Strategies to Refine Gastric Stimulation and Pacing Protocols: Experimental and Modeling Approaches

Needle-Like Multifunctional Biphasic Microfiber for Minimally Invasive Implantable Bioelectronics

Implantable bioelectronics has attracted significant attention in electroceuticals and clinical medicine for precise diagnosis and efficient treatment of target diseases. However, conventional rigid implantable devices face challenges such as poor tissue-device interface and unavoidable tissue damage during surgical implantation. Despite continuous efforts to utilize various soft materials to address such issues, their practical applications remain limited. Here, a needle-like stretchable microfiber composed of a phase-convertible liquid metal (LM) core and a multifunctional nanocomposite shell for minimally invasive soft bioelectronics is reported. The sharp tapered microfiber can be stiffened by freezing akin to a conventional needle to penetrate soft tissue with minimal incision. Once implanted in vivo where the LM melts, unlike conventional stiff needles, it regains soft mechanical properties, which facilitate a seamless tissue-device interface. The nanocomposite incorporating with functional nanomaterials exhibits both low impedance and the ability to detect physiological pH, providing biosensing and stimulation capabilities. The fluidic LM embedded in the nanocomposite shell enables high stretchability and strain-insensitive electrical properties. This multifunctional biphasic microfiber conforms to the surfaces of the stomach, muscle, and heart, offering a promising approach for electrophysiological recording, pH sensing, electrical stimulation, and radiofrequency ablation in vivo.

Vagus nerve stimulation in bursts can efficiently modulate gastric contractions and contraction frequency at varying gastric pressures

OBJECTIVE

There has been recent clinical interest in the use of vagus nerve stimulation (VNS) for treating gastrointestinal disorders as an alternative to drugs or gastric electrical stimulation. However, effectiveness of burst stimulation has not been demonstrated. We investigated the ability of bursting and continuous VNS to influence gastric and pyloric activity under a range of stimulation parameters and gastric pressures. The goals of this study were to determine which parameters could optimally excite or inhibit gastric activity.

MATERIALS AND METHODS

Data were collected from 21 Sprague-Dawley rats. Under urethane anesthesia, a rubber balloon was implanted into the stomach, connected to a pressure transducer and a saline infusion pump. A pressure catheter was inserted at the pyloric sphincter and a bipolar nerve cuff was implanted onto the left cervical vagus nerve. The balloon was filled to 15 cmH2O. Stimulation trials were conducted in a consistent order; the protocol was then repeated at 25 and 35 cmH2O. The nerve was then transected and stimulation repeated to investigate directionality of effects.

RESULTS

Bursting stimulation at the bradycardia threshold caused significant increases in gastric contraction amplitude with entrainment to the bursting frequency. Some continuous stimulation trials could also cause increased contractions but without frequency changes. Few significant changes were observed at the pylorus, except for frequency entrainment. These effects could not be uniquely attributed to afferent or efferent activity.

SIGNIFICANCE

Our findings further elucidate the effects of different VNS parameters on the stomach and pylorus and provide a basis for future studies of bursting stimulation for gastric neuromodulation.

Optimization of pacing parameters to entrain slow wave activity in the pig jejunum

Pacing has been proposed as a therapy to restore function in motility disorders associated with electrical dysrhythmias. The spatial response of bioelectrical activity in the small intestine to pacing is poorly understood due to a lack of high-resolution investigations. This study systematically varied pacing parameters to determine the optimal settings for the spatial entrainment of slow wave activity in the jejunum. An electrode array was developed to allow simultaneous pacing and high-resolution mapping of the small intestine. Pacing parameters including pulse-width (50, 100 ms), pulse-amplitude (2, 4, 8 mA) and pacing electrode orientation (antegrade, retrograde, circumferential) were systematically varied and applied to the jejunum (n = 15 pigs). Pulse-amplitudes of 4 mA (p = 0.012) and 8 mA (p = 0.002) were more effective than 2 mA in achieving spatial entrainment while pulse-widths of 50 ms and 100 ms had comparable effects (p = 0.125). A pulse-width of 100 ms and a pulse-amplitude of 4 mA were determined to be most effective for slow wave entrainment when paced in the antegrade or circumferential direction with a success rate of greater than 75%. These settings can be applied in chronic studies to evaluate the long-term efficacy of pacing, a critical aspect in determining its therapeutic potential.

