Reflectance spectrophotometry for the assessment of mucosal perfusion in the gastrointestinal tract

Mucosa-interfacing electronics

The surface mucosa that lines many of our organs houses myriad biometric signals and, therefore, has great potential as a sensor-tissue interface for high-fidelity and long-term biosensing. However, progress is still nascent for mucosa-interfacing electronics owing to challenges with establishing robust sensor-tissue interfaces; device localization, retention and removal; and power and data transfer. This is in sharp contrast to the rapidly advancing field of skin-interfacing electronics, which are replacing traditional hospital visits with minimally invasive, real-time, continuous and untethered biosensing. This Review aims to bridge the gap between skin-interfacing electronics and mucosa-interfacing electronics systems through a comparison of the properties and functions of the skin and internal mucosal surfaces. The major physiological signals accessible through mucosa-lined organs are surveyed and design considerations for the next generation of mucosa-interfacing electronics are outlined based on state-of-the-art developments in bio-integrated electronics. With this Review, we aim to inspire hardware solutions that can serve as a foundation for developing personalized biosensing from the mucosa, a relatively uncharted field with great scientific and clinical potential.

Quantification of gastric mucosal microcirculation as a surrogate marker of portal hypertension by spatially resolved subdiffuse reflectance spectroscopy in diagnosis of cirrhosis: a proof-of-concept study

BACKGROUND AND AIMS

Portal pressure can be used to identify patients with chronic liver disease who have progressed to cirrhosis. Portal pressure can also provide accurate prognostication for patients with cirrhosis. However, there are no practical means for assessment of portal pressure. Although it is well established that the gastric mucosal blood supply increases in patients with cirrhosis, this has been difficult to quantify reproducibly. Our group has developed a novel spectroscopic technology called spatially resolved subdiffuse reflectance spectroscopy (SRSRS), which enables quantification of mucosal microcirculation. We aim to ascertain if quantification of the gastric mucosal microcirculation with SRSRS correlates with clinical evidence of portal hypertension.

METHODS

Patients undergoing EGD for clinical indications had 10 measurements taken in the endoscopically normal gastric fundus via SRSRS probe to assess the microcirculation. Cases were defined as patients with cirrhosis (n = 18), and controls were those without evidence of liver disease (n = 18); this was corroborated with transient elastography.

RESULTS

The blood volume fraction (P = .06) and subdiffuse reflectance (P = .02) from a shallow depth in the gastric fundus were higher in patients with cirrhosis than those without. These markers were combined to yield an overall optical marker that can differentiate patients with cirrhosis from controls with a sensitivity of 72% and specificity of 94% (area under receiver operating curve, 0.82).

CONCLUSIONS

Spectroscopic quantification of gastric fundal mucosal microcirculation is a promising surrogate of clinical correlates of portal hypertension. This approach may represent a less-intrusive surrogate biomarker for liver disease prognostication and potentially response to therapy.

A Multilayer Monte Carlo Model for the Investigation of Optical Path and Penetration Depth at Different Perfusion States of the Colon

There is a great interest in monitoring the oxygen supply delivered to the colon. Insufficient oxygen delivery may lead to hypoxia, sepsis, multiorgan dysfunction and death. For assessing colonic perfusion, more information and understanding is required relating to the light-interaction within the colonic tissue. A multilayer Monte Carlo model of a healthy human colon has been developed to investigate the light-tissue behavior during different perfusion states within the mucosal layer of the colon. Results from a static multilayer model of optical path and reflectance at two wavelengths, 660 nm and 880 nm, through colon tissue, containing different volume fractions of blood with a fixed oxygen saturation are presented. The effect on the optical path and penetration depth with varying blood volumes within the mucosa for each wavelength has been demonstrated. The simulation indicated both wavelengths of photons penetrated similar depths, entering the muscularis layer.

Evaluation of endoscopic visible light spectroscopy: comparison with microvascular oxygen tension measurements in a porcine model

BACKGROUND

Visible light spectroscopy (VLS) is a technique used to measure the mucosal oxygen saturation during upper gastrointestinal endoscopy to evaluate mucosal ischemia, however in vivo validation is lacking. We aimed to compare VLS measurements with a validated quantitative microvascular oxygen tension (μPO₂) measurement technique.

