Orthogonal Polarization Spectral Research Articles

Non-invasive techniques to access in vivo the skin microcirculation in patients.

The microcirculation is composed of blood vessels with mean internal diameter smaller than 100 μm. This structure is responsible for survival of cells and in the last 50 years its study has become increasingly interesting because it often participates in the pathophysiology of several diseases or can determine better or worse prognosis for them. Due to the growing importance of knowing more about the microcirculation, several techniques have been developed and now it is possible to study its structure or function. In the last 25 years, the cutaneous microcirculation has emerged as an accessible and representative portion of generalized vascular bed allowing the examination of mechanisms of microcirculatory function and dysfunction. This mini review presents several techniques used for non-invasive access to skin microcirculation, such as Nailfold Videocapillaroscopy, Orthogonal Polarization Spectral Imaging, Sidestream Dark Field Imaging, Incident Dark field Illumination, Laser Doppler Flowmetry, and Laser Speckle Contrast Imaging applied. The techniques presented will describe which types of variables (structural or functional) can be evaluated, their limitations and potential uses.

The response of the microcirculation to mechanical support of the heart in critical illness.

Critical illness associated with cardiac pump failure results in reduced tissue perfusion in all organs and occurs in various conditions such as sepsis, cardiogenic shock, and heart failure. Mechanical circulatory support (MCS) devices can be used to maintain organ perfusion in patients with cardiogenic shock and decompensated chronic heart failure. However, correction of global hemodynamic parameters by MCS does not always cause a parallel improvement in microcirculatory perfusion and oxygenation of the organ systems, a condition referred to as a loss of hemodynamic coherence between macro- and microcirculation (MC). In this paper, we review the literature describing hemodynamic coherence or loss occurring during MCS of the heart. By using Embase, Medline Cochrane, Web of Science, and Google Scholar, we analyzed the literature on the response of MC and macrocirculation to MCS of the heart in critical illness. The characteristics of patients, MCS devices, and micro- and macrocirculatory parameters were very heterogenic. Short-term MCS studies (78%) described the effects of intra-aortic balloon pumps (IABPs) on the MC and macrocirculation. Improvement in MC, observed by handheld microscopy (orthogonal polarization spectral (OPS), sidestream dark-field (SDF), and Cytocam IDF imaging) in line with restored macrocirculation was found in 44% and 40% of the studies of short- and long-term MCS, respectively. In only 6 of 14 studies, hemodynamic coherence was described. It is concluded that more studies using direct visualization of the MC in short- and long-term MCS by handheld microscopy are needed, preferably randomized controlled studies, to identify the presence and clinical significance of hemodynamic coherence. It is anticipated that these further studies can enable to better identify patients who will benefit from treatment by mechanical heart support to ensure adequate organ perfusion.

Sidestream versus incident dark field imaging: How to compare two different technologies to study the microcirculation?

