• Doppler ultrasound;
  • manometry;
  • transpyloric flow


  1. Top of page
  2. Abstract
  8. References

The motor mechanisms responsible for transpyloric flow of gastric contents are still poorly understood. The aim of our study was to investigate the relationship between luminal pressures and gastric wall motion and between gastroduodenal pressure gradients and pressure waves, and ante- and retro-grade transpyloric flow. In eight healthy volunteers, intraluminal pressures were recorded from the antrum and proximal duodenum. Transpyloric flow was monitored simultaneously using duplex ultrasonography, before, during and after ingestion of 300 mL meat soup. Transpyloric emptying occurred as sequences of alternating periods of emptying–reflux–emptying. Approximately one-third of the sequences were not associated with peristalsis. The antroduodenal pressure gradients were significantly lower during nonperistaltic-related emptying than during peristaltic-related emptying (0.15 (0–0.3) kPa, and 1.7 (0.2–2.0) kPa, respectively [mean ± (range)], P < 0.005). The duration of emptying episodes not associated with peristalsis were significantly longer than those associated with peristalsis at (6.5 (3–8.7) s and 4.4 (2–6) s, respectively, P=0.059). Manometry detected only 56% of the antral contractions seen on ultrasound. We concluded that gastric emptying of a low-calorie liquid meal occurs both during peristaltic and nonperistaltic antral activity. In spite of lower antroduodenal pressure gradients, the emptying episodes were longer for nonperistaltic emptying, which is likely to be caused by low pyloric resistance. Considerable flow seems to occur without peristalsis during gastric emptying of a low-calorie, liquid meal in humans.


  1. Top of page
  2. Abstract
  8. References

Although several studies have suggested that the incidence of gastrointestinal contractions and their amplitude are important determinants of luminal flow, the motor mechanisms responsible for gastric emptying are still poorly understood.1[2][3][4]–5 In surgical experimental models in which antrectomy and extrinsic denervation were performed, no change in gastric emptying rate of liquids was observed.6,7 On the basis of these observations, a two-compartment model of gastric emptying was proposed in which the proximal stomach was assigned to play a dominant role for receipt and storage of food and for control of gastric emptying of liquids. The distal stomach was considered to exert the major role in retention, grinding, and propulsion of larger-sized particles. However, these observations did not take into account the pulsatile pattern of gastric emptying.8[9]–10 Several studies have shown that antral motor events increase the rate of liquid emptying, and that the antrum can expel fluids independently of proximal tone and pressure.11 Therefore, the relative contribution of antral peristalsis to gastric emptying of liquids is still unclear.

In order to study the relationship between motility and flow in detail, techniques with a high temporal and spatial resolution are required for the assessment of antropyloroduodenal pressure waves and transpyloric flow.

By the use of combined manometry and electromagnetic flowmeter in dogs12,13 and by using force transducers applied in isolated cat stomachs,14 the motor activity responsible for single emptying pulses has been studied. However, at present, there have been no reliable methods to study in detail the relationship between motility and flow in humans.

Recently, we reported on the feasibility of recording of gastrointestinal pressure waves from multiple closely spaced side holes (1.0–1.5 cm apart) with a perfused manometric system, and the use of locally developed software for the analysis of these manometric data.15[16]–17 This manometric technique allows studies of antropyloroduodenal pressure waves with high time and space resolution.

Information concerning movement of luminal contents in humans can be obtained by fluoroscopy, scintigraphy, magnetic resonance imaging (MRI) and duplex sonography. The disadvantages of fluoroscopy are the radiation exposure and the nonphysiological stimulus of barium18. The disadvantages of scintigraphy are the radiation exposure and the relatively low time resolution that can be obtained using this technique. Studies based on scintigraphy and standard ultrasound at the gastric and duodenal level indirectly measure overall rates of gastric emptying, but these methods do not have the temporal resolution to assess on a second-by-second basis. Promising new MRI methods have recently been developed. Echoplanar imaging, an ultrafast variant of MRI, can provide excellent images of both gastric wall movements and movements of solid and liquid meals19[20]–21. The ultrasound Doppler technique provides temporal resolution for assessment of flow. In several studies, Hausken et al. showed that the direction, timing and velocity of flow could be calculated using duplex sonography.22,23

In the present study, the manometric and the ultrasound Doppler techniques are applied simultaneously to investigate the relationship between luminal flow and antropyloroduodenal pressure waves. The aim of the study was therefore to investigate the relationship between gastroduodenal pressure gradients and pressure waves, and ante- and retro-grade transpyloric flow. The relationship between luminal pressures and gastric wall motion was also investigated.