METHODS

Simultaneous VLS measurements and μPO₂ measurements were performed on the small intestine of five pigs. First, simultaneous measurements were performed at different FiO₂ values (18%-100%). Thereafter, the influence of bile was assessed by comparing VLS measurements in the presence of bile and without bile. Finally, simultaneous VLS and μPO₂ measurements were performed from the moment a lethal dose potassium chloride intravenously was injected.

RESULTS

In contrast to μPO₂ values that increased with increasing FiO₂, VLS values decreased. Both measurements correlated poorly with R² = 0.39, intercept 18.5, slope 0.41 and a bias of - 16%. Furthermore, the presence of bile influenced VLS values significantly (median (IQR)) before bile application 57.5% (54.8-59.0%) versus median with bile mixture of the stomach 73.5% (66.8-85.8), p = < 2.2 * 10^-16; median with bile mixture of small bowel 47.6% (41.8-50.8) versus median after bile removal 57.0% (54.7-58.6%), p = < 2.2 * 10^-16). Finally, the VLS mucosal oxygen saturation values did not decrease towards a value of 0 in the first 25 min of asystole in contrast to the μPO₂ values.

CONCLUSIONS

These results suggest that VLS measures the mixed venous oxygen saturation rather than mucosal capillary hemoglobin oxygen saturation. Further research is needed to establish if the mixed venous compartment is optimal to assess gastrointestinal ischemia.

Colon Perfusion Patterns During Colorectal Resection Using Visible Light Spectroscopy

BACKGROUND

The impact of blood supply to the anastomosis on development of anastomotic leakage is still a matter of debate. Considering that bowel perfusion may be affected by manipulation during surgery, perfusion assessment of the anastomosis alone may be of limited value. We propose perfusion assessment at different time points during surgery to explore the dynamics of bowel perfusion during colorectal resection and its impact on outcome.

METHODS

In this prospective cohort study, patients undergoing elective colorectal resection were eligible. Colon perfusion was evaluated using visible light spectroscopy. Main outcome was the difference in colon perfusion, quantified by measuring tissue oxygen saturation (StO₂) in the colonic serosa, before and after anastomosis during surgery.

RESULTS

We included 58 patients between July 2013 and November 2015. Colon perfusion increased by an average of 5.9% StO₂ during surgery (95% confidence interval 3.1, 8.8; P < 0.001). The number of patients with abnormal perfusion (defined as StO₂ < 65%) decreased from 50% at the beginning to 24% by the end of surgery. Six patients (10%) developed anastomotic leaks (AL), of which five patients had abnormal perfusion at the beginning of surgery, whereas four patients had normal StO₂ at the anastomosis.

CONCLUSION

Colon perfusion significantly increased during colorectal surgery. Considering that one quarter of patients had suboptimal anastomotic perfusion without developing AL, impaired blood flow at the anastomosis alone does not seem to be critical. Further investigations including more patients are necessary to evaluate the impact of perioperative parameters on colon perfusion, anastomotic healing and surgical outcome.

Vascular conditioning of the stomach before esophageal reconstruction by gastric interposition

Gastric interposition with intrathoracic or cervical esophagogastrostomy is currently the preferred operation for reconstruction after esophagectomy. Anastomotic leaks however result from poor vascular supply to the proximal stomach. They are responsible for significant morbidity and mortality. 'Ischemic conditioning' of the interposed stomach has been proposed as a technique where the 'delay phenomenon' aims at improving the microcirculation of the gastric conduit and preventing anastomotic leakage. Experimental observations and clinical studies have been conducted to document the immediate effects and results of this approach. The aim of this work is to review the principles, pathophysiology, experimental, and clinical evidence related to vascular conditioning of the stomach prior to esophagectomy with gastric interposition and esophagogastric anastomosis. MEDLINE and PubMed were searched to identify articles related to vascular conditioning of the stomach. Cross references were added and reviewed to complete the reference list. The anatomic basis of ischemic conditioning, the prevalence of ischemic events on the gastric conduit, the methodology to assess the microcirculation before and after gastric devascularization, animal experiments, and clinical studies reported on this approach were reviewed. Ten experimental works, eleven clinical observations, four reviews, and two editorial commentaries addressing ischemic conditioning of the stomach were identified and reviewed. Experimental observations document improved microcirculation to the proximal stomach following partial gastric devascularization. Clinical reports show the feasibility and relative safety of gastric ischemic conditioning. Preliminary observations suggest potential improvements to the gastric microcirculation resulting from gastric ischemic conditioning. This approach may help prevent complications at the esophagogastric anastomosis. The actual level of evidence however cannot promote its use outside of clinical research protocols.

