Measuring implanted patient response to tone pips.

BACKGROUND: An electrical potential not previously reported-electrical cochlear response (ECR)-observed only in implanted patients is described. Its amplitude and growth slope are a measurement of the stimulation achieved by a tone pip on the auditory nerve. The stimulation and recording system constructed for this purpose, the features of this potential obtained in a group of 43 children, and its possible clinical use are described. The ECR is obtained by averaging the EEG epochs acquired each time the cochlear implant (CI) processes a tone pip of known frequency and intensity when the patient is sleeping and using the CI in everyday mode. The ECR is sensitive to tone pip intensity level, microphone sensitivity, sound processor gain, dynamic range of electrical current, and responsiveness to electrical current of the auditory nerve portion involved with the electrode under test. It allows individual evaluation of intracochlear electrodes by choosing, one at the time, the central frequency of the electrode as the test tone pip frequency, so the ECR measurement due to a variable intensity tone pip allows to establish the suitability of the dynamic range of the electrode current. RESULTS: There is a difference in ECR measurements when patients are grouped based on their auditory behavior. The ECR slope and amplitude for the Sensitive group is 0.2 µV/dBHL and 10 µV at 50 dBHL compared with 0.04 µV/dBHL and 3 µV at 50dBHL for the Inconsistent group. The clinical cases show that adjusting the dynamic range of current based on the ECR improved the patient's auditory behavior. CONCLUSIONS: ECR can be recorded regardless of the artifact due to the electromyographic activity of the patient and the functioning of the CI. Its amplitude and growth slope versus the intensity of the stimulus differs between electrodes. The relationship between minimum ECR detection intensity level and auditory threshold suggests the possibility of estimating patient auditory thresholds this way. ECR does not depend on the subject's age, cooperation, or health status. It can be obtained at any time after implant surgery and the test procedure is the same regardless of device manufacturer.

Effect of Feeding and Suction on Gastric Impedance Spectroscopy Measurements.

A specific device and system has been developed and tested for clinical monitoring of gastric mucosal reactance in the critically ill as an early warning of splanchnic hypoperfusion associated with shock and sepsis. This device has been proven effective in clinical trials and is expected to become commercially available next year. The system uses a combination nasogastric tube and impedance spectroscopy probe as a single catheter. Because this device has a double function, the question is: Does enteral feeding or suction affect the gastric reactance measurements? This study was designed to evaluate the effect of feeding and suction on the measurement of gastric impedance spectroscopy in healthy volunteers. Impedance spectra were obtained from the gastric wall epithelia of 18 subjects. The spectra were measured for each of the following conditions: postinsertion of gastric probe, during active suction, postactive suction, and during enteral feeding (236 ml of nutritional supplement). Impedance spectra were reproducible in all volunteers under all conditions tested. There was a slight increase in impedance parameters after suction, and a decrease in impedance after feeding; however, these observed differences were insignificant compared to patient-to-patient variability, and truly negligible compared with previously observed changes associated with splanchnic ischemia in critically ill patients. Our results demonstrate that suction or feeding when using the impedance spectro-metry probe/nasogastric tube does not significantly interfere with gastric impedance spectrometer measurements.

Incidence of gastric mucosal injury as measured by reactance in critically ill patients.

Gastric reactance has been proposed as a measure of mucosal ischemic injury in the critically ill. The purpose of this study was to evaluate the incidence of gastric mucosal injury as measured by gastric reactance in different subgroups of critical patients. We studied 100 adult patients admitted to 7 different hospital intensive care units, requiring a nasogastric tube. Gastric impedance measurements were continuously obtained from each patient for 24 hours. Patients were managed based on conventional protocols by hospital staff, blinded to the changes in gastric impedance parameters. The low-frequency central reactance (X L) reflects tissue edema caused by prolonged ischemia. The previously reported threshold of X L ≥ 13 - jΩ was used to classify injured mucosa; 80% of all patients had mean X L above this threshold. No significant differences were found in the incidence of mucosal ischemia between medical versus surgical, hemodynamic versus respiratory or neurological patients. Significant lower urine output was found in patients with X L above threshold (P < .01); also, there was a significant effect of fluid balance in those patients (P < .05). More complicated patients had higher average reactance. This study shows that gastric ischemia as estimated by gastric reactance has a very high incidence in the critically ill, independently of the reason for admission. High reactance is related with higher morbidity in agreement with other reports using different methods of assessing splanchnic hypoperfusion in this patient population.

Histomorphometric analysis with a proposed tissue lesion index in ischemia-reperfusion induced gastric mucosa damage.

Damage to the gastrointestinal mucosa caused by ischemia - reperfusion is a significant clinical problem associated with various physiopathological conditions. Our group has conducted various studies in patients in critical conditions and in animal models to identify early damage to the gastric mucosa under ischemia using impedance spectroscopy. It is important to perform a quantitative histopathological analysis which can be linked to changes in impedance of the gastric mucosa under conditions of ischemia and I/R. AIM: To propose a tissue lesion index which considers pathological alterations inherent to the inflammatory process and cell damage which may be directly related to changes in impedance under conditions of ischemia and I/R. METHODS: The animals were randomly distributed into 4 groups: control, ischemia (30 min), and I/R (30 and 60 min). Qualitative histopathological analysis was performed; the vascular area, glandular lumen area, the number of damaged cells, and the depth of the erosion were also quantified to obtain a scale to propose a tissue lesion index (TLI). RESULTS: Under ischemic conditions, histopathological analysis showed edema and necrosis in epithelial cells, and vascular congestion. In I/R (30 and 60 min) conditions, areas of epithelial erosion were generated. Damage was classified based on the TLI. A TLI threshold of 3 showed a predictive value of tissue lesion. CONCLUSION: The proposed gastric lesion index allows us to objectively quantify and classify damage to the gastric mucosa produced by I/R.

