Gastroenterologic Research Laboratories, Veterans Affairs Medical Center, Iowa City, Iowa 52242
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ABSTRACT |
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How the movements of the intestinal walls relate to luminal pressures and outflow remains incompletely understood. We triggered the peristaltic reflex in the isolated ileum of the guinea pig and quantified wall movements through computerized measurements of diameter changes. Contractions developed as indentations close to the upstream end of the loop. The indentations deepened and expanded in length. The downstream shoulder of contractions started and stopped to propagate before the upstream shoulder. Shoulders differed in their length and gradient over most of the duration of the contraction, and this gives the contraction an axial asymmetry. Over the course of individual contractions, the length of the indented segment correlated well with the luminal pressure. Contractions in response to large volumes generated long indented segments and high luminal pressures. The onset and the end of pressure waves and of outflow did not necessarily coincide with the onset and end of visual parameters of contractions. These findings indicate that objective visual parameters might be useful to describe and to classify contractions.
peristaltic reflex; intestinal flow; intestinal obstruction
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INTRODUCTION |
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DISTENSION OF THE GUINEA pig ileum triggers a contraction, which propagates through the circular muscle layer (19). This stereotypical response, called the peristaltic reflex, has served for many studies on the organization of contractions in the intestine and their control by myenteric neurons (1, 4, 7, 10, 25). Many preparations to study the peristaltic reflex have been described; some recorded luminal pressure, whereas others recorded muscle tension or the outflow of luminal contents (4, 10).
Contractions can also be recognized by the movements of the intestinal wall they produce. The peristaltic reflex produces a characteristic apposition of the ileal walls, which starts at the proximal end of the cut segment and advances in the distal direction (2, 19). We are not aware of detailed descriptions of the visual appearance of these contractions or of systematic studies on the factors that affect the appearance.
In the present study, we used video imaging (16-18) to track wall movements in isolated preparations of guinea pig ileum. We recorded easily identified visual features of contractions as digital data. We then related individual visual parameters to the mechanical and fluid-mechanical parameters of selected contractions.
The specific aims of the study were to 1) identify parameters useful to quantify the visual appearance of contractions, 2) determine how individual visual parameters change over the time course of single contractions, 3) correlate individual visual parameters to luminal pressure and outflow, and 4) determine how visual parameters are affected by the load conditions under which the intestine contracts.
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MATERIALS AND METHODS |
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Preparations. The experimental preparation is similar to one we have published before (16-18). Briefly, 15 guinea pigs of either sex, weighing 600-800 g, were euthanized by inhalation of pure CO2. The intestine was exposed through a midline incision. A loop of ileum was taken from 10 to 20 cm proximal to the ileocecal junction, flushed, and cleaned. Cannulas, 3 mm in diameter, were introduced into the proximal and distal ends, and the loop was mounted in a bath containing 500 ml Krebs solution. The composition of Krebs solution was (in mmol/l) 118 NaCl, 4.8 KCl, 2.5 CaCl2, 1.2 MgSO4, 25 NaH2CO3, and 11 glucose. The solution was bubbled with 95% O2-5% CO2 and kept at 37°C. This protocol was approved by the Animal Resource Committee of the Veterans Affairs Medical Center in Iowa City.
The upstream cannula was connected by a three-way stopcock to a syringe, which delivered boluses of specific volume to the lumen (Fig. 1). A three-way stopcock on the downstream cannula was attached to an outflow stub 5 cm high, which emptied into collecting cups. The length of the loop between the upstream and the downstream cannula was adjusted to 10 cm. This length was chosen to allow contractions to move over a maximal distance without having to constantly change the position and angle of the video camera. A force transducer attached to the cup measured the outflow volume (Fig. 1). Residual contents were removed from segments by gentle suction with the syringe, and segments were allowed to recover for a minimum of 5 min between injections. Because injection itself produced flow and movement of segments, the contractions that were regularly triggered by injection were not deemed suitable for image analysis. Rather, the first contraction occurring after the end of the injection was recorded.
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Definition of visual parameters of contractions.
Using diameter measurements at each raster column we defined
contractions by the following parameters (Fig.
2A). The
length of the contraction is given by the distance between its
downstream lead point and its upstream end point; the indented segment
is that part of the contraction where the diameter between the
mesenteric and antimesenteric walls of the ileum is maximally reduced.
In this segment, the two opposing walls run parallel to each other and
parallel to the walls of the resting intestine. The indented segment we
described by its length and its diameter. The diameter of the receiving
segment minus that of the indented segment constitutes the diameter
differential the contraction causes. We called shoulders those sections
of the contraction that connect the indented segment to noncontracting
intestine upstream and downstream. The downstream (leading) shoulder
connects the indented segment with the receiving segment; the upstream
(trailing) shoulder connects the indented segment with the
postcontraction segment. Shoulders are defined by their length
(distance from the indented segment to lead or end points of the
contraction, respectively) and their gradient. (Gradient is given by
the diameter differential divided by length.)