Endoscopic visible light spectroscopy: a new, minimally invasive technique to diagnose chronic GI ischemia

BACKGROUND

The diagnosis of chronic GI ischemia (CGI) remains a clinical challenge. Currently, there is no single simple test with high sensitivity available. Visible light spectroscopy (VLS) is a new technique that noninvasively measures mucosal oxygen saturation during endoscopy.

OBJECTIVE

To determine the diagnostic accuracy of VLS for the detection of ischemia in a large cohort of patients.

DESIGN

Prospective study, with adherence to the Standards for Reporting of Diagnostic Accuracy.

SETTING

Tertiary referral center.

PATIENTS

Consecutive patients referred for evaluation of possible CGI.

INTERVENTIONS

Patients underwent VLS along with the standard workup consisting of evaluation of symptoms, GI tonometry, and abdominal CT or magnetic resonance angiography.

MAIN OUTCOME MEASUREMENTS

VLS measurements and the diagnosis of CGI as established with the standard workup.

RESULTS

In 16 months, 121 patients were included: 80 in a training data set and 41 patients in a validation data set. CGI was diagnosed in 89 patients (74%). VLS cutoff values were determined based on the diagnosis of CGI and applied in the validation data set, and the results were compared with the criterion standard, resulting in a sensitivity and specificity of VLS of 90% and 60%, respectively. Repeated VLS measurements showed improvement in 80% of CGI patients after successful treatment.

LIMITATIONS

Single-center study; only 43% of patients had repeated VLS measurements after treatment.

CONCLUSIONS

VLS during upper endoscopy is a promising easy-to-perform and minimally invasive technique to detect mucosal hypoxemia in patients clinically suspected of having CGI, showing excellent correlation with the established ischemia workup.

Lung transplant airway hypoxia: a diathesis to fibrosis?

RATIONALE

Chronic rejection, manifested pathologically as airway fibrosis, is the major problem limiting long-term survival in lung transplant recipients. Airway hypoxia and ischemia, resulting from a failure to restore the bronchial artery (BA) circulation at the time of transplantation, may predispose patients to chronic rejection. To address this possibility, clinical information is needed describing the status of lung perfusion and airway oxygenation after transplantation.

OBJECTIVES

To determine the relative pulmonary arterial blood flow, airway tissue oxygenation and BA anatomy in the transplanted lung was compared with the contralateral native lung in lung allograft recipients.

METHODS

Routine perfusion scans were evaluated at 3 and 12 months after transplantation in 15 single transplant recipients. Next, airway tissue oximetry was performed in 12 patients during surveillance bronchoscopies in the first year after transplant and in 4 control subjects. Finally, computed tomography (CT)-angiography studies on 11 recipients were reconstructed to evaluate the post-transplant anatomy of the BAs.

MEASUREMENTS AND MAIN RESULTS

By 3 months after transplantation, deoxygenated pulmonary arterial blood is shunted away from the native lung to the transplanted lung. In the first year, healthy lung transplant recipients exhibit significant airway hypoxia distal to the graft anastomosis. CT-angiography studies demonstrate that BAs are abbreviated, generally stopping at or before the anastomosis, in transplant airways.

CONCLUSIONS

Despite pulmonary artery blood being shunted to transplanted lungs after transplantation, grafts are hypoxic compared with both native (diseased) and control airways. Airway hypoxia may be due to the lack of radiologically demonstrable BAs after lung transplantation.

