Nohra E. Beltran

Gastric impedance spectroscopy in elective cardiovascular surgery patients

Impedance spectroscopy has been proposed as a method of monitoring mucosal injury due to hypoperfusion and ischemia in critically ill patients. The present study characterizes human gastric impedance spectral changes under gastric hypoperfusion in patients undergoing cardiovascular surgery, and evaluates spectral differences between patients with no evidence of gastric ischemia and complications, and patients who developed ischemia and complications. Cole and Kun parameters were calculated over time to characterize changes as tissue injury progresses. Gastric ischemia was determined by air tonometry. Impedance spectroscopy spectra were obtained from 63 cardiovascular surgery patients. The recorded spectra were classified into three groups: group 1 for patients without ischemia or complications, group 2 for patients with a short period of ischemia (less than 2 h) and group 3 for patients with more than 4 h of gastric ischemia and complications. Two distinct dispersion regions of the spectra centered at about 316 Hz and 215 kHz become clearer as tissue damage develops. The average spectrum in group 3 shows a significant difference in tissue impedance at all frequencies relative to group 1. The parameters obtained for human gastric tissue show significant changes that occur at different times and at different frequencies as ischemia progresses, and could be correlated with patient outcome. This confirms our hypothesis that hypoperfusion and ischemia cause evident changes in the impedance spectra of the gastric wall. Therefore, this technology may be a useful prognostic and diagnostic monitoring tool.

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 This paper reviews previously published studies as well as unpublished experimental results, and puts the whole in context and perspective to help understand this technology.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.

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.

The predictive value of gastric reactance for postoperative morbidity and mortality in cardiac surgery patients

No useful method to directly monitor the level of end organ tissue injury is currently available clinically. Gastric reactance has been proposed to measure changes in a tissue structure caused by ischemia. The purpose of this study was to assess whether gastric reactance is a reliable, clinically relevant predictor of complications and a potentially useful tool to assess hypoperfusion in cardiovascular surgery patients. The value of gastric reactance measurements, standard hemodynamic and regional perfusion variables, and scores to predict postoperative complications were compared in 55 higher risk cardiovascular surgery patients with cardiopulmonary bypass. Low frequency gastric reactance, X(L), had a significant predictive value of postoperative persistent shock requiring more than 48 h of vasopressors and associated complications, before, during and after surgery (p < 0.05). Results suggest that reactance is an earlier predictor of patients at risk than all other variables tested. Patients with a high reactance (X(L) > 26) before surgery had a significantly higher incidence of complications, higher mortality and more days in the ICU than patients with a low reactance (X(L) < 13). X(L) was found to be a reliable and clinically relevant measurement. These results justify further clinical research to explore how this information may be used to improve patient management.

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.

Gastric Impedance Spectroscopy in Cardiovascular Surgery Patients vs. Healthy Volunteers

Impedance spectroscopy has been proposed as a method of monitoring mucosal injury due to hypoperfusion and ischemia in the critically ill. The present study evaluates spectral differences in elective cardiovascular surgery patients compared with the impedance spectra of the gastric mucosa in healthy adults. An impedance spectroscopy probe and nasogastric tube (ISP/NGT) was placed into the stomach of 77 heart surgery and 21 healthy volunteers. The recorded impedance spectra were classified into 3 groups: group 1 for healthy volunteers; group 2 for patients with neither ischemia nor complications; and group 3 for patients with ischemia and complications during the first 3 days in the recovery unit. There were statistical differences in resistance (p<0.001) and reactance (p<0.001) among the 3 groups. The results show that impedance spectra are distinct and significantly different in patients with probable ischemic mucosal injury compared to healthy subjects, and therefore this technology may be a useful prognostic and diagnostic monitoring tool

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.

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.