Jiande Chen, PhD, GI Research, Route 0632, Room 221, Microbiology Building, 1108 The Strand, Galveston, TX 77555-0632, USA. Tel: +1 409-747-3071; fax: +1 409-747-3084; e-mail: email@example.com
Abstract The aim of this study was to investigate the effects of short-pulse intestinal electrical stimulation (IES) on duodenal distention-induced delayed gastric emptying and vomiting in dogs and its possible mechanisms. The study was performed in 12 dogs with jejunal electrodes and a duodenal cannula in three separate experiments to investigate the effects of IES on duodenal distension (DD)-induced delayed gastric emptying and discomfort signs, vagal efferent activity, and jejunal tone. We found that: (i) IES significantly accelerated gastric emptying of liquid delayed by distension (18.05 ± 4.06%vs. 7.18 ± 1.99%, P = 0.036 at 60 min). (ii) IES significantly reduced vomiting and discomfort/pain induced by distension. The average signs score was 15.33 ± 1.37 during distension which decreased to 6.50 ± 0.91 (P = 0.0002) with IES. (iii) IES did not change vagal afferent activity, which was assessed by the spectral analysis of the heart rate variability. (iv) IES decreased jejunal tone. In conclusion, IES with parameters commonly used in gastric electrical stimulation for nausea and vomiting associated with gastroparesis improves DD-induced delayed gastric emptying and prevents DD-induced vomiting and discomfort signs. Further studies are warranted to investigate the therapeutic potential of IES for gastrointestinal symptoms associated with disturbances in motility and sensory function in small intestine.
Nausea and vomiting can be disabling symptoms of gastric and small bowel motility disorders. The most common gastric motility disorder associated with nausea and vomiting is gastroparesis. Similarly, nausea, vomiting and visceral pain/discomfort are commonly seen in patients with small bowel motility disorders, such as postoperative gastrointestinal (GI) complications, mechanical ileus and intestinal pseudo-obstruction which are characterized by abnormal motility in a dilated duodenum and proximal jejunum.1,2 The function of the duodenum and the rest of the small intestine is, to a large degree, mechanical.3 Dispensability is important for normal intestinal function, and altered mechanical properties are associated with GI diseases.4,5 It is well known that distension of the GI tract elicits reflex-mediated inhibition and induces visceral perception such as pain, nausea and vomiting.5 Mechanical duodenal distension (DD) inhibits gastric motility; it stimulates phasic and tonic pyloric pressure waves, suppresses antral contractions and induces gastric relaxation, thereby delaying gastric emptying.6–8 Duodenal distension also produces a generalized reaction, evoking the sympatho-adrenal system and results in disabling symptoms such as nausea and vomiting. This phenomenon is termed the viscero-visceral inhibitory reflex, and its consequences are believed to be restricted to the alimentary tract. The treatment options for disabling nausea and vomiting remain very limited. Recently, the use of gastric electrical stimulation (GES) has been found to have a therapeutic effect on these sysptoms.9–11
According to the width of the stimulation pulse, GES can be classified into short pulses (width in the order of microseconds) and long pulses (width in the order of milliseconds). Short-pulse GES has been approved by the US Food and Drug Administration (FDA) for humanitarian use in treating patients with gastroparesis. The most consistent improvement with this therapy is seen with nausea and vomiting in humans and animals.10,11 Its effects on gastric emptying are conflicting.10,12,13 While the mechanisms of short-pulse GES for nausea and vomiting related to gastroparesis remain largely unknown, a number of canine and clinical studies have suggested the involvement of vagal and central mechanisms. The involvement of the vagal pathway with the anti-emetic effect of short-pulse GES was implicated in a canine study, whereas the involvement of central and visceral perceptive mechanisms are suggested in humans.14,15 On the other hand, long-pulse GES or gastric pacing has been shown to be capable of normalizing gastric dysrhythmia and accelerating gastric emptying.16 Improvement in gastric emptying and dyspeptic symptoms was reported with long-pulse GES in patients with gastroparesis.17 The major mechanisms involved with long-pulse GES are believed to be largely myogenic.16
While GES is proposed to treat patients with gastric motility disorders or nausea and vomiting associated with gastroparesis, intestinal electrical stimulation (IES) has also been proposed for treating disorders associated with motility of the small intestine, such as dumping syndrome18,19 and delayed intestinal transit.20 Long-pulse IES has been shown to normalize gastric and intestinal dysrhythmia.21,22 A number of studies have been performed to investigate the effects of intestinal pacing on various conditions such as short bowel syndrome,23,24 Roux stasis syndrome,25,26 dumping syndrome18,19 and distension-induced intestinal myoelectrical dysrhythmia.22 A recent study in our laboratory showed that short-pulse IES reduces vomiting and behaviours suggestive of nausea induced by vasopressin in dogs.27 However, it is unknown whether short-pulse IES is effective in treating emetic symptoms and/or visceral pain/discomfort associated with small bowel motility disorders.
