Visualized Quantitative Evaluation of Gastrointestinal Activity in Healthy Volunteers Using a Noninvasive Single-Channel Electroamplifier

Background . Electrogastrography and electroenterography are noninvasive methods for measuring gastric and intestinal electrical activities, respectively. Few studies have measured electroenterography in healthy humans; however, no studies have measured electrogastrography and electroenterography simultaneously. This study was performed to provide basic electrogastrography and electroenterography data for comparison with future studies in patients. Methods . Simultaneous preprandial and postprandial measurements of electrogastrography and electroenterography were taken for 30 min each in 50 healthy volunteers. Power spectrum analysis was performed to calculate dominant frequency, dominant power, and power ratio. Results . Gastric and small intestinal dominant frequencies were not signi ﬁ cantly di ﬀ erent between preprandial and postprandial periods. In preprandial and postprandial periods, normogastria was seen in 49 (98%) and 44 (88%) patients ( p = 0 063 ), bradygastria in 1 (2%) and 6 (12%) patients ( p = 0 063 ), and tachygastria in 0 (0%) patients, respectively. Dominant power was signi ﬁ cantly increased in the stomach (828 [460 – 3203] μ V 2 vs. 1526 [759 – 2958] μ V 2 , p = 0 016 ) and small intestine (49 [27 – 86] μ V 2 vs. 68 [37 – 130] μ V 2 , p < 0 001 ). The power ratio was 1.6 (0.9 – 2.5) in the stomach and 1.4 (1.0 – 2.5) in the small intestine. Body mass index showed a negative correlation with the stomach and small intestinal dominant power in preprandial and postprandial periods ( r s = − 0 566 , p < 0 001 ; r s = − 0 534 , p < 0 001 ; r s = − 0 459 , p < 0 001 ; and r s = − 0 529 , p < 0 001 , respectively). The Bristol Stool Form Scale correlated positively with the small intestinal power ratio ( r s = − 0 430 , p = 0 002 ). Conclusion . There was no change in frequency in the stomach or small intestine, but power signi ﬁ cantly increased in both the stomach and small intestine.


Introduction
The gastrointestinal tract is an important organ and is responsible for the digestion of food, absorption of nutrients, and excretion of waste products.When gastrointestinal motility is impaired anywhere from the esophagus to the anus, symptoms such as indigestion, nausea, vomiting, bloating, constipation, abdominal pain, and diarrhea may occur, potentially reducing the patient's quality of life.Gastrointestinal motility occurs as a result of contraction and relaxation of smooth muscles, which involves the interaction of many cell groups, including cells of the enteric nervous system, interstitial cells of Cajal (ICC), and platelet-derived growth factor receptor alpha-positive cells [1,2].
ICC form a network throughout the gastrointestinal tract from the esophagus to the colon and function as a pacemaker in gastrointestinal motility [3].The ICC generate spontaneous and rhythmic electrical activity called slow waves, which are transmitted to smooth muscles, resulting in their spontaneous contraction [4].ICC are reduced or degenerated in patients with gastrointestinal diseases such as gastroparesis, ulcerative colitis, Crohn's disease, slow transit constipation, and gastrointestinal motility disorders associated with diabetes mellitus [5][6][7][8][9].
The frequency of slow waves varies with the site of the gastrointestinal tract, with slow waves occurring at a basic rhythm of approximately 3 cycles per minute (cpm) in the ICC of the stomach, 9-12 cpm in the small intestine, and 2-6 cpm in the large intestine [10,11].Gastric slow waves can be dysrhythmic; bradygastria is defined as slow waves slower than 2 cpm and tachygastria as slow waves faster than 4 cpm.Small intestinal slow wave dysrhythmias have not been observed in humans.Gastrointestinal electrography is capable of measuring the slow wave and includes electrogastrography (EGG) and electroenterography (EEnG) [10,12,13].
EGG is a noninvasive technique that monitors gastric electrical activity using electrodes placed on the abdominal surface.Previous studies have shown a good correlation between skin surface EGG recordings and electrical signals recorded from gastric serosal leads [14,15].Studies using EGG have shown a higher rate of gastric dysrhythmias and lower ratios of preprandial and postprandial gastric contractility parameters in patients with gastrointestinal disorders such as gastroparesis, chronic unexplained nausea and vomiting, and functional dyspepsia compared to healthy volunteers [16][17][18].EEnG, similar to EGG, monitors electrical activity in the small intestine using electrodes placed on the abdominal surface, but few studies have been conducted in humans, and data from healthy volunteers are lacking [19].
This study was aimed at simultaneously assessing the electrical activity of the stomach and small intestine in healthy volunteers using EGG and EEnG and at providing basic data for comparison with future studies in patients.

Methods and Materials
2.1.Participants.This study was approved by the institutional review board of the University of Tsukuba (approval #R03-039).The study was registered in the University Hospital Medical Information Network Clinical Trials Registry on July 1, 2021 (UMIN 000044720).All participants provided written informed consent prior to the study, in accordance with the Declaration of Helsinki.
This experimental study recruited 50 healthy volunteers aged 20-65 years (25 men and 25 women).Exclusion criteria were gastrointestinal disorders, autonomic neuropathy, diabetes mellitus, and gastrointestinal peristalsis medication usage within 48 h.Data were collected on age, sex, height, weight, body mass index (BMI), gastrointestinal electrical activity, time since last meal, and stool properties.The participants assessed stool properties using the Bristol Scale Form Scale (BSFS) score [20,21].