Optimization of Gastric Pacing Parameters Using High-Resolution Mapping

OBJECTIVE

Abnormal slow-wave activity has been associated with functional motility disorders. Gastric pacing has been investigated to correct slow-wave abnormalities, but clinical therapies are yet to be established. This study aimed to define optimal parameters to advance the application of gastric pacing.

METHODS

High-resolution gastric mapping was utilized to evaluate four pacing parameters in in-vivo pig studies: (i) orientation of the pacing electrodes (longitudinal vs circumferential), (ii) pacing energy (900 vs 10,000 ms mA2), (iii) the pacing location (corpus vs antrum), and (iv) pacing period (between 12 and 36 s).

RESULTS

The probability of slow-wave initiation and entrainment with the pacing electrodes oriented longitudinally was significantly higher than with electrodes orientated circumferentially (86 vs 10%). High-energy pacing accelerated entrainment over the entire mapped field compared to low-energy pacing (3.1±1.5 vs 7.3±2.4 impulses, p < 0.001). Regardless of the location of the pacing site, the new site of slow-wave initiation was always located 4-12 mm away from the pacing site, between the greater curvature and negative pacing electrode. A pacing period between 14-30 s resulted in stable slow-wave initiation and entrainment.

CONCLUSION

These data will now inform effective application of gastric pacing in future studies, including human translation.

Functional and anatomical gastric regions and their relations to motility control

The common occurrence of gastric disorders, the accelerating emphasis on the role of the gut-brain axis, and development of realistic, predictive models of gastric function, all place emphasis on increasing understanding of the stomach and its control. However, the ways that regions of the stomach have been described anatomically, physiologically, and histologically do not align well. Mammalian single compartment stomachs can be considered as having four anatomical regions fundus, corpus, antrum, and pyloric sphincter. Functional regions are the proximal stomach, primarily concerned with adjusting gastric volume, the distal stomach, primarily involved in churning and propelling the content, and the pyloric sphincter that regulates passage of chyme into the duodenum. The proximal stomach extends from the dome of the fundus to a circumferential band where propulsive waves commence (slow waves of the pacemaker region), and the distal stomach consists of the pacemaker region and the more distal regions that are traversed by waves of excitation, that travel as far as the pyloric sphincter. Thus, the proximal stomach includes the fundus and different extents of the corpus, whereas the distal stomach consists of the remainder of the corpus and the antrum. The distributions of aglandular regions and of specialized glands, such as oxyntic glands, differ vastly between species and, across species, have little or no relation to anatomical or functional regions. It is hoped that this review helps to clarify nomenclature that defines gastric regions that will provide an improved basis for drawing conclusions for different investigations of the stomach.

Evaluation of Pacing Parameters to Induce Contractions in the Small Intestine

Postoperative ileus and chronic intestinal pseudo-obstruction are intestinal motility disorders that can compromise bowel function resulting in a significant reduction in quality of life and prolonged hospital stays. While medication and nutritional support provides relief for some patients, a significant patient population remains untreated. Therefore, alternative treatment options are required. A novel framework that enables small intestine pacing and video mapping of the contractile response was developed. Pacing pulse parameters (pulse-period: 2.7, 10 s, pulse-width: 100, 400 ms, and pulse-amplitude: 4, 6, 8 mA) were systematically varied to investigate the effect of pacing on the small intestine contractility. The contractile response was quantified by computing the strain of the intestinal diameter at the pacing site. The framework was applied in vivo on porcine jejunal loops (n=4) where segmental contractions were induced in response to pacing pulses. Strain increased with increasing pulse-amplitude and pulse-width, while pacing at a period of 2.7 s elicited higher contractile strains compared to pacing at a period of 10 s at all settings (e.g., -0.18 ± 0.06 vs 0.12 ± 0.06 at 8 mA, 400 ms). For a pulse-width of 100 ms, the contractile strain continued to increase with increasing pulse-amplitude, while the induced strain was comparable for all pulse-amplitudes when paced with high pulse-width (400 ms). Therefore, pacing is an effective tool in modulating the intensity of segmental contractions.Clinical Relevance- Different pacing parameters can define contraction intensity and frequency in the small intestine. This is of therapeutic potential for treating motility disorders such as post-operative ileus and chronic intestinal pseudo-obstruction.