Proper perfusion of tissues and organs is necessary for the maintenance of physiologic functioning and homeostasis. The microcirculation, comprised of the smallest vessels such as capillaries, venules, and arterioles, is responsible for the delivery of oxygen and nutrients. Microhemodynamic changes during disease states can impair proper microcirculatory function leading to a hypoxic state and a buildup of waste products in certain body tissue [1]. From a clinical perspective, monitoring of microcirculatory parameters in patients is challenging. Primarily, non-invasive access to microcirculatory beds is usually limited to mucosal surfaces exposed to the environment, e.g. oral cavity. Technologies (hardand software) to observe and quantify microcirculatory function are still being developed, and have not been established in the clinical routine. Prior to the twenty-first century, bulky microscopes were used to study the microcirculation in patients, through techniques such as trans-illuminating tissue at oblique angles [2], epi-illumination of tissue using incident darkfield illumination (IDF) [3], and cross-polarized light [4]. The first handheld video microscope for microcirculatory observation was introduced in 1999, using orthogonal polarization spectral (OPS) imaging technology [5]. OPS imaging involved illuminating the tissue of interest with linearly polarized light and recording remitted light that had been orthogonally polarized through a polarizing filter. Only depolarized photons scattered in deep tissue contributed to the image, thereby allowing visualization of capillaries under mucosal membranes. In 2005, a second generation handheld video microscope was introduced using sidestream dark field (SDF) imaging [6, 7]. This technology illuminates tissue using a central probe surrounded by green light emitting diodes (530 nm). The green light is absorbed by hemoglobin in red blood cells, thereby illuminating surrounding tissue and allowing for the visualization of blood cells flowing through capillaries. The MicroScan video microscope (MicroVision Medical, Amsterdam, Netherlands) is the most commonly used portable handheld SDF device for research purposes. Until recently the main limitation in incorporating this technology at bedside diagnosis was obtaining quantifiable microcirculatory parameters in a timely manner. Semiquantitative analysis software (AVA 3, MicroVision Medical, Amsterdam, Netherlands) was specifically developed for the MicroScan video microscope to support offline analysis of the recorded videos. Recently, an updated software (AVA 4, MicroVision Medical, Amsterdam, Netherlands) was released, allowing automated real time analysis of the recorded videos. Validation studies for AVA 4 have not yet been published, therefore AVA 3 still remains the accepted ‘‘gold standard’’ for analyzing microcirculatory videos. Recently, a third generation of handheld microscope (CytoCam , Braedius Medical, Huizen, Netherlands) was introduced using IDF technology. The CytoCam system offers video resolutions of 4416 9 3312 pixels, while the MicroScan system offers resolutions of 720 9 580 pixels. Automated video analysis software is provided by the manufacturer (CytoCamTools , Braedius Medical, Huizen, Netherlands). & Christian Lehmann chlehmann@dal.ca

Monitoring Microcirculatory Blood Flow with a New Sublingual Tonometer in a Porcine Model of Hemorrhagic Shock.

Tissue capnometry may be suitable for the indirect evaluation of regional hypoperfusion. We tested the performance of a new sublingual capillary tonometer in experimental hemorrhage. Thirty-six anesthetized, ventilated mini pigs were divided into sham-operated (n = 9) and shock groups (n = 27). Hemorrhagic shock was induced by reducing mean arterial pressure (MAP) to 40 mmHg for 60 min, after which fluid resuscitation started aiming to increase MAP to 75% of the baseline value (60–180 min). Sublingual carbon-dioxide partial pressure was measured by tonometry, using a specially coiled silicone rubber tube. Mucosal red blood cell velocity (RBCV) and capillary perfusion rate (CPR) were assessed by orthogonal polarization spectral (OPS) imaging. In the 60 min shock phase a significant drop in cardiac index was accompanied by reduction in sublingual RBCV and CPR and significant increase in the sublingual mucosal-to-arterial PCO2 gap (PSLCO2 gap), which significantly improved during the 120 min resuscitation phase. There was significant correlation between PSLCO2 gap and sublingual RBCV (r = −0.65, p < 0.0001), CPR (r = −0.64, p < 0.0001), central venous oxygen saturation (r = −0.50, p < 0.0001), and central venous-to-arterial PCO2 difference (r = 0.62, p < 0.0001). This new sublingual tonometer may be an appropriate tool for the indirect evaluation of circulatory changes in shock.

