Electrogastrography in experimental pigs
The power analysis

Overview

Electrogastrography (EGG) is a reliable diagnostic technique providing information on the frequency and relative amplitude of antral contractions of stomach using cutaneous electrodes. The aim of this study was to analyze the power ratio obtained during the EGG recording. Eight mature pigs entered our study. EGG was recorded using a Digitrapper EGG equipment (Synectics Medical AB, Stockholm, Sweden). The experiment was carried out under general anaesthesia. We performed three separate recordings in each animal. First was a baseline measurement of EGG under general anaesthesia, after 24 hours of fasting. The second was after administration of 100 mg itopride and the third recording was after intragastric volume challenge (360 mL of water per animal). Each recording lasted for 30 minutes. Power analysis did not provide further improvement of differentiation between particular EGG tests (basic, volume challenge and itopride) compared to the running spectrum percent activity of amplitude. The antral motor index and complex myoelectrical activity index correlate closely with each other. How­ever, neither antral motor index nor complex myoelectrical activity index are essentially helpful in final evaluation of particular EGGs. Only non-significant trends could be found (basic EGG had the highest values of both indices). We conclude that running spectrum percent activity (dominant frequencies of gastric slow waves) remains the most important criterion for EGG evaluation in experimental pigs.

Key words:
electrogastrography – experimental pigs – running spectrum percent activity – power analysis – gastric slow waves

Electrogastrography (EGG) is a reliable diagnostic technique used for recording gastric myoelectrical activity using cutaneous electrodes placed on the anterior abdominal wall overlying the stomach. Electrogastro­graphy provides surrogate information on the frequency and relative amplitude of antral contractions [1,2]. These acti­vities originate from the gastric pace­maker located in the mid portion of the stomach which fires at three cycles per min (cpm) with aboral propagation of spike bursts to the pylorus in humans [1,3]. Since its first application in the 1920s [4] numerous studies have demonstrated a close association of cutaneously monitored electric abnormalities with gastro­intestinal motility disorders [1,3,5–19]. Many studies have shown that the EGG is an accurate measurement of the gastric slow wave frequency, and that the relative change in amplitude of the EGG signals might reflect the contractility of the stomach [2,3,8,16].

It is now well established that neuro­muscular activities of the stomach generate electrical phenomena termed “gastric slow waves” [3,20]. It is also believed that the interstitial cells of Cajal originate the rhyth­mic de­polarization of the gastric slow wave. Under quiescent conditions, slow waves partially depolarize gastric smooth muscle but do not cause contraction. Additional de­polarization provided by neurohumoral stimulation is the trigger for phasic gastric contractions which follow the spread of the electrical slow waves and can result in peristalsis. Thus, gastric electrical slow waves control the maximal frequency and the direction of contractions in the distal ­stomach [1,21,22].

The major parameters of EGG and gastric emptying measures (e.g. half-life of elimination in 13C-octanoic acid breath test) mutually correlate in healthy humans [23]. EGG has been advocated as a diagnostic test for the clinical evaluation of patients with unexplained nausea, vomiting and other dyspeptic symptoms to gain insight into mechanisms of symptom generation [10,14,18,19,24].

Spectral analysis of the signal from EGG identifies two components of the surface electrical signal: frequency and power which is related to the amplitude of the signal [25]. Numerous studies have demonstrated that frequency is related to the gastric slow waves recorded by serosal or mucosal electrodes [13,15,26]. Conversely, the physio­logical significance of the power (or amplitude) and its fluctuations has not been clearly established. The increase in the amplitude of the signal observed after a meal may be due to increased contractile activity or to gastric ­distension [8,26].

Even though there are distinct differences between the anatomy and physiology of humans and pig [27,28], they are a very close representative in relation to gastro-intestinal functions. The shape of porcine stomach is like a pouch and the gastric cardia is close to the pylo­rus and they have a special transverse pyloric fold called torus pyloricus which controls the flow of contents into the duodenum. Significant amount of food remnant is found in the porcine stomach even after 36–48 hours of fasting showing that the emptying is much slower than that in humans [28].

To the best of our knowledge, there are no previous reports on EGG in pigs in available literature. Our group set up and worked out the methods for EGG in experimental pigs recently [22]. The aim of this study was to evaluate power analysis of EGG recording in experimental pigs and to compare its diagnostic yield with the running spectrum analysis (dominant frequencies of gastric slow waves) under experimental conditions.

