Small intestine slow wave activity defined through in vivo high-resolution electrical mapping

Abstract

The small intestine is characterised by underlying rhythmic bioelectrical potentials that propagate along the organ in a coordinated fashion; these propagating biopotentials are termed slow waves, and they serve a vital role in the regulationand organisation of intestinal motility. Most previous studies of intestinal slow waves utilised low-resolution techniques, where few electrodes were sparsely placed along the intestine. The lack of spatial resolution allowed for analysis of temporal slow wave characteristics (e.g., frequency), but prevented spatial propagation analysis. More recently, high-resolution intestinal mapping has been introduced, allowing the addition of spatiotemporal resolution to our understanding of slow wave activity. However, the methods to date have been limited to at, rigid electrode arrays covering a relatively small area of the intestine. These methods lacked circumferential coverage, and were not applicable for human use, severely limiting the clinical applicability of intestinal slow wave mapping. This thesis aimed to: i) develop improved methods of high-resolution mapping and analysis of intestinal slow wave activity in vivo, ii) utilise those methods to de ne spatial slow wave characteristics, and iii) translate the developed methods to human studies. First, flexible, sterilisable printed-circuit-board (PCB) electrode arrays (256 electrodes, 4 mm spacing) were developed for recording intestinal slow wave activity in vivo. These PCBs were held in contact around the intestinal circumference with gauze-padded silicone cradles, and were validated in a porcine model. Digital filtering methods were compared and optimised, establishing a Gaussian moving median filter (20 s window) in combination with a Savitzky-Golay filter (polynomial-order 9; window size 1.7 s) as ideal filtering for small intestine slow wave recordings. An automated algorithm for marking slow wave activation times (ATs) was adapted and tuned for intestinal use, achieving a sensitivity of 0.87 0.04 and positive predictive value of 0.90 0.07. The quantitative data analysis and visualisation process was largely automated, including the calculation of slow wave frequency, velocity, and amplitude, and the generation of AT, velocity field, and amplitude maps. During the course of this thesis, the validity of extracellular gastrointestinal recording techniques was critically questioned with claims that these techniques misrepresented contractile artifacts as bioelectrical signals. In response, in vivo extracellular PCB recordings and video capture were performed in the porcine jejunum, before and after intra-arterial nifedipine administration to inhibit contractile movement, gastric extracellular recordings were simultaneously recorded using conventional serosal contact and suction electrodes, and biphasic and monophasic extracellular potentials were simulated in a biophysical model. Contractile movement was abolished by nifedipine, but extracellular slow waves persisted with no significant change in amplitude, downstroke rate, velocity, and downstroke width (p 0.10 for all). However, slow waves occurred at reduced frequency after administration of nifedipine (24% lower; p=0.03). Simultaneous suction and conventional serosal extracellular recordings were identical in phase (frequency and activation-recovery interval), but varied in morphology (monophasic vs biphasic; downstroke rate and amplitude: p<0.0001). Simulations demonstrated the field contribution of current flow to extracellular potential and quantified the effects of localised depolarisation due to suction pressure on extracellular potential morphology. In total, this study demonstrated the bioelectrical basis and validity of gastrointestinal extracellular recordings. The high-resolution electrical mapping methods developed in the first portion of this thesis were applied around the circumference of the porcine jejunum to examine the range of slow wave propagation patterns and wavefront initiation mechanisms occurring in the small intestine in vivo. Focal pacemakers were analysed, and two types of slow wave re-entry were discovered, functional and circumferential reentry. Both types of re-entry operated as pacesetting mechanisms with similar frequency (17.0 0.3 cpm circumferential vs 17.2 0.4 cpm functional; p=0.5), which was greater than that of focal pacemakers (12.7 0.8 cpm; p<0.001). Intestinal slow wave velocity was anisotropic, with faster circumferential propagation than longitudinal (12.9 0.7 mm s 1 vs 9.0 0.7 mm s 1; p<0.05), contributing to the initiation and maintenance of re-entry. The physical properties of re-entry were elucidated through mathematical analysis, and closely agreed with the experimental results, further validating the occurrence of intestinal re-entry and characterising the complex relationship between the size of the re-entrant circuit, refractory period, and anisotropic slow wave velocity. In the final component of this thesis, in vivo high-resolution electrical mapping was successfully translated to the healthy human jejunum, serving as the rst successful high-resolution recordings of human intestinal slow wave activity. Jejunal frequency and amplitude were analysed (9.9 0.4 cpm; 68 7 V), and closely accorded with previous data from low-resolution human recordings. Slow wave velocity was defined for the first time in the human intestine, and was found to be anisotropic (10.7 0.5 mm s 1 circumferential vs 9.6 0.5 mm s 1 longitudinal; p<0.001). Human intestinal slow wave propagation was mapped in spatial detail for the first time, and focal pacemakers were analysed, operating at a frequency of 10.1 0.02 cpm. No re-entry was observed, but based on the recorded velocity, re-entry is theoretically possible in the human intestine and should be further investigated in future studies. In summary, this work served as the first high-resolution study of human small intestine electrical activity, defined human intestinal slow wave velocity, provided the first spatial propagation analysis of human intestinal slow waves, and established a foundation for future clinical intestinal mapping. In total, the work detailed in this thesis provides a new avenue for investigating small intestine slow wave activity in high spatiotemporal resolution. The experimental results of this thesis serve to substantially improve our understanding of intestinal slow wave activity. The successful human translation also provides a foundation for the development of intestinal mapping as a clinical investigative tool. Future translation and continued development of the presented work will hopefully lead to the development of a diagnostic and treatment platform for the many intestinal motility disorders currently lacking suitable therapeutic options.