Quantification of mucosa oxygenation using three discrete spectral bands of visible light

Quantification of the mucosa oxygenation levels during Endoscopic imaging provides useful physiological/diagnostic information. In this work a method for non-contact quantification of the oxygen saturation index during Endoscopic imaging using three discrete spectral-band in the blue, the green, and the red parts of the spectrum (RGB bands) has been investigated. The oxygen saturation index (TOI_rgb) was calculated from the three discrete RGB spectral bands using diffusion approximation modeling and least-square analysis. A parametric study performed to identify the optimum band width for each of the three spectral bands. The quantification algorithm was applied to in vivo images of the endobronchial mucosa to calculate (TOI_rgb) from selected areas within the image view. The results were compared to that obtained from the full visible spectral (470-700 nm, 10 nm) measurements. The analysis showed that a band width of at least 20 nm in the blue and the green is required to obtain best results. The results showed that the method provides accurate estimation of the oxygenation levels with about 90% accuracy compared to that obtained using the full spectra. The results suggest the potential of quantifying the oxygen saturation levels from the three narrow RGB spectral bands/images.

Using an oblique incident laser beam to measure the optical properties of stomach mucosa/submucosa tissue

Background

The purpose of the study is to determine the optical properties and their differences for normal human stomach mucosa/submucosa tissue in the cardiac orifice in vitro at 635, 730, 808, 890 and 980 nm wavelengths of laser.

Methods

The measurements were performed using a CCD detector, and the optical properties were assessed from the measurements using the spatially resolved reflectance, and nonlinear fitting of diffusion equation.

Results

The results of measurement showed that the absorption coefficients, the reduced scattering coefficients, the optical penetration depths, the diffusion coefficients, the diffuse reflectance and the shifts of diffuse reflectance of tissue samples at five different wavelengths vary with a change of wavelength. The maximum absorption coefficient for tissue samples is 0.265 mm^-1 at 980 nm, and the minimum absorption coefficient is 0.0332 mm^-1 at 730 nm, and the maximum difference in the absorption coefficients is 698% between 730 and 980 nm, and the minimum difference is 1.61% between 635 and 808 nm. The maximum reduced scattering coefficient for tissue samples is 1.19 mm^-1 at 635 nm, and the minimum reduced scattering coefficient is 0.521 mm^-1 at 980 nm, and the maximum difference in the reduced scattering coefficients is 128% between 635 and 980 nm, and the minimum difference is 1.15% between 890 and 980 nm. The maximum optical penetration depth for tissue samples is 3.57 mm at 808 nm, and the minimum optical penetration depth is 1.43 mm at 980 nm. The maximum diffusion constant for tissue samples is 0.608 mm at 890 nm, and the minimum diffusion constant is 0.278 mm at 635 nm. The maximum diffuse reflectance is 3.57 mm^-1 at 808 nm, and the minimum diffuse reflectance is 1.43 mm^-1 at 980 nm. The maximum shift Δx of diffuse reflectance is 1.11 mm^-1 at 890 nm, and the minimum shift Δx of diffuse reflectance is 0.507 mm^-1 at 635 nm.

Conclusion

The absorption coefficients, the reduced scattering coefficients, the optical penetration depths, the diffusion coefficients, the diffuse reflectance and the shifts of diffuse reflectance of tissue samples at 635, 730, 808, 890 and 980 nm wavelengths vary with a change of wavelength. There were significant differences in the optical properties for tissue samples at five different wavelengths (P < 0.01).

Is mean blood saturation a useful marker of tissue oxygenation?