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Programs to measure visual parameters. We stored contraction sequences on computer disks (JPEG format, sampling rate 4 images/s). To reconstruct the visual parameters of contractions, we used the diameters of all raster columns from the upstream to the downstream end of preparations. We identified the end point of the contraction by the site at which a negative slope is first produced by decreasing diameters. From there we identified the lead point as the site where the slope returns to zero. The edge coordinates are recalled to measure the wall diameters of each image in the sequence. We used an algorithm to determine the ends of the segment and to measure each wall diameter per pixel column along the horizontal length of the segment. We stored all these data in ASCII format on disk. From the digital dates we constructed virtual contractions.
Hardware and programs. Our image analysis system is based on a RasterOps VideoLive Card, a HP 9000 model 735 workstation, and a VCR. We wrote our software in the C programming language using the X Window system and Motif widgets. We extracted the edge of all images in each sequence. We segmented images to a single level threshold. In the resulting binary image the ileum appeared white and the background black. Through morphological outlining we removed all interior pixels of the segment except those that defined its contour. We then used an edge-tracking algorithm to store the contour of the image as a set of Cartesian coordinates to an ASCII file.
Comparisons between experimental conditions were made using Student's t-test. P < 5% was considered statistically significant. ![]() |
RESULTS |
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Phases of contractions: relationship of visual parameters to pressure and outflow profiles. Contractions develop close to the upstream end of the loop of ileum and propagate toward its downstream end. Contractions are eccentric around the circumference of the ileum and primarily indent the antimesenteric border (Fig. 2B).
We defined three sequential phases to characterize the waxing and waning of contractions: 1) the developing, 2) the expanding, and 3) the vanishing contraction (Fig. 3). During the developing phase, the antimesenteric wall moves toward the mesenteric wall; this phase ends when the diameter is maximally reduced and the indented segment is established (Fig. 3B). During the expanding phase of the contraction, the contraction moves along the length of the ileum and the length of the indented segment increases (Fig. 3C). The diameter of the indented segment remains the same during the expanding phase and widens when the contraction vanishes (Fig. 3B). The length of the contraction may still increase to drop off abruptly once the diameter returns to baseline (Fig. 3C). Pressures and outflow cease well before the visual end of the contraction.
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Visual parameters of contractions as function of time or location.
The appearance of contractions changes with time and with their
location along the segment of ileum. Figure
4 shows how a contraction forms a short
indentation close to the upstream end of the segment during the
upstroke of the pressure wave and a long indentation close to the
downstream end of the segment during the pressure peak.
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Contraction in response to varying loads. Increases in bolus volume lead to changes in the visual parameters of contractions, which parallel changes in luminal pressures. An example characteristic of the responses to injecting 0.8- and 1.6-ml boluses into the same closed preparations is shown in Fig. 4. Pressures are higher, the contracting segment is longer, and the distance the contraction propagates is less with the larger bolus. The increasing length of the contraction reflects an increase in the lengths of the downstream shoulder and of the indented segment (Fig. 5, Table 1).
Contractions in response to small bolus volumes against low resistance generate shallow, short, and symmetric indentations that move quickly along the preparation (Fig. 5A). Contractions in response to large bolus volumes are characterized by long and deep indentations with a long leading shoulder (Fig. 5B). In a closed preparation, propagation appears slow at first until it gets a sudden boost as the bolus escapes retrograde (Fig. 5C). Alternatively, in a closed segment, there may be massive bulging of the receiving segment (Fig. 5D). Also, in closed segments, the length of the indenting segment increases to a maximum at between 1.5 and 2.0 s and declines thereafter (see Table 1). This is unlike in open preparations where emptying mediated by the contraction may produce a lasting reduction of the diameter (Fig. 3, B and C, for example). ![]() |
DISCUSSION |
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In the present study, we describe intestinal contractions by visual criteria, which can be constantly monitored and analyzed as digital data. We then studied how visual parameters change over the course of individual contractions in the isolated guinea pig ileum and how they relate to simultaneous recordings of luminal pressure and outflow. We tested how the parameters are affected by the bolus volume injected into the intestinal loop and by whether the end of the loop is open or closed. Our data suggest that objective visual parameters can be used to characterize the phases of individual contractions and to compare different contractions.
The configuration of contractions described in our analysis may have implications for the biomechanical properties of the intestinal wall (9), the neurosensory mechanisms controlling the contraction responses (1, 4, 7, 10), and the mechanics of luminal flow (2, 11-14, 24). We found that contractions indent primarily the antimesenteric wall of the guinea pig ileum. Several recent studies on the neuronal controls of the peristaltic reflex used videotapes and experimental conditions similar to ours (20-23). However, Tsuji et al. (20) restricted visual analysis of contractions to their propagation in full-thickness intestine compared with that in intestinal tubes devoid of mucosa and submucosa. Waterman and co-workers (21, 23) restricted visual analysis to diameters at select points of the segment at specific points in time. Neither group commented on the eccentric nature of the indentation caused by contractions. That contractions of the small bowel often take an eccentric configuration has been shown before, including in the in situ duodenum of the human and the cat (3, 6, 8, 15-18, 23, 25). One possible explanation for the circumferential asymmetry of contractions is the eccentric thickness of ileal muscle coat recently reported by Cue et al. (5).
