Factors influencing reflectance spectrophotometric measurements of gastrointestinal mucosal blood flow

Assessment of photobleaching during endoscopic autofluorescence imaging of the lower GI tract

BACKGROUND AND OBJECTIVES

In autofluorescence endoscopy, the difference in the fluorescence of intrinsic fluorophores is imaged to help visualize pre-malignant lesions, as in the system evaluated here. In this, blue light is used for excitation and the green autofluorescence is normalized by the red diffuse reflectance and presented using a false color scale. The present study was designed to quantify the degree of fluorescence photobleaching induced by the excitation light during use in the colon, since significant photobleaching could lead to false interpretation of the images, particularly false-positive lesions.

STUDY DESIGN

Measurements were made ex vivo and in vivo, both using the endoscopic imaging system and a separate fiberoptic spectroscopy probe in externalized rat jejunum and in patients undergoing routine colonoscopy, using exposures typical of autofluorescence endoscopic examination.

RESULTS

Photobleaching could be potentially caused at blue light exposure. However, at light intensities and exposure times that are typically used in clinical practice, the average photobleaching (% loss of peak fluorescence intensity) was <1% and <6% in the rat and human tissues, respectively. Nevertheless, the range was large: from -17% to +18% in rats and -33% to +43% in patients, where negative values denote an apparent increase in fluorescence. Both the large positive and negative deviations are believed in part to be due to a measurement artifact caused by uncontrollable tissue motility.

SUMMARY AND CONCLUSIONS

It is concluded that, using exposures typically encountered in clinical practice, there is minimal photobleaching during fluorescence endoscopy at exposure such as are used in the Onco-LIFE and comparable systems. The small changes in fluorescence intensity and spectral shift that do occur are not likely to be detectable by eye, and so should not impact significantly on the diagnostic accuracy of the technique.

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.

Reduced conscious blood flow in the stomach during non-steroidal anti-inflammatory drugs administration assessed by flash echo imaging

OBJECTIVE

Harmonic flash echo imaging, which is an intermittent second harmonic imaging technique, has recently been developed for the evaluation of blood flow. The present study was designed to investigate human gastric blood flow during administration of non-steroidal anti-inflammatory drugs (NSAIDs) by harmonic flash echo imaging.

MATERIAL AND METHODS

Eight healthy volunteers were enrolled in the study. After an overnight fast, the volunteers were requested to drink 400 ml water and remain in the sitting position. Seven milliliters (300 mg/ml) Levovist (SHU508A) was then injected intravenously at the rate of 1 ml/s, and intermittent harmonic scanning was carried out at 1-s intervals. A similar examination was carried out one hour after each subject took an oral tablet of diclofenac sodium (50 mg). To minimize the effect of variations in acoustic attenuation among patients, the ratio of the maximum amplitude in the gastric wall to that in the portal vein was expressed as the gastric perfusion index.

RESULTS

Strong ultrasonographic contrast enhancement of the gastric wall and portal vein was observed. The area under the curve was significantly reduced in the images obtained after ingestion of the diclofenac sodium tablet. The gastric perfusion index was significantly reduced from 0.617+/-0.114 to 0.480+/-0.127 in the antrum and from 0.659+/-0.103 to 0.509+/-0.107 in the lower gastric corpus after ingestion of the diclofenac sodium tablet (p<0.05).

CONCLUSIONS

A reduction in the human conscious gastric transmural blood flow following ingestion of an NSAID is revealed by harmonic flash echo imaging.

Effect of acid on duodenal blood flow and mucus secretion measured by reflectance spectrophotometry: a prospective, randomized-controlled study

BACKGROUND

In animals, hydrochloric acid increases blood flow and mucus secretion in the duodenal mucosa. A significant correlation between index of haemoglobin oxygen saturation and mucosal blood flow, and between change in index of haemoglobin concentration and mucus thickness, respectively, has been demonstrated by reflectance spectrophotometry.

AIM

To examine the effect of topical hydrochloric acid upon mucosal blood flow and mucus secretion in the human duodenum.

METHODS

This prospective study of 120 patients undergoing routine upper endoscopy, examined the effect of topical 0.1 n hydrochloric acid or 0.9% saline on the duodenal bulb in a randomized, double-blind fashion. Duodenal mucosal index of haemoglobin oxygen saturation and index of haemoglobin concentration were measured by endoscopic reflectance spectrophotometry before and after hydrochloric acid or saline.

RESULTS

Baseline index of haemoglobin oxygen saturation, calculated blood flow and index of haemoglobin concentration measurements were comparable between hydrochloric acid (n = 60) and saline (n = 60) treated groups. A history of current use of non-steroidal anti-inflammatory drug was associated with a significantly lower baseline index of haemoglobin oxygen saturation and calculated blood flow. Hydrochloric acid resulted in a significant increase in index of haemoglobin oxygen saturation and calculated blood flow, but a decrease in index of haemoglobin concentration, reflecting an increase in mucus thickness compared with saline.