Increasingly we are monitoring the distribution of oxygen through the microcirculation using optical techniques such as optical reflectance spectroscopy (ORS) and near-infrared spectroscopy. Mean blood oxygen saturation (SmbO2) and tissue oxygenation index measured by these two techniques, respectively, evoke a concept of the measurement of oxygen delivery to tissue. This study aims to establish whether SmbO2 is an appropriate indicator of tissue oxygenation. Spontaneous fluctuations in SmbO2 observed as changes in concentration of oxyhemoglobin ([HbO2]) and deoxyhemoglobin ([Hb]) were measured by ORS in the skin microcirculation of 30 healthy subjects (15 men, age 21–42 yr). Fourier analysis identified two distinctly different spontaneous falls in SmbO2. The first type of swing, thought to be induced by fluctuations in arterial blood volume, resulted from the effects of respiration, endothelial, sympathetic, and myogenic activity. There was no apparent change in [Hb]. In contrast, a second type of swing resulted from a fall in [HbO2] accompanied by a rise in [Hb] and was only induced by endothelial and sympathetic activity. Thus the same fall in SmbO2 can be induced by two distinct responses. A “type I” swing does not suggest an inadequacy in oxygen delivery whereas a “type II” swing may indicate a change in oxygen delivery from blood to tissue. SmbO2 alone cannot therefore be accepted as a definitive marker of tissue oxygenation.

Surgeons lack predictive accuracy for anastomotic leakage in gastrointestinal surgery

BACKGROUND

The dramatic clinical consequences of anastomotic leakage in gastrointestinal surgery can be reduced by a diverting stoma or drainage of the peri-anastomotic area. Currently, the surgeons' clinical judgement is of major importance in decision making, but reliable data of the diagnostic accuracy are lacking. In this prospective clinical study, the surgeons' predictive accuracy for anastomotic leakage was evaluated.

MATERIALS AND METHODS

In 191 patients undergoing colorectal resection with anastomosis, the risk for anastomotic leakage was determined by the surgeon on the basis of a visual analogue scale (VAS). This risk assessment was compared to the actual occurrence of anastomotic leakage post-operatively.

RESULTS

A total of 26 (13.6%) patients showed anastomotic leakage. The surgeons' median predicted leakage rate was 7.1% in anastomoses >15 cm from the anal verge and 9.5% <or=15 cm (sensitivity 38/62%, specificity 46/52%). Diagnostic accuracy was not influenced by the surgeons' training level (VAS score, surgeons 7.8% vs assistant surgeons 8.5%, p = 0.96, sensitivity 41% vs 44%, specificity 59% vs 48%, p = 0.20).

CONCLUSION

The surgeons' clinical risk assessment appeared to have a low predictive value for anastomotic leakage in gastrointestinal surgery. The low a priori risk of anastomotic leakage of 14% resulted in a low post-test odds (11%) of correct prediction of anastomotic leakage. This warrants the ongoing search for a better diagnostic test of anastomotic leakage to prevent morbidity and mortality.

Real-time probe measurement of tissue oxygenation during gastrointestinal stapling: mucosal ischemia occurs and is not influenced by staple height

BACKGROUND

Tension, ischemia, and technical error are factors leading to anastomotic complications such as leak, stricture, and ulceration with bleeding. Currently, surgeons evaluate tissue ischemia without any simple routine measurement technique. A new tissue surface probe, T-Stat 303, provides continuous measurement of tissue hemoglobin oxygen saturation (StO(2)) and may have clinical utility for intraoperative assessment of blood flow in areas of surgical anastomosis. This pilot study aimed to determine local StO(2) during gut stapling using various staple sizes for the purpose of assessing the tool's ability to measure changes and the reproducibility of those changes with stapling.

METHODS

Measurements were made in nine anesthetized adult swine during laparotomy. Various staple heights were used to transect small bowel and colon. Serosal and mucosal surface measurements were obtained at baseline and on each side of the transection using the T-Stat device adjacent to the staple line and 2 cm away from it.

RESULTS

Both small bowel and colon mucosal StO(2) adjacent to the staple line showed significant ischemia compared with baseline (p < 0.001) and 2 cm away from the staple line (p < 0.001) using all staple heights. The serosa of both small bowel and colon adjacent to the staple line was not significantly different from baseline serosa (p > 0.11) except when the grey stapler was used (baseline, 58 +/- 6.6 vs. staple line, 51 +/- 15.1; p = 0.022). The baseline mucosa of the small bowel and colon did not differ from mucosa 2 cm away from the staple line (p > 0.08). The small bowel serosa 2 cm away from the staple line did not differ from baseline mucosa, whereas the colon serosa 2 cm away significantly increased after stapling compared with baseline mucosa (p < 0.012). No statistically significant StO(2) difference was found between the various staple load sizes.

