High-resolution Electrophysiological Mapping of the Mid-gut in Rabbits

Abstract

Functional gastrointestinal (GI) disorders are common in New Zealand, affecting up to 25% of the population. Patients suffer a poor quality of life with symptoms of pain, nausea, bloating and vomiting. This problem is exacerbated due to a lack of reliable and specific diagnostic tests. Diagnostic difficulties are particularly prevalent for mid-gut disorders, such as Hirschsprung’s disease, intestinal pseudo-obstruction, irritable bowel syndrome and post-operative ileus, where the pathophysiological mechanisms remain largely unknown. There are also limited therapies available for these disorders, and the lack of remediation significantly burdens both the healthcare system and patients. The GI tract is coordinated by several interacting mechanisms, including bioelectric, myogenic, neuronal and hormonal activities, which cooperate to initiate and govern peristalsis. A synergistic electrophysiological system is created by the multi-layered muscularis of the GI tract, where the myogenic activity of the smooth muscle cells is initiated by the interstitial cells of Cajal. Here, a multicellular syncytium is formed, which allows bioelectrical events, such as slow waves (SWs) and spikes, to conduct throughout the GI tract. Recent applications of high-resolution (HR) mapping techniques on the stomach have provided the spatiotemporal detail of the bioelectrical events by accurately resolving, quantifying and classifying their pacemaking mechanisms, propagating dynamics and functional interactions. A major barrier in translating similar HR mapping techniques to the intestines is the complexity of capturing bioelectrical events from these deformable and irregular organs. In this thesis, a novel inflatable cuff was developed to conform to the irregular topological surfaces of the intestine, whilst providing uniform support for HR electrode arrays. To reduce invasive animal experiments used whilst refining techniques, the design was validated on in vivo porcine small intestine. Multiple inflation pressures were applied to the cuff to investigate the effects on the resultant SW signals with inflation pressure. SW frequency was uninfluenced by the inflation pressure of the cuff (P>0.05) and amplitude was reduced with inflation of the cuff (variance reduced from 48 μV to less than 12 μV, P<0.001). However, the variability of SW morphology was also reduced and SW detectability improved within the biopotential signal. In addition, the application of inflation pressure (above 1 mm Hg) significantly increased the number of channels that recorded viable SW events (from 57% to 74%). Utilising this novel in vivo HR mapping technique in a rabbit model, we aimed to improve the understanding of the electrical conduction system within the mid-gut. Multiple SW characteristics and morphologies was recorded along the different regions that comprise the mid-gut (distal ileum, sacculus rotundus, ampulla coli, distal caecum and proximal colon). This reflected the influence of the mechanical function and tissue structure of each region. To define the SW dynamics and morphology, a quantitative comparison of HR recordings was carried out for the distribution of SW frequency, amplitude and speed along the mid-gut. The frequency distribution along the mid-gut was relatively consistent (mean values between 12.1 to 13.7 cpm), whilst the amplitude and velocity reduced in from the sacculus rotundus (SR) to the ampulla coli (AC) (mean velocity: 16.3 to 8.4 mm/s and mean amplitude: 0.19 to 0.03 mV, p<0.001). Intermittent propagating spike bursts was detected in the AC. The spike bursts propagated circumferentially with a mean amplitude of 0.17 ± 0.04 mV and speed of 8.2 ± 0.5mm/s. The frequency of spikes bursts in the AC was consistent within all rabbits but varied between rabbits (range: 2.06 ± 0.37 cpm and 0.23 ± 0.02 cpm). The distal caecum also had intermittent uncoordinated spike-like biopotentials, which occurred in alternating muscles bundles separated by gyrus folds in the musculature. The electrical activity behaviour between the intestinal regions was also investigated, revealing coordination along the mid-gut. The ileum consistently entrained the sacculus rotundus (SR), but the activity did not propagate from the SR into the ileum. The SR and AC had some coordination through spike burst activity, which may play a role in emptying digesta from the ileum into the distal caecum. The ileocecal junction and the cecocolic junction were electrically quiescent and acted as isolators of the SW activity between the three regions of the mid-gut. The structure and function of the mid-gut are complex and under-researched, leaving much of the underlying electrophysiological mechanisms undefined. For the first time, an investigative HR mapping study was performed on the mid-gut. Novel inflatable cuffs was designed and used to support the HR arrays, and the technique improved the electrophysiological recordings of the mid-gut. This enabled the unique and integrated electrical activities within the distal ileum, sacculus rotundus, AC, distal caecum and proximal colon to be recorded. This thesis has provided a technological platform for HR mapping of the mid-gut; revealed new electrophysiological understanding with functional implications associated with each region of the mid-gut and the coordination between these regions. These methods now enable further electrophysiological research to be performed to enhance our understanding of the mechanisms that underlie normal and abnormal functioning of the mid-gut.