Orthogonal polarization spectral imaging of conjunctival microcirculation

Orthogonal polarization spectral (OPS) imaging is an optical imaging technique that uses a handheld microscope and green polarized light to visualize the red blood cells in the microcirculation of organ surfaces. The purpose of this study was to evaluate whether OPS imaging can be used for the functional and morphological evaluation of microcirculation in the conjunctiva. To accomplish the aforementioned purpose, 21 eyes of 21 volunteer patients were examined. OPS images of the vasculature of the inferior conjunctiva and the nasal part of the bulbar conjunctiva were taken from each eye. The images were subsequently analyzed using a computer, and the following parameters were assessed: red blood cell velocity, blood vessel diameter, and functional capillary density. In addition, distinct qualitative aspects of the conjunctival microvasculature were characterized. OPS imaging facilitated both the observation of red blood cells that were flowing through conjunctival vessels on a white background, and the measurement of other quantitative and qualitative microvascular parameters. Significant differences between several measures of the inferior and nasal bulbar conjunctival microcirculations were found, including differences in the configurations of the vessel segments, the number of vessel segments, the number of bifurcations, the mean diffusion distance, and the functional capillary density. OPS imaging can be used to measure the diameters of microvessels, functional capillary density, and other parameters. Significant differences between the microcirculations of the inferior conjunctiva and the nasal bulbar conjunctiva were found, which indicates the necessity of using a standardized approach to examine the conjunctival vasculature. OPS imaging is suitable for both the functional and morphological evaluation of the conjunctival microcirculation.

Depth selectivity for the assessment of microstructure by polarization studies

A polarimetric imaging system capable of continuously selecting imaging depth in a turbid media is demonstrated. The proposed system is based on the orthogonal polarization spectral (OPS) technique, and is able to detect microstructure and microvessel. First, we compare the performance of four polarization imaging channels on a biological phantom, and find that there is a linear relation between the degrees of ellipticity and image contrast in co-linear/co-elliptical channels. In addition, the cross-linear channel has the best image contrast. We then prove the performance of depth selectivity of microvessel in a mouse ear.

Monitoring microcirculatory blood flow with a new sublingual tonometer in a porcine model of haemorrhagic shock

Tissue capnometry may be suitable for the indirect evaluation of regional hypoperfusion. We tested the performance of a new sublingual capillary tonometer in experimental hemorrhage. Thirty-six anesthetized, ventilated mini pigs were divided into sham-operated (n = 9) and shock groups (n = 27). Hemorrhagic shock was induced by reducing mean arterial pressure (MAP) to 40 mmHg for 60 min, after which fluid resuscitation started aiming to increase MAP to 75% of the baseline value (60-180 min). Sublingual carbon-dioxide partial pressure was measured by tonometry, using a specially coiled silicone rubber tube. Mucosal red blood cell velocity (RBCV) and capillary perfusion rate (CPR) were assessed by orthogonal polarization spectral (OPS) imaging. In the 60 min shock phase a significant drop in cardiac index was accompanied by reduction in sublingual RBCV and CPR and significant increase in the sublingual mucosal-to-arterial PCO2 gap (PSLCO2 gap), which significantly improved during the 120 min resuscitation phase. There was significant correlation between PSLCO2 gap and sublingual RBCV (r = -0.65, p < 0.0001), CPR (r = -0.64, p < 0.0001), central venous oxygen saturation (r = -0.50, p < 0.0001), and central venous-to-arterial PCO2 difference (r = 0.62, p < 0.0001). This new sublingual tonometer may be an appropriate tool for the indirect evaluation of circulatory changes in shock.

The effects of non-leukoreduced red blood cell transfusions on microcirculation in mixed surgical patients

BackgroundThe impact of the storage process on oxygen-carrying properties of red blood cells and the efficacy of red blood cell (RBC) transfusions concerning tissue oxygenation remain an issue of debate in transfusion medicine. Storage time and leukocyte content probably interact since longer storage duration is thought to cause greater accumulation of leukocyte-derived cytokines and red blood cell injury. ObjectivesThe aim of this study was to investigate the effects of storage and the efficacy of fresh (stored for less than 1week) versus aged (stored for more than 3weeks) non-leukoreduced RBC transfusions on sublingual microvascular density and flow in mixed surgical patients. MethodsEighteen surgical patients were included in this study. Patients were randomly assigned into two groups receiving fresh (Group A) and aged (Group B) RBC transfusions. Sublingual microcirculatory functional capillary density (FCD) and microvascular flow index (MFI) were assessed using orthogonal polarization spectral (OPS) imaging. Measurements and collection of blood samples were performed after induction of general anesthesia, before RBC transfusion and 30min after the RBC transfusion ended. ResultsIn both groups RBC transfusions caused an increase in hemoglobin concentration (p<0.001). RBC transfusions increased FCD in Group A (p<0.001), while FCD remained unaffected in Group B. Changes in MFI following RBC transfusion in both groups remained unaltered. ConclusionsFresh non-leukoreduced RBC transfusions but not RBCs stored for more than 3weeks, were effective in improving microciruculatory perfusion by elevating the number of perfused microvessels in mixed surgical patients.