CONCLUSION

Mucosal ischemia occurs after gastrointestinal stapling and is not affected by various staple heights. The T-Stat probe provides a real-time method for assessment of gut ischemia by surgeons during surgical procedures.

Preoperative irradiation with 5 x 5 Gy in a murine isolated colon loop model does not cause anastomotic weakening after colon resection

INTRODUCTION

There are conflicting studies on the influence of fractionated preoperative 5 days of 5 Gy irradiation on tissue oxygenation and subsequent colonic anastomotic strength. To elucidate the effect of preoperative irradiation on anastomotic strength, an isolated colon loop model was developed.

METHODS

Male Wistar rats (n = 164) were randomly divided into three groups. One group remained untreated (control). In the other two groups, a loop of descending colon was exteriorized to create a hernia of the abdominal wall. After 4 weeks' recovery, this loop was locally irradiated with 5 x 5 Gy of gamma-rays or sham irradiated. One week after (sham-) irradiation, an anastomosis was performed in all groups. Tissue oxygenation (StO2) was determined with visible light spectroscopy. The animals were sacrificed 3 or 7 days after the operation and the anastomosis was tested for bursting pressure and breaking strength.

RESULTS

Irradiated rats showed significantly more weight loss (90% SD 4.3 of initial body weight vs. 96% SD 2.8, p < or = 0.05) and enteritis (18% vs. 5%, p = 0.013) compared to sham and control animals. StO2 was not influenced by irradiation and was not predictive for anastomotic strength. The control group showed significantly lower bursting pressure and breaking strength compared to (sham-) irradiated animals.

CONCLUSION

We developed a new isolated loop model for intermittent irradiation of the colon. Preoperative irradiation of the distal part of a colon anastomosis was successfully administered with acceptable side effects and did not cause reduced tissue oxygenation nor clinical signs of anastomotic weakening, nor objective reduction in bursting pressure and breaking strength.

Endoscopic reflectance spectrophotometry and visible light spectroscopy in clinical gastrointestinal studies

The use of reflectance spectrophotometry (RS) for mucosal hemodynamic measurement relies on the recognition of changes in indexes of mucosal hemoglobin concentration and oxygen saturation. Endoscopic application in clinical studies has confirmed important observations demonstrated in animal experiments. The vasoconstriction induced by propranolol, vasopressin, glypressin, or somatostatin in the portal hypertensive gastric mucosa and the reduction of gastroduodenal mucosal perfusion by nonsteroidal anti-inflammatory drugs (NSAIDs) or smoking, mesenteric venoconstriction associated with systemic hypoxia, and acid-induced duodenal hyperemia are important examples. Prognostic predictions include the development of stress-induced gastric ulcerations in patients with significant reductions in gastric perfusion after thermal or head injury, or the demonstration of delayed gastric or duodenal ulcer healing when the hyperemia at the ulcer margin fails to materialize. In mechanical-ventilator-dependent patients with sepsis, a significantly reduced gastric mucosal RS measurement portends a grave prognosis (mortality >80%). Recent advances in technology resulted in the construction and validation of instruments for visible light spectroscopy. Measurements focused on tissue oxygen saturation demonstrated epinephrine and vessel-ligation-induced vasoconstriction, the absence of ischemia in radiation-induced rectal telangiectasias, and gut ischemia responsive to revascularization treatment. Endoscopic RS and visible light spectroscopy are suitable for assessing the role of blood flow in conditions with a lesser degree of ischemia and for testing the hypothesis that functional dyspepsia and dysmotility syndromes may be due to gut ischemia.