Computational models of autonomic regulation in gastric motility: Progress, challenges, and future directions

The stomach is extensively innervated by the vagus nerve and the enteric nervous system. The mechanisms through which this innervation affects gastric motility are being unraveled, motivating the first concerted steps towards the incorporation autonomic regulation into computational models of gastric motility. Computational modeling has been valuable in advancing clinical treatment of other organs, such as the heart. However, to date, computational models of gastric motility have made simplifying assumptions about the link between gastric electrophysiology and motility. Advances in experimental neuroscience mean that these assumptions can be reviewed, and detailed models of autonomic regulation can be incorporated into computational models. This review covers these advances, as well as a vision for the utility of computational models of gastric motility. Diseases of the nervous system, such as Parkinson’s disease, can originate from the brain-gut axis and result in pathological gastric motility. Computational models are a valuable tool for understanding the mechanisms of disease and how treatment may affect gastric motility. This review also covers recent advances in experimental neuroscience that are fundamental to the development of physiology-driven computational models. A vision for the future of computational modeling of gastric motility is proposed and modeling approaches employed for existing mathematical models of autonomic regulation of other gastrointestinal organs and other organ systems are discussed.

A novel framework for the removal of pacing artifacts from bio-electrical recordings

BACKGROUND

Electroceuticals provide clinical solutions for a range of disorders including Parkinson's disease, cardiac arrythmias and are emerging as a potential treatment option for gastrointestinal disorders. However, pre-clinical investigations are challenged by the large stimulation artifacts registered in bio-electrical recordings.

METHOD

A generalized framework capable of isolating and suppressing stimulation artifacts with minimal intervention was developed. Stimulation artifacts with different pulse-parameters in synthetic and experimental cardiac and gastrointestinal signals were detected using a Hampel filter and reconstructed using 3 methods: i) autoregression, ii) weighted mean, and iii) linear interpolation.

RESULTS

Synthetic stimulation artifacts with amplitudes of 2 mV and 4 mV and pulse-widths of 50 ms, 100 ms, and 200 ms were successfully isolated and the artifact window size remained uninfluenced by the pulse-amplitude, but was influenced by pulse-width (e.g., the autoregression method resulted in an identical Root Mean Square Error (RMSE) of 1.64 mV for artifacts with 200 ms pulse-width and both 2 mV and 4 mV amplitudes). The performance of autoregression (RMSE = 1.45 ± 0.16 mV) and linear interpolation (RMSE = 1.22 ± 0.14 mV) methods were comparable and better than weighted mean (RMSE = 5.54 ± 0.56 mV) for synthetic data. However, for experimental recordings, artifact removal by autoregression was superior to both linear interpolation and weighted mean approaches in gastric, small intestinal and cardiac recordings.

CONCLUSIONS

A novel signal processing framework enabled efficient analysis of bio-electrical recordings with stimulation artifacts. This will allow the bio-electrical events induced by stimulation protocols to be efficiently and systematically evaluated, resulting in improved stimulation therapies.

Systematic review of small intestine pacing parameters for modulation of gut function

BACKGROUND AND PURPOSE

The efficacy of conventional treatments for severe and chronic functional motility disorders remains limited. High-energy pacing is a promising alternative therapy for patients that fail conventional treatment. Pacing primarily regulates gut motility by modulating rhythmic bio-electrical events called slow waves. While the efficacy of this technique has been widely investigated on the stomach, its application in the small intestine is less developed. This systematic review was undertaken to summarize the status of small intestinal pacing and evaluate its efficacy in modulating bowel function through preclinical research studies.

METHODS

The literature was searched using Scopus, PubMed, Ovid, Cochrane, CINAHL, and Google Scholar. Studies investigating electrophysiological, motility, and/or nutrient absorption responses to pacing were included. A critical review of all included studies was conducted comparing study outcomes against experimental protocols.

RESULTS

The inclusion criteria were met by 34 publications. A range of pacing parameters including amplitude, pulse width, pacing direction, and its application to broad regional small intestinal segments were identified and assessed. Out of the 34 studies surveyed, 20/23 studies successfully achieved slow-wave entrainment, 9/11 studies enhanced nutrient absorption and 21/27 studies modulated motility with pacing.