Microcirculatory Changes in Term Newborns with Suspected Infection: An Observational Prospective Study

Background. In adults severely disturbed microcirculatory flow can be observed by Orthogonal Polarized Spectral (OPS) imaging techniques during sepsis. Therefore we set out to assess for microcirculatory changes in term newborns with suspected early onset infection using OPS. Methods. OPS images were obtained prospectively from the vascular bed of the ear conch and upper arm of 47 newborns on their 1st, 2nd, and 3rd day of life. OPS sequences were analyzed semiquantitatively offline and blinded to clinical status of the infant. Flow in vessels was classified as continuous or noncontinuous flow and given as proportion of total vessels per image as in the studies in adults. Results. The proportion of vessels with continuous flow was significantly lower in the infants with infection (69% [56–81] versus 90% [87–94] (P = 0.0003)). None of the infants with infection was in shock or severely septic. Conclusion. In term neonates the microcirculatory flow is impaired in a large proportion of vessels even in mild to moderate infection. These changes can be observed at the onset of disease at the external ear, an optimal site for microcirculatory measurements in term infants.

Microvascular Distribution in the Ocular Conjunctiva and Digestive Tract in an Experimental Setting

Recently improved microcirculatory imaging techniques, such as orthogonal polarization spectral (OPS) and its technical successor sidestream dark field (SDF) imaging, in handheld devices have allowed a direct observation of the microcirculation at the bedside. Usually a cut-off of 20 µm in diameter is used to differentiate small vessels (mainly capillaries) from large vessels (mainly venules) during this technique. We hypothesized that it was possible to measure the small vessels with a considerably smaller inner diameter. Images of the sublingual, conjunctival, jejunal, and rectal mucosa microcirculation were obtained with SDF videomicroscopy (Microscan®, Microvision Medical, Amsterdam, the Netherlands). Using the validated software, the length and diameter of microvessels were manually traced with a computer-generated line. All vessels were divided into the groups according to the inner diameter. A total of 156 SDF images of the sublingual, ocular conjunctival, jejunal, and rectal mucosa were taken in 13 pigs. The length of microscopic vessels progressively increased with a decrease in the vessel diameter less than 8 mm in all the lodges, such as sublingual (80.6% of total vessel length), ocular conjunctival (76.5% of total vessel length), jejunal (99.8% of total vessel length), and rectal (97.8% of total vessel length), due to capillary network formation. There was no significant difference in the distribution of vessels from 0 to 10 µm in diameter comparing sublingual and eye conjunctival as well as jejunal and rectal mucosa. In pigs, small-diameter microscopic vessels (<10 µm) dominated in all the studied lodges (sublingual, ocular conjunctival, jejunal, and rectal mucosa), and this is evidence to establish a new cut-off for capillaries in microcirculatory analysis of SDF imaging in experimental and clinical studies.