We also found that contractions were asymmetric along the axis of the ileum. The axial asymmetry reflected the different rates at which the indenting segment expanded and at which the contraction propagated along the intestine. Thus the lead point moved downstream before the end point; once the lead point stopped the end point would catch up. Circumferential and axial asymmetry are likely to have implications for luminal flow. The analytic work that has been done on the fluid-mechanical implications of intestinal contractions made the assumption that indentations of the intestinal wall are concentric and symmetric (9, 11-14, 24).
Our observations indicate that the configuration of contractions changes with volume load and outflow resistance. It is likely that factors such as the specific gravity, compressibility, and viscosity have similar or additional effects on the configuration of contractions. It remains possible that different experimental conditions would eliminate the axial or circumferential asymmetry observed here. For the sake of imaging we prevented segments from shortening. Longitudinal contraction is an important part of the peristaltic response (1, 4, 7, 19, 25), and shortening might well change the configuration of the contraction.
The changes that contractions imparted on the intestinal configuration went through three characteristic phases: 1) they developed as shallow indentations that deepened until the lumen was maximally reduced, 2) they expanded as the diameter reduction included an increasingly long segment of the intestine, and 3) they vanished by the incremental increase of the luminal diameter toward but not necessarily fully back to baseline. Narrowing of the lumen resulted in outflow and luminal pressure.
The timing between the visual and the mechanical changes was complex. Visual parameters identified contractions over longer time periods than did pressure or outflow parameters. For instance, indentations typically preceded the rise of luminal pressure, and some contractions aborted during this initial phase. Thus the beginning of contractile activity appears to be reflected more accurately in the indentation than in the luminal pressure, which is dampened at first by outflow or downstream accommodation.
Even greater divergence was observed for the cessation of contractile activity. Pressure dropped back to baseline apparently before the end of contractile activity, whereas visual changes persisted until after their end. Pressures were again affected by accommodation and outflow. The visual end of contractions depends largely on refilling of the postcontraction segment. As it advances, the contraction leaves the postcontraction segment behind it empty and collapsed. If the contraction clears the lumen permanently of contents, the indenting and postcontraction segment might increase in length and merge until they involve the entire length of the intestinal preparation. Thus the postcontraction segment assumes the resting diameter of the empty intestine. The situation is different if the segment continues to contain or to receive luminal contents. For instance, if contractions are prevented from emptying contents from the distal end of the segment, redistribution of volume occurs. In that situation, refilling of the postcontraction segment actually reduces the length of the indenting segment long before the end of the contraction.
Pressure changes parallel changes in the length of the indented segment. Contractions that produce high luminal pressures also produce long indented segments. This finding reflects the expected close association between lumen occlusion and the mechanical effects of contractions. It implies that a primary mechanism by which the intestine copes with increased loads is to recruit additional lengths of the intestine to contract. We have previously demonstrated contractions of increasing length and force with retrograde perfusion of the duodenum (16). To what extent correlations between force of contractions and length of indenting segment apply in other gut segments and across a variety of preload and afterload conditions remains to be studied.
The length and the gradients of the shoulders of the contraction were also influenced by the conditions in the receiving and the postcontraction segment. If a great amount of fluid accumulated in either segment, the slopes connecting the indented segment to them could become quite long or steep.
Isolated loops of guinea pig ileum have long been used to study various aspects of intestinal contractions (1, 4, 7, 10, 16-23, 25). One limitation of these preparations is their comparatively short length. Contractions are likely to differ if they and the luminal contents they drive can advance unimpeded over long distances or if they come to a rapid end at a mechanical barrier (24). As a compromise, we used here segments as long or longer than used in most studies of the peristaltic reflex. This ensured that our data can be compared with that literature (1, 4, 7, 10, 16-23, 25). In humans and in intact animals, non-lumen-occluding contractions are common and may represent the majority of contractions after meals (6, 8, 15). To what extent our observations can be extended to contractions that do not produce occlusion of the lumen as did apparently most contractions in our preparation remains to be studied. We do not suggest, in this regard, that the term indented segment be reserved only for contractions that occlude the lumen.
We conclude that parameters derived from computerized measurements of intestinal diameters can be used to define contractions in objective visual terms. Visual parameters can be used to describe the temporal phases of contractions and their geographic progression along the intestine or to compare contractions that occur under different conditions. Of select visual parameters, the length of the indented segment shows particular promise as a predictor of the mechanical effectiveness of contractions.
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ACKNOWLEDGEMENTS |
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John Raab, Dept. of Surgery, Univ. of Iowa, wrote the software and did data processing. Bob Hermann performed experiments and data collection. Drs. Siroos Shirazi, Dept. of Surgery, and Bruce P. Brown, Dept. of Radiology, provided critical support.
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FOOTNOTES |
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This work was supported by a Merit Review Grant from the Veterans Affairs Medical Center.
Address for reprint requests and other correspondence: K. S. Schulze, Digestive Disease Center, 4551 JCP, Univ. of Iowa, Iowa City, IA 52242 (E-mail: konrad-schulze{at}mail.int-med.uiowa.edu).
Received 10 July 1997; accepted in final form 1 March 1999.
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