Material and methods

Study subjects

Our study was performed on eight mature female pigs. They were 4–5 ­months old and weighed 30.5 ± 2.6 kg. All the animals were fed twice a day on standard diet (assorted food A1) and were allowed access to water ad libitum. The animals entered the study were after 24 hours of fasting. Sus scrofa f. domestica, the hybrid of Czech white and Landrace breeds where proved to be suitable for the experiment from our previous study.

Study protocol

The experiment was carried out under general anaesthesia. Intramuscular injection of ketamine (20 mg per kg; Narkamon, Spofa Praha, Czech Republic) was used at the beginning. A repeated dose of the drug was administered intra­muscularly as and when required for maintaining the sedation to avoid any kind of motion artefacts. All the animals were in a right lateral position during the experiment. The epi­gastric area was shaved before application of electrodes to decrease impedance in signal conduction through the skin. Electrodes were placed at a similar site in all the animals to avoid positional errors. The first electrode was placed within 5 cm of the processus xiphoideus in the centre; the other two were then placed at a distance of 10 cm from the central electrode on left and right hypochondria.

The Project was approved by the Institutional Review Board of Animal Care Committee of the Institute of Experimental Biopharmaceutics, Academy of Sciences of the Czech Republic, Record Number 1492006.

Recording of gastric myoelectric activity

EGG was recorded using a Digitrapper EGG (Synectics Medical AB, Stockholm, Sweden).The EGG system comprises a portable unit for electronic recording, a pair of recording electrodes along with a reference electrode, all connected to the portable recording unit. In each of the eight animals, we performed three separate recordings. First was a baseline mea­surement under general anaesthesia, after 24 hours of fasting. Then we administered of 100 mg itopride, a prokinetic drug (Ganaton, Abbott Laboratories) and the second measurement was done. The third recording was immediately after intragastric volume challenge (360 mL of water per animal). Each recording lasted for 30 minutes.

During the whole experiment, the animals were closely monitored for any movement or muscle tremor. If there were signs of movement like limb shaking or restlessness, the mea­surement was paused and the necessary steps were taken to calm the animal down, notes were made to identify these arte­facts and their subsequent removal during analysis. In the case of any such unforeseen event, intramuscular bolus of anaesthesia was administered. Our experiment was carried out in a peaceful manner, barring two events.

The amplitude of stomach slow wave is very weak, while many visceral organs also produce rhythmic electric signals, like small and large intestine, heartbeat, respiration, and even body movements. Thus noise other than the slow wave had to be carefully monitored for and removed. The best signal among all recordings was selected to compute EGG parameters based on spectral analysis. Prior to subjecting the visually recognizable waveforms to computer analysis, periods which exhibit clear artefacts were identified and excised. Motion artefacts, if included in the quantitative computer software, may lead to erroneous determinations. Obtained EGG parameters include dominant frequency, power, % normal rhythm, % bradygastria, % tachy­gastria, instability coefficient and power ratio. Power analysis was based on the measurement of height of all amplitudes obtained at each EGG recording independently.

Along with the evaluation of power analysis, we also calculated the antral motor index (AMI) in all our animals using the EGG readings [29]. The antral motor index is defined as the sum of height of all waves × number of waves at a single EGG recording. To get comparable results, we exclude all the artefact intervals before calculating the AMI. We have worked out our own new parameter – the complex myoelectrical activity index (the Varayil index). This was calculated as the sum (of height of all waves × number of waves at a single EGG recording) divided by total time of assessable recording (in seconds).

Statistical analysis

The data obtained after removing the visible artefacts were statistically trea­ted. We performed descriptive statistics, Mann-Whitney rank sum test and paired t test. Because of great inter-individual variability in power in particular animals, power analyses were referred to the mean values of power at basic EGG recording and expressed in percentage.

Results

All obtained EGG recordings in all animals were fully relevant and assessable Power analyses of the EGG recordings were feasible. Both indices (AMI and CMAI) were calculated in all measurements. Results are summarised in Tables 1–3. There was no significant difference using power analysis, AMI and CMAI between three measurements (basic EGG recording, EGG after itopride administration and EGG after volume challenge).