  1. Top of page
  2. Abstract
  8. References


Eight healthy students without abdominal symptoms or signs of previous gastrointestinal surgery or disease were included in the study. None of the volunteers used medication known to influence gastrointestinal motor activity.

All subjects gave written informed consent. The protocol was approved by the Research Ethics Committee of the University Hospital Utrecht and was conducted according to the Declaration of Helsinki.

Experimental design

On the study day, following an overnight fast, the subjects were intubated with a 21-lumen manometric assembly. At 08.30 hours the assembly was introduced transnasally and positioned in the antropyloroduodenal region using fluoroscopy. The positioning of the catheter was monitored during the test using measurement of the transmucosal potential difference (TMPD) between two sideholes located in the distal antrum and proximal duodenum. Established criteria (antral TMPD < –40 mV, duodenal TMPD > 0 mV, minimal difference between the two, 15 mV) were used to define the position of the proximal side holes positioned in the gastric antrum and more distally in the duodenum.

Once the catheter was in the correct position the subjects were placed in a comfortable chair leaning slightly backwards for the remainder of the study. When phase II activity in the small intestine was present, a standardized liquid nutrient meal was ingested within 3 min.

Duplex sonography was used to monitor antral contractions and transpyloric flow. The synchronization of manometric and Doppler/ultrasound data started 5 min before the meal, and continued during the 3 min of ingestion and for 10 min postprandially.


The test meal consisted of a liquid meal of commercial meat soup (Drinkbouillon®; Knorr, the Netherlands). The soup (300 mL) was prewarmed to 37 °C, and the soup was gradually ingested during 3 min. The meal contained 0.9 g protein, 0.9 g fat, and 0.9 g carbohydrate (9 kcal). Fat, protein and carbohydrate were all soluble in water. In addition, the soup contained salt, monosodium glutamate, peppers, and soluble and nonsoluble seasoning.

The emptying curve for this meal follows an exponential curve, with the greater part of the emptying occurring initially and 50% of the meal emptied after 22 min.24

Manometric technique

Antropyloroduodenal manometric recordings were obtained using a water-perfused multiple-lumen manometric assembly. The silicone rubber assembly with a length of 160 cm, incorporated 21 (+ 1) lumina of 0.4 mm and a core of 1.0 mm. The assembly incorporated a 4.0-cm transpyloric sleeve sensor with three side holes spaced along the sleeve, four antral side holes and 12 duodenal side holes. The assembly also incorporated three tungsten weights to facilitate passage through the stomach. A balloon located 5 cm from the tip was inflated with 8 mL of air to facilitate aboral migration as soon as one of the side holes had entered the small intestine. Pressures were recorded from 11 of the 21 side holes (seven antral and pyloric at 1 cm intervals, four duodenal at 1.5 cm intervals) channels via external transducers (Abbott, Chicago, IL, USA) and used for subsequent analysis. The pressure data were stored in a digital format data logger (MMS, Enschede, the Netherlands) with a memory capacity of 4 mB, using a sample frequency of 8 Hz. At the end of each manometric study the data logger was connected to a personal computer and data were transferred to the hard disk for subsequent analysis.

Manometric analysis

The phases of the interdigestive migrating motor complex in the small bowel were determined visually and classified as:

1 Phase 1: quiescence starting immediately after completion of phase 3

2 Phase 2: pressure waves > 1.0 kPa (7.4 mmHg) occurring at a frequency greater than 2 per 10 min, but less than 10–12 min−1

3 Phase 3: pressure waves at the maximum frequency (10–12 min−1) for at least 2 min, which propagated over more than two sideholes and were followed by motor quiescence.

In the postprandial period, any pressure rise that was less than 20 s in duration was scored, provided it occurred as an independent event not attributable to respiration, straining or change of posture. The pressure waves were then classified according to site, extent and their timing in relation to each other. Amplitudes and directions of antropyloroduodenal contractions were analysed.

