New developments in defecatory studies based on biomechatronics

Abstract

Introduction: Defecation is a complex process that is difficult to study and analyze directly. In anorectal disease conditions, the defecation process may be disturbed, resulting in symptoms including fecal incontinence and constipation. Current state-of-the-art technology measures various aspects of anorectal function but detailed analysis is impossible because they are stand-alone tests rather than an integrated multi-dimensional test.

Objectives: The need for physiologically-relevant and easy-to-use diagnostic tests for identifying underlying mechanisms is substantial. We aimed to advance the field with integrated technology for anorectal function assessment.

Methods: We developed a simulated stool named Fecobionics that integrates several tests to assess defecation pressures, dimensions, shape, orientation and bending during evacuation. A novelty is that pressures are measured in axial direction, i.e. in the direction of the trajectory. Using this novel tool, we present new analytical methods to calculate physiologically relevant parameters during expulsion in normal human subjects.

Results: Data are reported from 28 human subjects with progressively more advanced versions of Fecobionics. A new concept utilizes the rear-front pressure (preload-afterload) diagram for computation of novel defecation indices. Fecobionics obtained physiological data that cannot be obtained with current state-of-the-art technologies.

Conclusion: Fecobionics measures well known parameters such as expulsion time and pressures as well as new metrics including defecation indices. The study suggests that Fecobionics is effective in evaluation of key defecatory parameters and well positioned as an integrated technology for assessment of anorectal function and dysfunction.

Keywords: Anal sphincter relaxation; Anorectal physiology; Bionics; Defecation; Fecobionics.

Graphical abstract

Fig. 1

Sketches of the three Fecobionics probes used in the studies. A: First prototype was wired and contained pressure sensors and 6-axis motion processing units (MPUs). The core of the probe was 10 cm long and 12 mm diameter. B: Second prototype had impedance electrodes added to the surface of the core for cross-sectional area measurements. It was wired and the core of the probe was 12 cm long and 12.5 mm diameter. The data were collected in two programs. C: Latest stage probe is wireless and has upgraded circuitry and sensors including 9-axis MPUs. The core of the probe is 10 cm long and 10 mm diameter. The data were collected in a single program with a novel graphical user interface D: Photo of the latest stage probe. The device transmits wirelessly to the WTU (black box). The syringe is used for filling the bag and the tube can be detached after filling, making the device completely untethered.

Fig. 2

Examples of pressure recordings as function of time (A, B) and the front pressure plotted as function of the rear pressure in two normal subjects (C, D). The line of pressure unity is plotted. Defecation cannot take place if the front pressure is above the line of unity, i.e. against a positive pressure gradient. The front and rear pressures and the difference (delta pressure) between the rear and front are displayed (A, B). The defecation attempt starts with simultaneous increase in rear and front pressures. The first contraction usually follows the line of pressure unity where after the front pressure progressively decreases due to anal sphincter relaxation. In some subjects this happened in one contraction (C) whereas others use several contractions (D). Eventually the front will be outside anus and the front pressure drops to zero. The rear is expelled within 1–2 s after the front. The delta pressure is a measure of the rectoanal pressure gradient. See Fig. 3 for further explanation of the front-rear pressure plot.

Fig. 3

A) Graphical illustration of the front pressure as function of rear pressure (preload-afterload plot) that is used for computation of defecation indices (DIs). In this example, the front and rear pressures increase during the abdominal wall contraction initiating defecation. After reaching maximum front pressure, it declines, reflecting the anal relaxation and movement of the device front away from the high-pressure abdominal cavity into the anal canal. The rear pressure temporarily declines before the final contractions that expels Fecobionics completely. For the DIs relating to the front pressure (DI-F), the area under curve is computed, including areas that may be counted several times due to multiple contractions. The first contraction sums the areas 1, 2, and 3. The pressure decline sums area 2 and 3. The second contraction sums area 3 and 4. For the DIs based on the rear pressure (DI-R), similar areas were computed relative to the Y-axis. Some DIs were divided by the urge-to-defecate volume and some DIs were normalized with duration. B and C. The Subfigures show the Fecobionics probe and rectoanal anatomy in the pre-contraction/contraction phase (B) and in the relaxation and expulsion phase (C).

Fig. 4

Box-Whisker plots of measured and computed Fecobionics parameters from 16 normal subjects (box and whiskers) and three presumed normal subjects with abnormal BET expulsions (•). A total of 11 parameters were tested where the two first were the urge-to-defecate volume and expulsion duration (A). Four Defecation Indices (DIs) were the integrated areas for the front and rear, respectively and normalized to expulsion duration or not (B). The lower right diagram (D) shows the same four DIs after multiplying with the urge-to-defecate volume (divided by 1000 for scaling purposes). The last DI was the ratio between the DI-R/vol and DI-F/vol, which is a proxy of the propulsive workload relative to anal resistance (C). See the main text for detailed explanation of the DIs. The box represents the interquartile range, which contains the middle 50% of the records. The line across the box indicates the median. The whiskers are lines that extend from the upper and lower edge of the box to highest and lowest values which are no greater than 1.5 times the IQ range. Outliers are cases with values between 1.5 and 3 times the IQ range; i.e., beyond the whiskers. The three subjects with BET>2 min were all abnormal on Fecobionics expulsion duration too (2 min). The urge volume is not a good parameter to differentiate patients from normal subjects. The same accounts for the defecation indices based on the DI parameters that were normalized to expulsion duration. However, the DIs not normalized to expulsion duration were quite different; i.e. generally higher for the rear sensor (indicating excessive contraction force) and with large spread indicating that they may belong to different subgroups.

Fig. 5

Color topography plot of diameter recordings from the multiple impedance electrodes (top). The forefront of the blue area shows the movement of the electrodes at the front into the anal canal and finally passing the anal canal. The backside of the blue area represents the rear electrodes passing the anal canal. Blue color indicates low diameter. The front, bag and rear pressures as well as the bending angle are shown in the lower diagram. The pressures are clearly associated with the changes in diameters and the bending angle is associated with the final defecation.

Fig. 6

Rectal preload-afterload plots from two subjects in terms of bag pressure as function of the lowest diameter recorded by the rear electrodes (left) or as function of volume (right). The curve is counterclockwise. The first part represents filling of the bag. When urge is reached at a diameter of 30 mm, the subject initiates several contractions that are inefficient for inducing defecation. These can be considered isometric contractions though there is a slight translation to the left. The last contraction is successful, first represented by an isobaric contraction before the loop is completed when the device is finally expelled.

Fig. 7

Pre-defecation and defecation tracings. The defecation attempt starts at time 35 s. The measures front, bag and rear pressures, the computed delta pressure and the friction force are shown.

Fig. 8

Screen dumps of the GUI showing an unsuccessful attempt to defecate (top) and defecation (bottom). Color contours of the CSA (blue represents low diameter and red large diameter), pressures and bending angle. The diagrams to the right show 3D representation of the shape, orientation and bending (see also the Supplementary video).