Acute duodenal distension (DD) with high volume has been confirmed to induce delayed gastric emptying, vomiting and visceral pain;28,29 it is widely used as an acute visceral and dysmotility animal model. The aim of this study was to investigate the effects of short-pulse IES on DD-induced vomiting, discomfort signs and delayed gastric emptying and possible mechanism in dogs.
Materials and methods
A total of 12 healthy female dogs (22–27 kg) were used in the study. After an overnight fast, the dogs were anaesthetized with an initial intravenous infusion of sodium thiopental (5 mg kg−1; Abbott Laboratories, North Chicago, IL, USA) and maintained on IsoFlo (1.5% isoflurane, inhalation anaesthesia; Abbott) in oxygen–nitrous oxide (1 : 1) carrier gases delivered from a ventilator following endotracheal intubation. Laparotomy was performed. Two pairs of wires were implanted in the small intestinal serosa 35 and 40 cm below the pylorus. The electrodes in each pair were arranged circumferentially with a distance of about 0.5–1.0 cm. The electrodes were allowed to penetrate into the subserosal layer and were affixed to the serosa by non-absorbable sutures. The connecting wires of the electrodes were tunnelled through the anterior abdominal wall subcutaneously along the right side of the trunk and placed outside the skin around the right hypochondrium for attachment to the recorder [World Precision Instruments (WPI), Sarasota, FL, USA). A cannula was placed in the duodenum, 20 cm beyond the pylorus. The dogs were transferred to recovery cages after receiving medications for postoperative pain control. The study was initiated after the dogs recovered completely (usually 2 weeks after the surgery). The study was approved by the Institutional Animal Care and Use Committees of the University of Texas Medical Branch at Galveston and performed at the University of Texas Medical Branch.
Experiment 1 was performed to study the effects of short-pulse IES on delayed gastric emptying and signs of discomfort induced by DD in six of the 12 dogs. It included two sessions on two separate days in a randomized order: DD with and without IES; the two sessions were at least 1 week apart. In the DD session, after an overnight fast, a catheter with a 3-cm-long rubber balloon was inserted into the small bowel via the duodenal cannula, and the balloon was placed 5 cm distal to the cannula (see Fig. 1A). Animal behaviours were recorded for 20 min. The balloon was inflated with 40–50 mL of air for a period of 60 min immediately after the ingestion of 237 mL of liquid meal (240 kcal; Boost; Novartis Medical Nutrition, Minneapolis, MN, USA). This volume of distension was found to consistently induce vomiting in all six dogs. Gastric effluent was collected from the cannula every 15 min for 60 min. Signs of pain or discomfort in the animals were continuously observed and noted during the 60-min postprandial period. The IES session was the same as the DD session except that IES was performed during the experiment.
Experiment 2 was performed to investigate the effects of IES on vagal tone in the same six dogs after the completion of experiment 1. It included 20 min of baseline, 20 min of IES and 20 min of recovery in fasting state (after an overnight fast). The electrocardiogram (ECG) was recorded using a special one-channel amplifier with a cut-off frequency of 100 Hz (Model 2283 Fti Universal Fetrode Amplifier; UFI, Morro Bay, CA, USA) from two separate leads and one ground electrode. The two leads were attached to the left and right superclavicular fossae of the subjects and the ground electrode to the left leg. The data were digitized online at 1000 Hz using a PC and a data acquisition package (Alice 3; Healthdyne Technologies, Inc., Marietta, GA, USA). The heart rate variability (HRV) signal was derived from the ECG recording using a special program developed and validated in our laboratory30 by identifying R peaks, calculating R–R intervals, interpolating the R–R intervals so that the time interval between consecutive samples was equal, and finally downsampling the interpolated data to a frequency of 1 Hz.
Experiment 3 was designed to study the effects of IES on jejunal tone in seven of the 12 dogs. Jejunal tone was recorded for 60 min at baseline. The bag was immediately deflated and maintained thus for 20 min. Finally, another 60-min period of recording of jejunal tone was performed with IES.