Procedure.
To prepare for the study, participants fasted for at least 4 h and drank no water for at least 2 h according to the guide [10].Measurements were taken for 30 min in a preprandial state and 30 min in a postprandial state.Participants were instructed to lie in the supine position with minimal body movement.They were also instructed not to talk, sleep, or breathe deeply during all processes of measurement.
After the preprandial measurement, each participant consumed a meal (rice balls) of 250-400 kcal in approximately 10 min.Subsequently, the measurements were performed in the postprandial state, paying attention to the points mentioned above.
2.3.Electrode Placement.Ag/AgCl adhesive gel for electrocardiography was used as the disposable electrode (Vitrode F; Nihon Kohden, Japan).Four surface electrodes, a reference electrode, and a ground electrode were placed on the abdominal skin surface.The electrode placement sites for EGG and EEnG are shown in Figure 1.To measure the electrical signals in the stomach, an electrode was placed 8 cm above the navel along the line connecting the navel and sternum, with a second electrode placed 35 °to the upper left and 8 cm away from it [22].For the small intestine, based on the results of the preliminary study, two electrodes were placed 2.5 cm above the umbilicus, 5 cm apart, symmetrically at the umbilicus (Appendix I-V).
2.4.Recording Setting.We used a biological recording device (Polymate Pocket MP208; Miyuki Giken, Japan) to measure at a sampling rate of 500 Hz.Gastrointestinal electrical activity is 1-9 cpm in the stomach and 9-12 cpm in the small intestine [10,23].In addition to these fundamental frequencies, harmonic recordings are recommended [10].Hence, the frequency setting during data collection was in the range of 1 to 60 cpm (or 0.016 to 1 Hz).

EGG and EEnG Data Analysis. EGG and EEnG data
were analyzed using data-compatible low-frequency analysis software (Low Frequency Analysis Pro; NoruPro Light Systems, Japan).Power spectral analysis was performed for each EGG and EEnG segment using a fast Fourier transform.Power spectral analysis is the most common analytical method for quantifying slow wave variability in the gastrointestinal tract.This method can convert the time domain series data into the frequency domain and generate a spectrum.A running spectral analysis was performed, for which a fast Fourier transform was applied to consecutive 409.6 s signal stretches with approximately 80% of overlap.Dominant frequency (DF) and dominant power (DP) were used as indicators of gastrointestinal electrical activity.DF is the frequency that represents the peak power in the overall power spectrum.DP is the value corresponding to DF from the spectrum and is expressed in μV 2 .For EGG, gastric DF was defined as the frequency of the stomach showing peak power in the range of 1-9 cpm [10,24,25].The frequency 2 GastroHep ranges were classified into bradygastria (1-2 cpm), normogastria (2-4 cpm), and tachygastria (4-9 cpm).For EEnG, small intestinal DF was defined as the frequency of the small intestine showing peak power in the range of 9-12 cpm, according to the typical intestinal frequency in different small intestinal segments [10].DF and DP for each segment were produced from a 30 min running power spectral analysis where preprandial and postprandial values were averaged.Power ratio (PR) is the ratio of power before and after the intervention.A ratio > 1 0 reflects an increase in gastric contractility due to the intervention, and a ratio < 1 0 reflects a decrease in gastric contractility [10,23].
Gastric and small intestinal DF and DP and classification of gastric DF were compared in both preprandial and postprandial periods.Additionally, we confirmed the correlation between the participants' characteristics and each parameter to identify factors that influence gastrointestinal electrical activity.
2.6.Statistics.All continuous data were tested for normality, but normality was not found.Therefore, descriptive data are expressed as numbers and percentages and continuous data as median and interquartile ranges (IQR).For continuous variables, the Wilcoxon signed-rank test was used.Correlations were calculated using Spearman's rank correlation coefficient.Differences with p values of < 0.05 were considered statistically significant.All analyses were conducted with IBM SPSS Statistics 27 (IBM Corp., Armonk, NY, USA).

