-correlation, a fixed significance threshold is generally used (for the entire image) to detect activation assuming constant noise over time and a normal distribution. As a consequence, statistical significance can become strong or weak due to temporal differences in baseline LD noise, which can possibly deviate from a normal distribution. The main emphasis of this study was the application of bootstrap resampling in conjunction with cross-correlation to estimate the confidence intervals on a pixel-by-pixel basis to avoid distributional specifications on the additive measurement error leading to reliable whisker activation-induced CBF changes. At a 95% confidence level, bootstrap resampling followed by confidence intervals for the correlation coefficient distribution increased the number of active pixels by almost 45% when compared to conventional cross-correlation. These pixels were mostly confined to areas with intermediate and large baseline LD flux with considerable deviation from normality. It is suggested that confidence intervals of the bootstrap estimates can lead to unbiased detection of CBF change in the cerebral cortex, particularly in regions with large temporal variation in noise and low CNR.