Intestinal electrical stimulation was performed with pulse trains with train on-time of 0.1 s, train off-time of 4.9 s, a pulse frequency of 14 Hz, a pulse width of 300 μs and pulse amplitude of 6 mA (see Fig. 1B). The electrical stimuli were generated by a universal pulse generator (WPI, Sarasota) and delivered via the first pair of intestinal electrodes. This set of parameters was the same as that used in gastric electrical stimulation for the treatment of nausea and vomiting in patients with gastroparesis.10
Measurement and analysis
Gastric emptying The test meal was composed of 237 mL of boost (240 kcal) evenly mixed with 100 mg of phenol red. Gastric emptying was determined by the assessment of the amount of phenol red in each collection obtained from the duodenal cannula. For each collection of the gastric effluent, the volume was recorded and a sample of 5 mL was taken and stored in a freezer. The samples were analysed all together at the end of the study using a spectrophotometer.31
Signs of discomfort Signs of discomfort of the animals were observed during the experiment, including licking tongue, closing eyes, yawning, belching, murmuring, rapid breathing and movement. These signs were assessed based on their severity and/or frequency (0, never; 1, seldom; 2, often; 3, continues or intolerable).11 Vomiting was scored differently from other signs as they were the most severe discomfort signs, and was scored 3, 4 or 5 if presented one, two or three times, respectively. To make the evaluation objective, the person who evaluated the signs was blinded from the study design and objectives.
Heart rate variability Overall power spectral analysis was applied to the HRV signal and the power in each frequency sub-band was calculated using a previously validated method.29 It has been well established that the power in the low-frequency (LF) band (0.04– 0.15 Hz), represents mainly sympathetic activity and the power in the high-frequency (HF) band (0.15–0.50 Hz) stands purely for parasympathetic or vagal activity.32 LF was defined as the area under the curve in the frequency range of 0.04–0.15 Hz and HF was defined as the area under the curve in the frequency range of 0.15–0.50 Hz. The LF/HF ratio reflects the balance between sympathetic activity and vagal activity.
Jejunal tone An electronic barostat (Synectics Visceral Stimulator, Synectics Medical, Stockholm, Sweden) was used to measure small bowel tone. After an overnight fast, a catheter with a polyethylene balloon (300 mL maximum capacity) on its tip was connected to a barostat device and placed into the intestine via the duodenal cannula with the caudal end of the attached balloon 10 cm distal to the cannula. Small bowel tone was measured as follows: after the determination of the minimal distending pressure (MDP; a pressure level needed to overcome intra-abdominal pressure), intestinal volume was measured at a constant pressure of 2 mmHg above the MDP.33 Jejunal tone is inversely correlated with intestinal volume.
Results are reported as mean ± SE. Paired t-test was used to investigate the differences between any two sessions/periods if the analysis of variance (anova) revealed a significant difference among the three sessions/periods or more. Chi-squared analysis was applied for the assessment of certain individual uncomfortable signs score, such as vomiting.
Effects of IES on delayed gastric emptying induced by DD
Compared with the control session without distention, DD significantly and substantially inhibited gastric emptying at 15, 30, 45 and 60 min (P < 0.001). IES significantly accelerated DD-induced delayed gastric emptying. Gastric emptying at 60 min was 7.18 ± 1.99% in the distension session, which increased to 18.05 ± 4.06% (P = 0.036 vs. distension session) in the session with IES (Fig. 2).
Effects of DD and IES on signs of discomfort
Duodenal balloon distension caused significant changes in animal behaviours. During the 20-min when the balloon was inserted into the intestine but was deflated, no significant changes were noted in animal behaviours. The balloon’s distension of 40–50 mL, however, induced signs suggestive of severe pain/discomfort. These signs were linked to the action of the nociceptive factors such as an increase in movement, licking tongue, closing eyes, yawning, belching, murmuring, rapid breathing and vomiting. Intestinal electrical stimulation significantly improved the signs of discomfort induced by distension, especially vomiting. The total episodes of vomiting was 18 in the distension session, which significantly decreased to five in the IES session. The mean sign score was 15.3 ± 1.4 in the DD session, which reduced to 6.5 ± 0.9 (P = 0.0002 vs. distension session) in the session with IES (Fig. 3).
Effects of IES on vagal tone
Vagal activity assessed by the spectral analysis of the HRV showed no difference among baseline, stimulation and recovery periods (anova, P > 0.05). The vagal activity (HF) was 0.71 ± 0.04 at baseline, 0.67 ± 0.06 during the stimulation period (P = 0.63 vs. baseline) and 0.69 ± 0.08 during the recovery period (P = 0.81 vs. baseline) (Fig. 4A). The sympatho-vagal balance (LF/HF) was 0.43 ± 0.08 at baseline, 0.57 ± 0.21 during the stimulation period (P = 0.46 vs. baseline) and 0.62 ± 0.27 during the recovery period (P = 0.42 vs. baseline) (Fig. 4B).
Effects of IES on jejunal tone
Basal jejunal tone was significantly decreased following short-pulse IES. As shown in Fig. 5, in the fasting state, the average jejunal volume was 100.9 ± 3.0 mL in the control session, which significantly increased to 119.4 ± 6.9 mL following IES (P = 0.026).
In the present study, we found that (i) short-pulse IES significantly accelerated delayed gastric emptying of liquid induced by DD; (ii) IES significantly improved vomiting and animal behaviours suggestive of visceral pain or discomfort induced by distension; (iii) IES significantly decreased jejunal tone; (iv) IES did not alter vagal efferent activity assessed by the spectral analysis of the HRV.