Evaluation of HIF-1α and iNOS in ischemia/reperfusion gastric model: bioimpedance, histological and immunohistochemical analyses.

Gastrointestinal ischemia/reperfusion (I/R) generates pathological alterations that could lead to death. Early ischemic damage markers could be used to guide therapy and improve outcomes. AIM: To relate hypoxia-inducible factor 1α (HIF-1α) activation and inducible nitric oxide synthase (iNOS) expression to gastric impedance changes due to I/R damage. METHODS: Experimental animals were randomly distributed into 3 groups: control, ischemia (30 min) and I/R (60 min). Gastric ischemia was generated by celiac artery clamping for 30 min, and then blood flow was restored for 60 min. Impedance spectra and biopsies of the glandular portion were obtained for histological and immunohistochemical analyses. Immunodetection of both HIF-1α and iNOS was performed. RESULTS: Under ischemia and I/R conditions, there was an increase (p<0.05) in the impedance parameters. Histologically, under ischemic conditions, edema and necrosis were observed in epithelium and significant vascular congestion. In I/R condition, alterations of the glandular and luminal integrity were found, which generated areas of epithelial erosion. Immunohistochemical analysis of HIF-1α revealed an increase (p<0.01) in the number of immunoreactive cells in the ischemia (35.7±13.9) and I/R (119.9±18.8) conditions compared to the control (0.8±1.2). Immunodetection of iNOS showed an increase (p<0.01) in the number of cells expressing iNOS under the ischemia (5.4±2.9) and I/R conditions (27.4±11.3) was observed compared to the control (0.4±0.8). CONCLUSION: Early changes in impedance in response to I/R is related to histopathological changes, the nuclear stabilization and translocation of HIF-1α as well as expression of iNOS.

Gastrointestinal ischemia monitoring through impedance spectroscopy as a tool for the management of the critically ill.

Impedance spectroscopy (IS) has been proposed as a tool for monitoring mucosal tissue ischemia and damage in the gut of critically ill patients resulting from shock and hypoperfusion. A specific device and system have been developed and tested for this specific application over the past 12 years by our research group. This paper reviews previously published studies as well as unpublished experimental results, and puts the whole in context and perspective to help understand this technology. Results presented include summaries of gastric reactance measurement understanding, in vivo measurements in animal models, clinical significance of the measurement, and future perspectives of clinical use of this technology. All of the experimental work done to date has been designed to determine the evolving device prototypes' performance and limitations from an instrumentation point of view. Although there are still questions to be answered with regard to the IS measurement, we conclude that we have reached enough confidence in the measurement and the device's performance and safety to begin clinically oriented research to learn how this technology may be useful in the diagnosis and management of different populations of the critically ill.

Gastric tissue damage analysis generated by ischemia: bioimpedance, confocal endomicroscopy, and light microscopy.

The gastric mucosa ischemic tissular damage plays an important role in critical care patients' outcome, because it is the first damaged tissue by compensatory mechanism during shock. The aim of the study is to relate bioimpedance changes with tissular damage level generated by ischemia by means of confocal endomicroscopy and light microscopy. Bioimpedance of the gastric mucosa and confocal images were obtained from Wistar male rats during basal and ischemia conditions. They were anesthetized, and stain was applied (fluorescein and/or acriflavine). The impedance spectroscopy catheter was inserted and then confocal endomicroscopy probe. After basal measurements and biopsy, hepatic and gastric arteries clamping induced ischemia. Finally, pyloric antrum tissue was preserved in buffered formaldehyde (10%) for histology processing using light microscopy. Confocal images were equalized, binarized, and boundary defined, and infiltrations were quantified. Impedance and infiltrations increased with ischemia showing significant changes between basal and ischemia conditions (P < 0.01). Light microscopy analysis allows detection of general alterations in cellular and tissular integrity, confirming gastric reactance and confocal images quantification increments obtained during ischemia.

Algorithm for characteristic parameter estimation of gastric impedance spectra in humans.

Impedance spectroscopy has been proposed as a method of monitoring mucosal injury due to hypoperfusion and ischemia in the critically ill. The present paper presents an algorithm developed to calculate the characteristic electrical values that best describe human gastric impedance measurements and simplify the information obtained with this method. An impedance spectroscopy probe and nasogastric tube (ISP/NGT) was placed into the stomach of healthy volunteers, cardiovascular surgery and critically ill patients, and a database with 16199 spectra was obtained. The gastric spectrum forms two semi circles in the complex domain, divided into low frequency (F < 10 kHz) and high frequency (F > 10 kHz). A fitting algorithm was developed based on the Cole model, and central characteristic parameters were calculated. The parameters were validated using the normalized mean squared error and 0.66% of the spectra were discarded. From the experimental data obtained in humans, the greatest changes observed as the gastric mucosa becomes ischemic occur at low frequencies, which are specific and sensitive to tissue damage, and vary with the degree of hypoperfusion.