Gastrointestinal perfusion in septic shock

Septic shock is characterised by vasodilation, myocardial depression and impaired microcirculatory blood flow, resulting in redistribution of regional blood flow. Animal and human studies have shown that gastrointestinal mucosal blood flow is impaired in septic shock. This is consistent with abnormalities found in many other microcirculatory vascular beds. Gastrointestinal mucosal microcirculatory perfusion deficits have been associated with gut injury and a decrease in gut barrier function, possibly causing augmentation of systemic inflammation and distant organ dysfunction. A range of techniques have been developed and used to quantify these gastrointestinal perfusion abnormalities. The following techniques have been used to study gastrointestinal perfusion in humans: tonometry, laser Doppler flowmetry, reflectance spectrophotometry, near-infrared spectroscopy, orthogonal polarisation spectral imaging, indocyanine green clearance, hepatic vein catheterisation and measurements of plasma D-lactate. Although these methods share the ability to predict outcome in septic shock patients, it is important to emphasise that the measurement results are not interchangeable. Different techniques measure different elements of gastrointestinal perfusion. Gastric tonometry is currently the most widely used technique because of its non-invasiveness and ease of use. Despite all the recent advances, the usefulness of gastrointestinal perfusion parameters in clinical decision-making is still limited. Treatment strategies specifically aimed at improving gastrointestinal perfuision have failed to actually correct mucosal perfusion abnormalities and hence not shown to improve important clinical endpoints. Current and future treatment strategies for septic shock should be tested for their effects on gastrointestinal perfusion; to further clarify its exact role in patient management, and to prevent therapies detrimental to gastrointestinal perfusion being implemented.

Microcirculatory function monitoring at the bedside--a view from the intensive care

Microcirculatory dysfunction plays a key role in the pathophysiology of various disease states and may consequently impact patient outcome. Until recently, the evaluation of the microcirculation using different measurement techniques has been mostly limited to animal and human research. With technical advances, microcirculatory monitoring nowadays becomes more and more available for application in clinical praxis. Unfortunately, measurements within the microcirculation are mostly limited to easily accessible surfaces, such as skin, muscle and tongue. Due to major differences in the physiologic regulation of microcirculatory blood flow and in metabolism between organs and even within different tissues in one organ, the clinical importance of regional microcirculatory measurements remains to be determined. In addition, technical methods available demonstrate large differences in the measured parameters and sampling volume, making interpretation of data even more difficult. Nonetheless, the monitoring of the microcirculation may, ahead of time, alert physicians that tissue oxygen supply becomes compromised and it may lead to a better understanding of basic pathophysiological aspects of disease. In the present review, we describe available non-invasive microcirculatory measurement techniques which can be applied clinically at the bedside. After a short discussion of physiologic and pathophysiologic basics related to microcirculatory monitoring, the measuring principles, applications, strengths and limitations of different monitoring systems are discussed.

Optical diagnostic technology based on light scattering spectroscopy for early cancer detection

This article reviews the application of optical diagnostic technology based on light scattering spectroscopy for minimally invasive detection of precancerous and early cancerous changes in a variety of organs. Optical spectroscopic techniques have shown promising results in the diagnosis of diseases at the cellular scale. They do not require tissue removal, can be performed in vivo and allow for real-time diagnosis. While fluorescence and Raman spectroscopy are most effective in revealing the molecular properties of tissue, the novel technique, light scattering spectroscopy, is capable of characterizing the structural properties of tissue at the cellular and subcellular scale.

Diagnosis of chronic mesenteric ischemia by visible light spectroscopy during endoscopy

BACKGROUND

Chronic mesenteric ischemia can be difficult to diagnose by means of currently available clinical techniques. We developed a novel endoscopic device for objective measurement of GI mucosal ischemia.

OBJECTIVE

Our purpose was to evaluate the performance of the device in patients with chronic mesenteric ischemia.

DESIGN

A fiberoptic catheter-based visible light spectroscopy oximeter (T-Stat 303 Microvascular Oximeter, Spectros, Portola Valley, Calif) was used to evaluate 30 healthy control subjects and 3 patients with chronic mesenteric ischemia before and after successful percutaneous stenting.

SETTING

Veterans Affairs Palo Alto Health Care System hospital.

RESULTS

Normal mucosal (capillary) hemoglobin oxygen saturation was 60% to 73% in the duodenum and jejunum. In the 3 patients with chronic mesenteric ischemia, ischemic areas in the duodenum or proximal jejunum were found with mucosal saturations of 16% to 30%. After successful angioplasty and stent placement of the celiac, superior mesenteric, or inferior mesenteric arteries, the mucosal saturation in these areas increased to 51% to 60%.