CONCLUSION

Small intestine pacing shows therapeutic potential in treating disorders such as short bowel syndrome and obesity. This systematic review proposes standardized protocols to maximize research outcomes and thereby translate to human studies for clinical validation. The use of novel techniques such as high-resolution electrical, manometric, and optical mapping in future studies will enable a mechanistic understanding of pacing.

High-resolution in vivo monophasic gastric slow waves to quantify activation and recovery profiles

BACKGROUND

Gastric bio-electrical slow waves are, in part, responsible for coordinating motility. Spatial dynamics about the recovery phase of slow wave recordings have not been thoroughly investigated due to the lack of suitable experimental techniques.

METHODS

A high-resolution multi-channel suction electrode array was developed and applied in pigs to acquire monophasic gastric slow waves. Signal characteristics were verified against biphasic slow waves recorded by conventional surface contact electrode arrays. Monophasic slow wave events were categorized into two groups based on their morphological characteristics, after which their amplitudes, activation to recovery intervals, and gradients were quantified and compared. Coverage of activation and recovery maps for both electrode types were calculated and compared.

KEY RESULTS

Monophasic slow waves had a more pronounced recovery phase with a higher gradient than biphasic slow waves (0.5 ± 0.1 vs. 0.3 ± 0.1 mV·s^-1 ). Between the 2 groups of monophasic slow waves, there was a significant difference in amplitude (1.8 ± 0.5 vs. 1.1 ± 0.2 mV), activation time gradient (0.8 ± 0.2 vs. 0.3 ± 0.1 mV·s^-1 ), and recovery time gradient (0.5 ± 0.1 vs. 0.3 ± 0.1 mV·s^-1 ). For the suction and conventional contact electrode arrays, the recovery maps had reduced coverage compared to the activation maps (4 ± 6% and 43 ± 11%, respectively).

CONCLUSIONS AND INFERENCES

A novel high-resolution multi-channel suction electrode array was developed and applied in vivo to record monophasic gastric slow waves. Slow wave recovery phase analysis could be performed more efficiently on monophasic signals compared with biphasic signals, due to the more identifiable recovery phases.

Validation of noninvasive body-surface gastric mapping for detecting gastric slow-wave spatiotemporal features by simultaneous serosal mapping in porcine

Gastric disorders are increasingly prevalent, but reliable noninvasive tools to objectively assess gastric function are lacking. Body-surface gastric mapping (BSGM) is a noninvasive method for the detection of gastric electrophysiological features, which are correlated with symptoms in patients with gastroparesis and functional dyspepsia. Previous studies have validated the relationship between serosal and cutaneous recordings from limited number of channels. This study aimed to comprehensively evaluate the basis of BSGM from 64 cutaneous channels and reliably identify spatial biomarkers associated with slow-wave dysrhythmias. High-resolution electrode arrays were placed to simultaneously capture slow waves from the gastric serosa (32 × 6 electrodes at 4 mm spacing) and epigastrium (8 × 8 electrodes at 20 mm spacing) in 14 porcine subjects. BSGM signals were processed based on a combination of wavelet and phase information analyses. A total of 1,185 individual cycles of slow waves were assessed, out of which 897 (76%) were classified as normal antegrade waves, occurring in 10 (71%) subjects studied. BSGM accurately detected the underlying slow wave in terms of frequency (r = 0.99, P = 0.43) as well as the direction of propagation (P = 0.41, F-measure: 0.92). In addition, the cycle-by-cycle match between BSGM and transitions of gastric slow wave dysrhythmias was demonstrated. These results validate BSGM as a suitable method for noninvasively and accurately detecting gastric slow-wave spatiotemporal profiles from the body surface.NEW & NOTEWORTHY Gastric dysfunctions are associated with abnormalities in the gastric bioelectrical slow waves. Noninvasive detection of gastric slow waves from the body surface can be achieved through multichannel, high-resolution, body-surface gastric mapping (BSGM). BSGM matched the spatiotemporal characteristics of gastric slow waves recorded directly and simultaneously from the serosal surface of the stomach. Abnormal gastric slow waves, such as retrograde propagation, ectopic pacemaker, and colliding wavefronts can be detected by changes in the phase of BSGM.