Antroduodenal pressure gradients at the onset of transpyloric flow were calculated as the maximum difference in pressure recorded between the distal antrum and proximal duodenum at the onset of emptying or duodenogastric reflux, respectively.

The pre- and post-prandial periods were separated into four periods: period 1 was the ingestion period, and periods 2–4 were the following postprandial periods (3.3 min each).

Duplex sonographic technique

A duplex scanner (Esaote AUS; Esaote Pie Medical, Maastricht, the Netherlands) with a 5-MHz curved array probe was used. This scanner allows real-time ultrasound images of antral motility and flow velocity curves of the Doppler recordings to be visualized simultaneously. For quantitative measurements of flow velocity and timing, a pulsed Doppler mode was used. All ultrasound and Doppler measurements were performed by one investigator (T.H.).

During the fasting period, antral contractions were recorded with the ultrasound probe in a standardized vertical section in which the antrum, the superior mesenteric vein and the aorta were visualized in one image.

Transpyloric flow and antral contractions

The study of the antropyloroduodenal region was performed with the ultrasound probe positioned at the level of the transpyloric plane, and the antrum, the pylorus and the proximal duodenum visualized simultaneously. The common chamber was defined as a volume entity created by a simultaneous relaxation of the proximal duodenum, pylorus and antrum, allowing liquid contents within the volume to be retained or passed back and forth across the pylorus. The sample volume of the pulsed Doppler was positioned across the pylorus, and the angle between the Doppler beam and the transpyloric direction of flow was always < 60°. With the probe in this position the following parameters were analysed:

1 Antral contraction, defined as an indentation of the gastric wall greater than one antral wall thickness, which was not due to respiration, pulsation transmitted from the aorta or heart, or to movements of adjacent intestine, and which was observed to propagate in space and time.

2 First gastric emptying, defined as the first occurrence of gastric emptying after start of ingestion of the soup. An episode of gastric emptying was defined as flow across the pylorus with a mean velocity of more than 10 cm s−1 lasting more than 1 s.

3 Occluding peristaltic-related transpyloric emptying, defined as gastric emptying associated with contractile activity in which the ultrasound image showed that the gastric walls occluded the lumen.

4 Non-occluding peristaltic-related emptying defined as transpyloric emptying of commen chamber contents associated with contractile activity in which the ultrasound image showed the gastric walls contracting but not occluding the lumen. During maximal contractions, transpyloric flow could still be seen passing to and fro through the open pylorus.

5 Non-peristaltic-related transpyloric emptying, defined as transpyloric emptying of common chamber contents, without contractions detected on ultrasound or manometry. Data on spatial and temporal resolution of the Doppler technique has been described previously8.

6 Emptying–reflux–emptying sequence was detined as the transpyloric flow seen as part of the emptying episode, consisting of continous flow backwards and forwards.

Synchronization of Doppler/ultrasound and manometric recordings

The UD-2000 Video-Mix and image digitizing software, (Version 5.10; MMS, Medical Measurements Systems, Enschede, the Netherlands) was used to synchronize the recorded data (Fig. 1). The image digitizing allowed simultaneous display of Doppler/ultrasound data and manometry signals on one PC monitor. The Doppler/ultrasound data were digitized during the investigation, and the digitized images were saved on disk together with the digitized manometry signals. The computer controlled video recorder recorded all the Doppler/ultrasound images. The computer kept a database with tape information. During recording, the tapes were formatted with coded information. This allowed the system to verify that the correct tape was used. The tape also contained timing information, to allow synchronized playback during analysis. Images could also be digitized from tape during analysis.


Figure 1.  Synchronization of Doppler/ultrasound and manometric recordings. In the ultrasound/Doppler image is the transpyloric region displayed. A line indicates the sample volume covering the pyloric channel. Below is the velocity curve of the Doppler recording visualized with emptying below the zero-line and duodenogastric reflux above the line. Manometry signals are displayed in the 11 channels. A vertical line (Pic) denotes the timing. A duodenal contraction is associated with reflux in the Doppler image.

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The data were analysed on two occasions. On the first occasion, contractions (manometry versus ultrasound) were analysed, while on the second, the relationship between flow and pressure gradients were analysed. On both occasions all three investigators took part in analysing the video tapes.