CONCLUSIONS

This preliminary study suggests that chronic mesenteric ischemia is detectable during endoscopy by use of visible light spectroscopy and that successful endovascular treatment results in near normalization of mucosal oxygen saturation.

Liver Tumor Gross Margin Identification and Ablation Monitoring during Liver Radiofrequency Treatment

PURPOSE

To determine whether tissue visible light spectroscopy (VLS) used during radiofrequency (RF) ablation of liver tumors could aid in detecting when tissue becomes adequately ablated, locate grossly ablated regions long after temperature and hydration measures would no longer be reliable, and differentiate tumor from normal hepatic tissue based on VLS spectral characteristics.

MATERIALS AND METHODS

Studies were performed on human liver in vivo and animal liver ex vivo. In three ex vivo cow livers, RF-induced lesions were created at 80 degrees C. A 28-gauge needle embedded with VLS optical fibers was inserted alongside an RF ablation array, and tissue spectral characteristics were recorded throughout ablation. In one anesthetized sheep in vivo, a VLS needle probe was passed through freshly ablated liver lesions, and ablated region spectral characteristics were recorded during probe transit. In two human subjects, a VLS needle probe was passed through liver tumors in patients undergoing hepatic tumor resection without ablation, and tumor spectral characteristics were recorded during probe transit.

RESULTS

In bovine studies, there was significant change in baseline absorbance (P < .0001) as a result of increased light scattering as liver was ablated. Liver exhibited native differential absorbance peaks at 550 nm that disappeared during ablation, suggesting that optical spectroscopy detects markers of tissue altered during ablation. In sheep, liver gross ablation margins were clearly defined with millimeter resolution during needle transit through the region, suggesting that VLS is sensitive to gross margins of ablation, even after the temperature has normalized. In humans, absorbance decreased as the needle passed from normal tissue into tumor and normalized after emerging from the tumor, suggesting that absence of native liver pigment may serve as a marker for the gross margins and presence of tumors of extrahepatic origin.

CONCLUSIONS

In human subjects, VLS during RF liver tumor ablation depicted gross hepatic tumor margins in real time; in animal subjects, VLS achieved monitoring of when and where RF ablation endpoints were achieved, even long after the tissue cooled. Real-time in vivo monitoring and treatment feedback may be possible with the use of real-time VLS sensors placed along side of, or embedded into, the RF probe, which can then be used as an adjunct to standard imaging during tumor localization and RF ablation treatment.

Optical detection of tumors in vivo by visible light tissue oximetry

Endoscopy is a standard procedure for identifying tumors in patients suspected of having gastrointestinal (G.I.) cancer. The early detection of G.I. neoplasms during endoscopy is currently made by a subjective visual inspection that relies to a high degree on the experience of the examiner. This process can be difficult and unreliable, as tumor lesions may be visually indistinguishable from benign inflammatory conditions and the surrounding mucosa. In this study, we evaluated the ability of local ischemia detection using visible light spectroscopy (VLS) to differentiate neoplastic from normal tissue based on capillary tissue oxygenation during endoscopy. Real-time data were collected (i) from human subjects (N = 34) monitored at various sites during endoscopy (enteric mucosa, malignant, and abnormal tissue such as polyps) and (ii) murine animal subjects with human tumor xenografts. Tissue oximetry in human subjects during endoscopy revealed a tissue oxygenation (StO2%, mean +/- SD) of 46 +/- 22% in tumors, which was significantly lower than for normal mucosal oxygenation (72 +/- 4%; P < or = 0.0001). No difference in tissue oxygenation was observed between normal and non-tumor abnormal tissues (P = N.S.). Similarly, VLS tissue oximetry for murine tumors revealed a mean local tumor oxygenation of 45% in LNCaP, 50% in M21, and 24% in SCCVII tumors, all significantly lower than normal muscle tissue (74%, P < 0.001). These results were further substantiated by positive controls, where a rapid real-time drop in tumor oxygenation was measured during local ischemia induced by clamping or epinephrine. We conclude that VLS tissue oximetry can distinguish neoplastic tissue from normal tissue with a high specificity (though a low sensitivity), potentially aiding the endoscopic detection of gastrointestinal tumors.