Orthogonal Polarization Spectral Imaging: A Novel Tool for Examination of Microcirculatory Changes in the Testis

Testicular torsion is a surgical emergency in urologic practice. Many models have demonstrated the relationship between the degree and duration of the torsion and the subsequent damages of the testis, but there are as yet no suitable methods for visualization of the testicular microcirculation in this condition. We aimed to use orthogonal polarization spectral (OPS) imaging to observe the microcirculatory changes in the testis after ischemia-reperfusion (I/R) challenge, and to compare this technique with that of fluorescence intravital video microscopy (IVM). Twelve rats subjected to 60-minute ischemia following 720° testicular torsion were divided into two groups for IVM or OPS imaging examinations of the microcirculatory network of the testis at 60, 120, 180, and 240 minutes after reestablishment of perfusion. Significant microcirculatory failure was demonstrated after I/R in both groups. The absolute values of the microcirculatory parameters recorded with the OPS and IVM imaging methods did not differ statistically. The microcirculatory disturbances are present during the later phases of testicular torsion. OPS imaging technique represents an accurate noninvasive method to detect physiologic and pathophysiologic changes in the microcirculation of the testis.

Orthogonal Polarization Spectral Imaging in Microcirculation

The microcirculation plays a crucial role in the interaction between blood and tissues in physiological pathophysiological pharmacological and clinical states. Despite its significant role in numerous diseases including, hypertension diabetes, sepsis or multiple organ failure, visible and near-infrared light, offer a window into human and animal tissues due to reduced scattering and absorption. We review orthogonal polarization spectral (OPS) imaging method for direct visualization and quantitative assessment of human microcirculation at the bedside are limited. OPS imaging is a relatively new noninvasive method for assessment of human microcirculation without using fluorescent dyes. Recent clinical studies using OPS imaging in various pathological states have shown a wide spectrum of different clinical applications with evident impact on the diagnosis, treatment or prognosis assessment. Thus, there is a great effort to validate OPS imaging for various clinical purposes. The principles of OPS imaging, validation studies, its advantages, limitations, methods of quantitative assessment and current experience in clinical practice are also discussed.

Sidestream dark field imaging: the evolution of real-time visualization of cutaneous microcirculation and its potential application in dermatology

Technological advances during the last years have enhanced the image quality of the microcirculation. Intravital microscopy (IM) has been considered the "gold standard" for many years, but it can be used mostly in anesthetized animals which is a disadvantage. The nailfold videocapillaroscopy, a non-invasive examination that includes a microscope with an epiillumination system, came afterward, but its major disadvantage is the restricted area available for investigation namely the nailfold capillary bed. The orthogonal polarization spectral (OPS) imaging technique, where reflected light allows the visualization of the microcirculation, was the next non-invasive exam, but it still presents some drawbacks such as suboptimal capillary visualization and image blurring due to red blood cell movements. Excessive probe pressure modifies red blood cell velocity. There is suboptimal imaging of capillaries due to motion-induced image blurring by movements of OPS device, tissue and/or flowing red blood cells. Sidestream dark field (SDF) imaging is the newest tool for microcirculatory research. Illumination is provided by concentrically placed light-emitting diodes to avoid image blurring and to enhance image contrast. It represents a simple and non-invasive imaging technique, with low cost, good portability and high sensitivity that provides fine, well-defined images. In addition, the microcirculation can be studied through laser Doppler flowmetry (LDF) or reflectance-mode confocal-laser-scanning microscopy (RCLM). However, LDF cannot show microcirculatory vessels and high cost of RCLM can be an inconvenience. New applications of SDF technique could include skin microcirculatory evaluation and allow dermatological studies on psoriasis, skin tumors and leprosy.

Monitoring the microcirculation in the critically ill patient: current methods and future approaches