Because of great inter-individual variability in power in particular animals, power analyses after itopride and volume challenge were referred to the mean values of power at basic EGG recording and expressed in percent. Difference in delta were not significant: group A (basic recording) vs. group B (EGG after itopride administration): delta plus 2% (p = 0.710); group A vs. group C (EGG after volume challenge): delta minus 38.5% (p = 0.209); group B vs. group C: delta minus 14.2% (p = 0.572).

The antral motor index and complex myoelectrical activity index correlate closely with each other (r = 1.0; p < 0.00001), however they did not cor­relate with the running spectrum percent activity (dominant frequencies of gastric slow waves). Only non-significant trends could be found (basic EGG had the highest values of both indices).

Discussion

Our initial experiences of EGG with experimental animals were successful. In our previous trial, we demonstrated that EGG in experimental pigs is feasible. Analyses of dominant frequencies were comparable to the normal frequencies in humans [22]. Thus EGG in experimental pigs is a suitable model for further preclinical studies [30,31].

The aim of this project was to appraise if power analysis and motor indices could further improve the evaluation of EGG recording and thus make its diagnostic yield more precise. Based on our current results, these tools failed to provide any additional discriminating information (to the “standard” running spectrum percent activity).

In humans, there is an increase of power ratio after water ingestion (gastric power is calculated by numeral integration of the power spectrum density curve; given in μV²) [32]. We decide another approach in our experimental setting. To be able to compare the power of control group (basal recording on fasting condition with no challenging action) with those after itopride and water administration, we evaluated the height of all amplitudes obtained at particular EGG recordings (in mV). Surprisingly, we got a trend (albeit a non-significant one) of decreased power after itopride and water challenge compared to controls. This phenomenon should be further clarified. It is questionable if it can be at least partly explained by different anatomy and physiology of the porcine stomach compared to humans [28].

Lack of standardized methodology in terms of electrode positions, recording periods, test meals, analytic software and normal reference values makes the significance of EGG recording controversial [22,24,34].

Dominant power analysis has also other limitations. Even though the dominant frequencies are not affected much with the positioning of the electrodes, minor changes can have a huge effect on the determination of the dominant powers [16,20,32,35]. We are aware that there can be changes in the power data due to this.

All our recordings were performed under general anaesthesia in pigs; otherwise EGG would not be practical. In humans anaesthesia have significant effect on the myoelectrical activity of the stomach, so the measurement may be influenced and affected by the anaesthesia ­administered [22].

Many obstacles are required to be overcome when EGG recording system is used to evaluate the stomach dysmotility [2,36,37]. First, the amplitude of surface-recorded slow waves is very weak, in the range of ­50–500 µV, compared to cardiac myoelectricity. Consequently, an amplifier is needed when this very low voltage signal is acquired by the EGG system. Second, the abdominal cavity is not silent because many visceral organs also produce rhythmic electric signals, for example heart­beat, respiration, other organs of the gastrointestinal tract and even body movements and success of the future studies also depends on how much these can be from stomach slow waves [1,8,11,17,26,32,33,36–38].

Conclusions

Power analysis and motor indices did not provide further improvement of differentiation between particular EGG tests (basic, volume challenge, itopride) compared to running spectrum percent activity (dominant frequencies of gastric slow waves).

The antral motor index and complex myoelectrical activity index (Varayil index) correlate closely with each other. However, neither antral motor index nor complex myoelectrical activity index (Varayil index) are essentially helpful in final evaluation of particular EGGs. Only non-significant trends could be found (basic EGG had the highest values of both indices).

For our further studies, more adjusted placement of electrodes should be used for each animal (to find out the highest possible amplitudes) rather than the standard placement used now before the start of EGG recording. We conclude that running spectrum percent activity (dominant frequencies of gastric slow waves) remains the most important criterion for EGG evaluation.

Acknowledgement

The study was supported by ­research project MZO 00179906 from the ­Ministry of Health of the Czech ­Republic and by research grant GAČR 305/080535, Czech Republic.

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Doručeno/Submitted: 26. 9. 2011

Přijato/Accepted: 16. 10. 2011

Professor Jan Bureš, MD, PhD

2^nd Department of Medicine Charles University Teaching Hospital, Sokolská 581500 05 Hradec Králové, Czech Republic

bures@lfhk.cuni.cz