Discussion
In this study, we compared the electrical activity of the stomach and small intestine between preprandial and postprandial periods in healthy volunteers.Both gastric and small intestinal DF did not change between preprandial and postprandial periods in healthy volunteers.However, there was a significant increase in both gastric and small intestinal DP between preprandial and postprandial periods.In addition, time since last meal and premeal DF, BMI and DP, and BSFS and PR were correlated.
The normal range of gastric DF in healthy individuals is reported to be 2-4 cpm, and the results of this study are generally consistent with this [10,24,25].However, 2% of the participants had bradygastria despite being healthy, with bradygastria increasing to 12% postprandially.Gastric DF was reduced by more than 30% for some of the participants.Gastric mechanoreceptors may have been activated by distension of the stomach due to the meal, triggering dysrhythmias via non-5-hydroxytryptamine-3, non-prostaglandin-dependent, and noncholinergic pathways [25,26].Gastric DP increased significantly between preprandial and postprandial periods, similar to previous studies [27,28].The reason may be that in addition to the accelerated contraction of the stomach by the meal, the distance between the stomach and the electrode was shortened by the distension of the stomach due to the meal [26,29].
Although there are few studies measuring noninvasive EEnG, they have been performed in healthy individuals and following total gastrectomy [12,19].Since no studies have measured the preprandial and postprandial EEnG in healthy individuals, only premeal parameters can be compared.The results for DF of healthy individuals were consistent with those of the present study [19].In patients who underwent total gastrectomy, there was no significant change between preprandial and postprandial DF, while DP was significantly increased after meals, similar to the results of the present study [12].These previous studies are similar to this study with regard to electrode placements.The median PR was lower in this study, which may have been influenced by differences in diet, but the values were within the range reported in these studies [10,23,29].
Interestingly, time since last meal and preprandial DF showed a negative correlation, but since 98% of preprandial DF was in the normal range, it is impossible to show any implications.It may be that the incidence of bradygastria may be higher in patients with gastrointestinal problems or in patients who have been fasting for a long period of time.BMI correlates negatively with preprandial and postprandial gastric and small intestinal DP, consistent with the results of previous studies [28].This may be due to the distance between the stomach and the electrodes being increased by the abdominal wall.However, DP is susceptible to other factors besides BMI, such as skin conductance and differences in stomach shape, and should not be treated in isolation.Therefore, PR showing changes in DP due to intervention should be considered together [23].The BSFS and PR of the small intestine showed a weak positive correlation.This may suggest that excessive intestinal peristalsis causes soft stools to be excreted without fully absorbing water.However, since it is mainly the large intestine that determines stool properties, caution should be exercised in interpreting the results because there may be a relationship between the electrical activity of the large intestine and fecal characteristics.Hence, further study of the electrogram of the large intestine is also needed.
In clinical practice, it may be possible to assess a patient's gastrointestinal motility function using EGG and EEnG.In patients whose defecation must be controlled, healthcare professionals check the status of defecation and administer gastrointestinal peristalsis stimulants.Real-time visualization of 5 GastroHep gastric and small intestinal movements using EGG and EEnG may allow consideration of nutrition and drug administration.The noninvasive measurement method may be particularly well suited for use in critically ill patients.However, there has been only one study on the use of an electrogastrogram in critically ill patients to date, and further research is warranted [30].
This study has some limitations.First, undiagnosed gastrointestinal or autonomic disorders could have affected the parameters.Regular check-ups are essential to diminish this factor.Second, since we were not able to control for emotion in this study, the parameters may have been affected if there was some emotionally stimulating event prior to the experiment [24,31].To address this, it may be effective to introduce an intervention such as a video that would override the emotion.However, caution should be exercised in implementing such protocols, as they involve prolonged restraint time.Third, the researchers were not blinded due to a lack of resources.The data analysts were aware that the sample comprised healthy individuals and understood the normal range of electrical activity in the gastrointestinal tract.This may have introduced analytical bias in the processing of the data.To solve such a problem, it is necessary to divide the work among the measurer, data manager, and analyst.However, no previous study has ever simultaneously measured preprandial and postprandial EGG and EEnG in as many as 50 healthy volunteers.EEnG can be measured as easily enough as EGG.This study may provide useful control data for future comparisons with patients with gastrointestinal dysmotility.
In conclusion, preprandial and postprandial EGG and EEnG measurements were performed in healthy volunteers.There was no change in the electrical activity frequency in the stomach or small intestine, but power significantly increased in both the stomach and the small intestine.Time since last meal was negatively correlated with preprandial DF, BMI was negatively correlated with DP, and stool properties were positively correlated with PR.EEnG can be measured easily and simultaneously with EGG.

Figure 2
shows a representable example of a running power spectrum of EGG.The horizontal axis represents frequency, the vertical axis represents power, and the depth axis represents the passage of time.It can be seen that DP increases significantly after meal ingestion.Gastric DF was not significantly different between preprandial and postprandial periods (3.0 [2.8-3.1]cpm vs. 3.0 [2.8-3.2]cpm, p = 0 466).Small intestinal DF was also not significantly different between preprandial and postprandial periods (10.4 [10.1-10.7]cpm vs. 10.2 [9.8-10.6]cpm, p = 0 065) (Figure Characteristics and Parameters of EGG and EEnG.Correlations between participant characteristics and parameters of EGG and EEnG are shown in

FrequencyFigure 2 :
Figure 2: Example of running power spectrum of EGG in a healthy individual.The horizontal axis is frequency, vertical axis is power, and depth axis is time.DP increases following meal intake.EGG: electrogastrography; DP: dominant power.

Figure 3 :Figure 4 :
Figure 3: Preprandial and postprandial changes in DF and DP in the stomach and small intestine.Shown is the change in (a) DF and (b) DP in the stomach and small intestine in preprandial and postprandial periods.DF did not change, but DP increased significantly in both the stomach and small intestine.DF: dominant frequency; DP: dominant power.