Nausea and vomiting are common symptoms associated with gastric and small bowel motility disorders in clinical practice. The primary peripheral system containing receptors that can induce nausea and vomiting is the digestive tract. Digestive tract emetic receptors are mechanoreceptors and chemoreceptors.34 Mechanical stimulation of the digestive tract from pharynx to small intestine by distension or obstruction can induce nausea and vomiting.35 Excessive distension of the gut induces pain via serosal stretch receptors whose output passes via sympathetic neurones to the central nervous system; it also activates both vagus and splanchnic nerves and induces retching accompanied by cardiovascular and respiratory responses with a short latency and high success rates.36 The stimuli arising from the stomach and small intestine are important in the aetiology of upper GI symptoms and disordered gastric motility.37,38 The experiment design of this study was considered to investigate the effects of IES on vomiting, visceral pain and delayed gastric emptying associated with small bowel dysmotility. Duodenal balloon distension is the one of models of intestinal motility disorder commonly used in the study of vomiting and visceral pain.28,29
In this study, delayed gastric emptying, vomiting and discomfort signs were induced by duodenal balloon distension. Short-pulse IES accelerated but did not normalize delayed gastric emptying, improved vomiting and discomfort signs. These findings demonstrated that short-pulse electrical stimulation applied to the small intestine had an anti-emetic effect similar to that when applied to the stomach.10,11 The anti-emetic effect of short-pulse IES was also reported in the canine model of vomiting induced by vasopressin.27 Short-pulse GES has been widely reported and its consistent improvement of nausea and vomiting has been shown in humans and animals.10,11 Gastric emptying was shown to be improved with short-pulse GES in some studies but not in others.9,10,13,39 The improvement in gastric emptying reported in some studies could be attributed to the improvement in overall clinical profiles of the patients rather than the direct effect of the short-pulse GES as it has never been reported in any controlled clinical studies or animal studies.16 During the initial stages of this study, short-pulse GES was also performed in a number of dogs and no improvement was noted in delayed gastric emptying induced by DD.
Vagal and central mechanisms are assumed to be involved in the anti-emetic effect of IES. Although the spectral analysis of the HRV signal did not reflect any significant alterations in vagal efferent activity, the involvement of the vagal efferent pathway could not be ruled out because of different efferent projects originating from the dorsal motor nucleus of vagi to the heart and the stomach.40,41 In a previous canine study, IES with the same parameters as those used in this study improved vasopressin-induced emesis, suggesting a central mechanism of IES as vasopressin-induced emesis is centrally mediated.27 In another study in rats, IES with parameters similar to those used in this study activated neurones in the nucleus tractus solitarii (NTS), suggesting the involvement of the vagal afferent pathway, as most of the neurones in the NTS receive inputs from the vagal afferent.42
Mechanisms involved in improved gastric emptying with IES are not known but might be attributed to the reduced pressure gradient across the pylorus and enhanced vagal efferent activity. Although the pressure of the intestine was not measured in the session of gastric emptying, it was shown to be decreased in the fasting state with IES (Experiment 3), reflected by an increase in the volume of the barostat balloon with IES. The decrease in intestinal pressure during IES increases the pressure gradient between the stomach and the proximal intestine or reduces intestinal resistance, thereby leading to an acceleration of gastric emptying. However, a confirmative conclusion could not be made in this study as the pressure of the distending balloon was not measured during gastric emptying. The involvement of the vagal efferent was purely speculative; previous studies with electrical stimulation of similar parameters demonstrated an increase in vagal efferent activity which was, however, not noted in the current study.43 It should also be noted that the distension-induced gastric emptying delay was dramatic whereas the improvement by IES was limited although significant as shown in Fig. 2. The mechanism involved in the IES-induced relaxation (or reduced tone) of the small intestine was not investigated in this study but the involvement of the sympathetic pathway was reported in a previous study.44
The findings of the present study indicate that stimulation parameters commonly used in GES for nausea and vomiting associated with gastroparesis can also be applied to IES and has a similar anti-emetic effect as applied to the stomach. IES with appropriate parameters may have a therapeutic potential for the treatment of GI symptoms associated with disturbances in motility and sensory function in the GI tract. Further studies are needed to investigate the role of IES in treating patients with intestinal disorders such as postoperation ileus or intestinal pseudo-obstructions.
In conclusion, IES with parameters commonly used in GES for nausea and vomiting associated with gastroparesis improves DD-induced delayed gastric emptying and prevents DD-induced vomiting and discomfort signs. Further studies are warranted to investigate the therapeutic potential of IES for GI symptoms associated with disturbances in motility and sensory function in small intestine.
This study was partially supported by a research grant from NIH (DK055437).