To discuss the techniques currently available to evaluate the microcirculation in critically ill patients. In addition, the most clinically relevant microcirculatory alterations will be discussed. Review of the literature on methods used to evaluate the microcirculation in humans and on microcirculatory alterations in critically ill patients. In experimental conditions, shock states have been shown to be associated with a decrease in perfused capillary density and an increase in the heterogeneity of microcirculatory perfusion, with non-perfused capillaries in close vicinity to perfused capillaries. Techniques used to evaluate the microcirculation in humans should take into account the heterogeneity of microvascular perfusion. Microvideoscopic techniques, such as orthogonal polarization spectral (OPS) and sidestream dark field (SDF) imaging, directly evaluate microvascular networks covered by a thin epithelium, such as the sublingual microcirculation. Laser Doppler and tissue O(2) measurements satisfactorily detect global decreases in tissue perfusion but not heterogeneity of microvascular perfusion. These techniques, and in particular laser Doppler and near-infrared spectroscopy, may help to evaluate the dynamic response of the microcirculation to a stress test. In patients with severe sepsis and septic shock, the microcirculation is characterized by a decrease in capillary density and in the proportion of perfused capillaries, together with a blunted response to a vascular occlusion test. The microcirculation in humans can be evaluated directly by videomicroscopy (OPS/SDF) or indirectly by vascular occlusion tests. Of note, direct videomicroscopic visualization evaluates the actual state of the microcirculation, whereas the vascular occlusion test evaluates microvascular reserve.

Orthogonal Polarization Spectral Imaging

The estimation of burn depths is still a challenge, and even experienced surgeons often fail. In search of an objective method for differentiation between deep- and partial-thickness burns, we investigated the use of orthogonal polarization spectral (OPS) imaging to visualize the microcirculation in burn wounds. Twenty-seven burned patients were included in the study, 81 burn areas were investigated at day 1 and 4 post burn. The final therapy was compared with the therapy that would have been carried out as a consequence of the OPS imaging result. The patients were treated conservatively with dressings or surgically with necrectomy and split skin grafts. A comparison of the estimation of the burn depths of OPS imaging with the final therapy showed a correlation of 76.5%, about 5% less than the clinical assessment. It has proved a useful tool in particular for the prognosis as to whether the burn will heal spontaneously within 14 days or not. The presented OPS-Imaging device provides additional qualitative and quantitative information about the perfusion of the skin and therefore facilitates decisions about the follow on therapy. It is not an alternative to an experienced burn surgeon but provides important additional information.

The temporal course of mucoperiosteal flap revascularization at guided bone regeneration‐treated implant sites: a pilot study

To investigate post-operative capillary density regeneration in healing mucoperiosteal flaps at guided bone regeneration-treated implant sites. A non-invasive post-operative investigation was performed in 10 patients using orthogonal polarization spectral (OPS) imaging for assessment of capillary density during the course of mucoperiosteal flap wound healing for 6 weeks in patients receiving dental implants. The greatest increase in capillary regeneration occurred in the early wound-healing phase, during weeks 1 and 2, and recovery to baseline was achieved between weeks 4 and 5. A comparison of adjacent OPS measurements indicated that differences between the time point immediately following administration of local anaesthesia and directly post-operatively ( p=0.002), between a directly post-operative time point and after 1 week (p=0.009), and between post-operative weeks 1 and 2 (p=0.036) were statistically significant. The early healing phase of mucoperiosteal flaps is characterized by rapid capillary regeneration. OPS imaging enabled the possibility to monitor and quantify the temporal development of mucoperiosteal flap revascularization following periodontal surgery.

Prostaglandin application improves macro- and microcirculation after aorto-hepato-mesenteric bypass in chronic mesenteric ischemia

Chronic mesenterial ischemia (CMI) is an uncommon cause of abdominal afflictions. The spectrum of therapeutic options ranges from mesenteric artery angioplasty and stenting to surgical revascularization. We used orthogonal polarization spectral (OPS) imaging to assess microcirculation after revascularization of the celiac artery and the superior mesenteric artery. Furthermore, we applicated a prostaglandin I2 derivate (iloprost, Ilomedin®, BayerVital GmbH, Leverkusen, Germany) after bypass reperfusion and demonstrated the effect of this vasodilatative agent to microcirculatory parameters. Our patient was a 52-year-old woman who suffered from a complete obstruction of the celiac artery as well as high-grade stenosis of the superior mesenteric artery (SMA). Therefore, we performed an open revascularization using the greater saphenous vein of the left thigh as graft (reversed vein bypass from the supraceliac aorta sequentially to the celiac artery (termino-lateral anastomosis) and the superior mesenteric artery (termino-terminal anastomosis)). After bypass reperfusion, we administered 3 μg iloprost as bolus directly into the bypass. The agreement of the patient was obtained before the aforementioned procedure and the measurements. Immediately following laparotomy, bypass reperfusion and iloprost administration, we measured the microcirculation of stomach, pancreas, small intestine and right hemi-colon. Microhemodynamic analysis included the quantitative analysis of capillary diameter (D), functional capillary density (FCD) and red blood cell velocity (RBCV). Using these parameters, we calculated the individual capillary volumetric flow rate (capillary blood flow (CBF); in picoliter/s; pl/s) and the perfusion index (PI).