CONCLUSIONS

Our observations in humans confirm data in animal studies that topical exposure to hydrochloric acid induces an increase in duodenal mucosal blood flow and mucus secretion. Post hoc analysis of the data also revealed that attenuation of basal duodenal mucosal blood flow is associated with a history of current non-steroidal anti-inflammatory drug use. Endoscopic reflectance spectrophotometry appears to be adequate to assess factors that influence duodenal defence mechanisms of blood flow and mucus secretion in humans.

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

Reflectance spectrophotometry (RS) is an optical technology that has been used for nearly three decades in the measurement of tissue hemoglobin oxygen saturation in the gastrointestinal tract. The technology has evolved substantially throughout this period,and commercial devices are now available for use in clinical trials. Numerous studies have used RS to investigate the importance of mucosal perfusion in disorders such as ulcer disease, portal hypertension, and septic shock. More recently, the technique has been applied to measure changes in perfusion in response to infusion of vasoactive medications and maneuvers such as cardiopulmonary bypass. The results of current trials investigating the application of RS in critical care monitoring and vascular interventions will likely determine whether the technique will evolve from predominantly a research tool to a clinically useful device.

Reflectance spectrophotometry in the gastrointestinal tract: limitations and new applications

OBJECTIVES

Reflectance spectrophotometry (RS) assesses blood flow changes by measuring an index of Hb concentration (IHB) and index of Hb oxygen saturation (ISO2). We tested the following hypotheses: 1) endoscopic RS measurements obtained by two observers and with the aid of fiber optic and video endoscopes are similar, and 2) the method is suitable for documenting mesenteric venoconstriction associated with systemic hypoxia and blood flow autoregulation associated with hemorrhagic hypotension.

METHODS

Study 1: two investigators obtained baseline gastric mucosal RS measurements in anesthetized rats (n = 3) before and after stepwise reduction of blood pressure induced by arterial hemorrhage. Study 2: subjects were examined by both fiber optic and video endoscopes. Endoscopic RS measurements were obtained at 20 cm from the anal verge. Study 3: video endoscope was used to obtain RS measurements in oxygen-dependent patients on and off oxygen treatment. Study 4: the procedures in study 1 were repeated in five additional rats by one of the investigators.

RESULTS

Study 1: there was good agreement between the measurements of IHB and ISO2 between the two investigators. Study 2: video endoscope-assisted measurements were consistently lower. Study 3: cessation of oxygen treatment produced a significant drop in oxygen saturation (pulse oximetry), decline in ISO2, and rise in IHB. Study 4: when blood pressure varied between 90% and 40% of baseline, gastric mucosal blood flow (IHB) was maintained at approximately 70% of baseline level.

CONCLUSIONS

We confirmed that reproducible measurement can be obtained by different investigators using standardized techniques. Standardization of endoscopic equipment is also necessary to overcome the significant limitation of endoscopic equipment on RS measurements. RS measurements can document mesenteric venoconstriction associated with systemic hypoxia and blood flow autoregulation associated with hemorrhagic hypotension.

Clinical evaluation of reflectance spectrophotometry for the measurement of gastric microvascular oxygen saturation in patients undergoing cardiopulmonary bypass

OBJECTIVE

To evaluate the impact of cardiopulmonary bypass (CPB) on gastric mucosal oxygen saturation assessed by reflectance spectrophotometry in patients undergoing coronary artery bypass graft surgery.

DESIGN

Prospective, observational study.

SETTING

A division of cardiothoracic anesthesia in a university hospital.

PARTICIPANTS

Twelve consecutive patients undergoing CPB.

INTERVENTIONS

Monitoring, anesthesia, surgical procedure, and CPB for the patients followed routine clinical protocol as established in the departments. Microvascular oxygen saturation in gastric mucosa was assessed by reflectance spectrophotometry before, during, and after CPB.

MEASUREMENTS AND MAIN RESULTS

Gastric mucosal oxygen saturation averaged 65 +/- 7% (mean +/- SD) before CPB, decreased significantly to 57 +/- 9% during CPB (p < 0.01), and was 59 +/- 8% after CPB. These changes in regional oxygen saturation were not mirrored in variables of systemic oxygenation. Gastric mucosal oxygen saturation always showed instantaneous reactions to various surgical and pharmacologic interventions.

CONCLUSION

Reflectance spectrophotometry allowed the authors to assess gastric mucosal oxygen saturation with a high repetition rate, regardless of spontaneous circulation with pulsatile flow or nonpulsatile flow during CPB. This technique provided the means to monitor on-line the course of tissue oxygen saturation throughout the operative procedure. Reflectance spectrophotometry is an appropriate and sensitive assessment tool to monitor gastric mucosal oxygen saturation in patients undergoing CPB.