A generalized framework for pacing artifact removal using a Hampel filter

Cardiac pacing is a clinical therapy widely used for treating irregular heart rhythms. Equivalent techniques for the treatment of gastric functional motility disorders hold great potential. Accurate analysis of pacing studies is often hindered by the stimulus artifacts which are superimposed on the recorded signals. This paper presents a semi-automated artifact detection method using a Hampel filter accompanied by 2 separate artifact reconstruction methods: (i) an auto-regressive model, and (ii) weighted mean to estimate the underlying signal. The developed framework was validated on synthetic and experimental signals containing large periodic pacing artifacts alongside evoked bioelectrical events. The performance of the proposed algorithms was quantified for gastric and cardiac pacing data collected in vivo. A lower mean RMS difference was achieved by the artifact segment reconstructed using the auto-regression ([Formula: see text]), method compared to the weighted mean ([Formula: see text]) method. Therefore, a more accurate artifact reconstruction was provided by the auto-regression approach. Clinical Relevance- The ability to efficiently and accurately isolate evoked bioelectrical events by eliminating large artifacts is a critical advancement for the analysis of paced recordings. The developed framework allows more efficient analysis of preclinical pacing data and thereby contributes to the advancement of pacing as a clinical therapy.

A Computational Model of Biophysical Properties of the Rat Stomach Informed by Comprehensive Analysis of Muscle Anatomy

An anatomically based 3D computational model of the rat stomach was developed using experimental muscle thickness measurements and muscle fiber orientations for the longitudinal muscle (LM) and circular muscle (CM) layers. First, 15 data points corresponding to the measurements were registered on the dorsal and ventral faces of the serosal surface of an averaged 3D rat stomach model. A thickness field representing the varying wall thickness was fitted to the surface and nodal points were projected outwards (for the LM layer) and inwards (for the CM layer) to create 2 new surfaces. In addition, a computational volume mesh was created and fiber orientation in each tetrahedral element was computed using a Laplace-Dirichlet rule-based algorithm and a simulation was performed to validate the model. The stomach model successfully represented the experimental measurements with a thickness in the range of 11.7-52.9 µm and 40.6-276.5 µm in the LM and CM layers, respectively, while the variation across the stomach was in agreement with the reported values. Similarly, the generated fiber orientations matched with the investigated fiber data and successfully resembled the observed properties such as the hairpin-like structure formed by the LM fibers in the fundus. Bioelectrical simulation using the developed model was successfully converged and reflected the properties of normal antegrade activity. In conclusion, a 3D computational model of the rat stomach was successfully developed and tested for in-silico studies. The model will be used in future studies to assess parameters in electrical therapies and to investigate the structure-function relationship in gastric motility. Clinical Relevance - Electrical stimulation is an emerging therapy for functional motility disorders. The 3D model of rat stomach developed in this study could provide accurate assessment of the efficacy of a vast range of stimulation parameters via in-silico studies and could aid in the adaptation of electrical therapies to clinical settings.

Gastric pacing response evaluated with simultaneous electrical and optical mapping

Gastric pacing is an attractive therapeutic approach for correcting abnormal bioelectrical activity. While high-resolution (HR) electrical mapping techniques have largely contributed to the current understanding of the effect of pacing on the electrophysiological function, these mapping techniques are restricted to surface contact electrodes and the signal quality can be corrupted by pacing artifacts. Optical mapping of voltage sensitive dyes is an alternative approach used in cardiac research, and the signal quality is not affected by pacing artifacts. In this study, we simultaneously applied HR optical and electrical mapping techniques to evaluate the bioelectrical slow wave response to gastric pacing. The studies were conducted in vivo on porcine stomachs ( n=3) where the gastric electrical activity was entrained using high-energy pacing. The pacing response was optically tracked using voltage-sensitive fluorescent dyes and electrically tracked using surface contact electrodes positioned on adjacent regions. Slow waves were captured optically and electrically and were concordant in time and direction of propagation with comparable mean velocities ([Formula: see text]) and periods ([Formula: see text]). Importantly, the optical signals were free from pacing artifacts otherwise induced in electrical recordings highlighting an advantage of optical mapping. Clinical Relevance- Entrainment mapping of gastric pacing using optical techniques is a major advance for improving the preclinical understanding of the therapy. The findings can thereby inform the efficacy of gastric pacing in treating functional motility disorders.