Alterations of tissue-dependent microcirculation in patients after successful resuscitation

The crucial role of the microcirculation in improved neurological outcome in patients after successful resuscitation has been discussed for many years. New noninvasive imaging techniques enable the visualization and analysis of the microcirculation in vivo. Our study utilised an orthogonal polarization spectral (OPS) noninvasive imaging technique to test microcirculation in patients after successful resuscitation.

Functional Vessel Density in the First Month of Life in Preterm Neonates

Changes in microcirculation have been recognized as central to many disease processes. The aim of this study was to evaluate factors, which influence the microcirculation of the skin during the first month of life in premature infants. Red blood cell (RBC) velocity, vessel diameter, and functional small vessel density (FSVD) were measured daily for the first 30 d on the upper arm in preterm infants with gestational age <30 wk. Orthogonal polarization spectral (OPS) images were analyzed off-line with the Capi- Scope-Image program. In 25 infants, FSVD decreased significantly from week 1 (mean +/- SD 236 +/- 33 cm/cm2) to week 4 (207 +/- 30 cm/cm2) and correlated directly with Hb levels and incubator temperature. Vessel diameters and RBC velocity did not change significantly, nor did clinical parameters such as blood pressure, heart rate or body temperature. Microvascular parameters were not dependent on gestational or postnatal age. The microcirculation of the skin might be an easily accessible window to obtain better understanding of circulatory changes in the postnatal period. Our data are essential as basis for further studies in this field. Hb levels and possible incubator temperatures have a substantial influence on functional small vessel density and therefore need to be taken in account.

Orthogonal polarization spectroscopy to detect mesenteric hypoperfusion

Orthogonal polarization spectral (OPS) imaging is used to assess mucosal microcirculation. We tested sensitivity and variability of OPS in the assessment of mesenteric blood flow (Qsma) reduction. University Animal Laboratory. In eight pigs, Qsma was reduced in steps of 15% from baseline; five animals served as controls. Jejunal mucosal microcirculatory blood flow was recorded with OPS and laser Doppler flowmetry at each step. OPS data from each period were collected and randomly ordered. Samples from each period were individually chosen by two blinded investigators and quantified [capillary density (number of vessels crossing predefined lines), number of perfused villi] after agreement on the methodology. Interobserver coefficient of variation (CV) for capillary density from samples representing the same flow condition was 0.34 (0.04-1.41) and intraobserver CV was 0.10 (0.02-0.61). Only one investigator observed a decrease in capillary density [to 62% (48-82%) of baseline values at 45% Qsma reduction; P = 0.011], but comparisons with controls never revealed significant differences. In contrast, reduction in perfused villi was detected by both investigators at 75% of mesenteric blood flow reduction. Laser Doppler flow revealed heterogeneous microcirculatory perfusion. Assessment of capillary density did not reveal differences between animals with and without Qsma reduction, and evaluation of perfused villi revealed blood flow reduction only when Qsma was very low. Potential explanations are blood flow redistribution and heterogeneity, and suboptimal contrast of OPS images. Despite agreement on the method of analysis, interobserver differences in the quantification of vessel density on gut mucosa using OPS are high.