Clinical application and research progress of extracellular slow wave recording in the gastrointestinal tract

The physiological function of the gastrointestinal (GI) tract is based on the slow wave generated and transmitted by the interstitial cells of Cajal. Extracellular myoelectric recording techniques are often used to record the characteristics and propagation of slow wave and analyze the models of slow wave transmission under physiological and pathological conditions to further explore the mechanism of GI dysfunction. This article reviews the application and research progress of electromyography, bioelectromagnetic technology, and high-resolution mapping in animal and clinical experiments, summarizes the clinical application of GI electrical stimulation therapy, and reviews the electrophysiological research in the biliary system.

Pediatric gastrointestinal neuromodulation: A review

Neuromodulation, also known as bioelectric neuromodulation or neurostimulation, is the therapeutic use of electrical stimulation of nerves or brain centers. Neuromodulation has been trialed in an increasing range of human diseases as well as gastrointestinal disorders. The application of neuromodulation to treat pediatric motility and functional disorders is an exciting recent development. This review aims to briefly discuss the use of neuromodulation for the treatment of pediatric gastroparesis, constipation, and visceral hyperalgesia.

Organisation of the musculature of the rat stomach

The strengths, directions and coupling of the movements of the stomach depend on the organisation of its musculature. Although the rat has been used as a model species to study gastric function, there is no detailed, quantitative study of the arrangement of the gastric muscles in rat. Here we provide a descriptive and quantitative account, and compare it with human gastric anatomy. The rat stomach has three components of the muscularis externa, a longitudinal coat, a circular coat and an internal oblique (sling) muscle in the region of the gastro-oesophageal junction. These layers are similar to human. Unlike human, the rat stomach is also equipped with paired muscular oesophago-pyloric ligaments that lie external to the longitudinal muscle. There is a prominent muscularis mucosae throughout the stomach and strands of smooth muscle occur in the mucosa, between the glands of the corpus and antrum. The striated muscle of the oesophageal wall reaches to the stomach, unlike the human, in which the wall of the distal oesophagus is smooth muscle. Thus, the continuity of gastric and oesophageal smooth muscle bundles, that occurs in human, does not occur in rat. Circular muscle bundles extend around the circumference of the stomach, in the fundus forming a cap of parallel muscle bundles. This arrangement favours co-ordinated circumferential contractions. Small bands of muscle make connections between the circular muscle bundles. This is consistent with a slower conduction of excitation orthogonal to the circular muscle bundles, across the corpus towards the distal antrum. The oblique muscle merged and became continuous with the circular muscle close to the gastro-oesophageal junction at the base of the fundus, and in the corpus, lateral to the lesser curvature. Quantitation of muscle thickness revealed gradients of thickness of both the longitudinal and circular muscle. This anatomical study provides essential data for interpreting gastric movements.

Gastric neurons in the nucleus tractus solitarius are selective to the orientation of gastric electrical stimulation

Objective.Gastric electrical stimulation (GES) is a bioelectric intervention for gastroparesis, obesity, and other functional gastrointestinal disorders. In a potential mechanism of action, GES activates the nerve endings of vagal afferent neurons and induces the vago-vagal reflex through the nucleus tractus solitarius (NTS) in the brainstem. However, it is unclear where and how to stimulate in order to optimize the vagal afferent responses.Approach.To address this question with electrophysiology in rats, we applied mild electrical currents to two serosal targets on the distal forestomach with dense distributions of vagal intramuscular arrays (IMAs) that innervated the circular and longitudinal smooth muscle layers. During stimulation, we recorded single and multi-unit responses from gastric neurons in NTS and evaluated how the recorded responses depended on the stimulus orientation and amplitude.Main results.We found that NTS responses were highly selective to the stimulus orientation for a range of stimulus amplitudes. The strongest responses were observed when the applied current flowed in the same direction as the IMAs in parallel with the underlying smooth muscle fibers. Our results suggest that gastric neurons in NTS may encode the orientation-specific activity of gastric smooth muscles relayed by vagal afferent neurons.Significance.This finding suggests that the orientation of GES is critical to effective engagement of vagal afferents and should be considered in light of the structural phenotypes of vagal terminals in the stomach.