Statistical analysis

As most of the data were not normally distributed, nonparametric tests were used. The Wilcoxen signed rank test was used for pairwise intra-individual comparisons and the Mann–Whitney U-test for comparison between groups. Data relating to fasting and pre- and post-prandial amplitudes as measured by manometry were evaluated, using Friedman's analysis of variance (ANOVA),with Dunn's post hoc test. The data were presented as mean values with intrerquartile ranges. If data were normally distributed, parametric tests were used. All statistical calculations and graphic designs were performed using commercially available software (SPSS version 10.0 and Prism 3.0 for Windows). P < 0.05 was accepted as significant in all analyses.


  1. Top of page
  2. Abstract
  8. References

Two subjects had to be excluded from the study due to displacement of the catheter during soup ingestion. Six subjects were then included (2F, 4M; median age 22 years, range 20–26 years). Both peristaltic-related emptying and emptying without manometric activity were recorded in all remaining six subjects. Time to first gastric emptying, first antral contraction, and time to peristaltic-related flow using ultrasound were 115 (39–40) s, 219 (23–347) s, and 221 (39–360) s, respectively, whereas time to first antral contractions using manometry was 172 (23–312) s.

Antral contractions

Fasting period.

In the fasting state (the 5-min period before soup ingestion) the total number of antral contractions for the six subjects together was 25 (ultrasound and manometry together), in which 84% were detected by manometry. The mean amplitude was 16.4 (2.5–32.7) kPa. One antral contraction was seen on manometry only (Table 1).

Table 1.   Total number of antral contractions detected on ultrasound and manometry in the fasting and pre- and post-prandial period Thumbnail image of
Pre- and post-prandial period.

Using ultrasound the total number of antral contractions during ingestion and postprandial (for the six subjects together) was 77, of which 77% were seen as occluding and 23% as nonoccluding contractions (Table 1). Fifty-six percent of the contractions were seen on both ultrasound and manometry with a mean amplitude of 3.5 (1.4–5) kPa, while 44% of the contractions were not detected by manometry (Table 1).

Only four (22%) of the 18 nonoccluding antral contractions were detected by manometry in the most distal antral channel (Table 2). A mean of 16 ± 10 antral contractions per subject was seen on ultrasound, whereas a mean of 9 (1–18) antral contraction were detected by manometry (P < 0.05).

Table 2.   Number of antral contractions detected on ultrasound and manometry during ingestion and the following postprandial period Thumbnail image of

Twelve nonoccluding contractions were seen during periods 3 and 4 vs. 6 nonoccluding contractions during periods 1 and 2. Using non-parametric, repeated measures ANOVA (Friedman) we found the main effect on the amplitudes in the fasting period, which were significantly higher than those in the following ingestion and post-prandial period (P < 0.05). The post hoc test (Dunn) could not find any statistical difference between the ingestion and post-prandial periods (Fig.  2).


Figure 2.  Amplitudes of antral contractions (mean [range]) in the fasting period, the ingestion and postprandial period. The pre- and post-prandial periods were separated into four periods: period 1 was the ingestion period, and periods 2–4 were the following postprandial periods (3.3 min each). The amplitudes in the fasting period were significantly higher (P < 0.05) than in the following ingestion and postprandial periods.

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Relationship between transpyloric flow and motor activity

Transpyloric emptying occurred as emptying–reflux–emptying sequences; 45.3 (18–68)% of the sequences were associated with occluding antral peristalsis, 21.5 (5–37)% were associated with nonoccluding peristaltic related antral contractions, and 35.7 (4–79)% of the sequences were not associated with peristalsis. The mean number of emptying episodes for each subject was 30.5 (24–38) and mean number of reflux episodes was 19.2 (11–26). Peristaltic-related flow occurred as emptying alone, emptying followed by reflux or as sequences of emptying–reflux–emptying pulses. The flow events always occurred in front of the peristaltic wave. The nonperistaltic–related flow was often seen as longer periods of alternating emptying–reflux sequences.

Relationship between transpyloric emptying and occluding peristaltic contractions.

Antroduodenal pressure gradients and duration of pyloric flow are presented in (Table 3). When peristaltic related flow was seen by ultrasound, propagated pressure waves were recorded manometrically over three or more side holes.

Table 3.   Antroduodenal pressure gradients and duration of transpyloric flow (means (quartiles) Thumbnail image of

A sequence of emptying–reflux–emptying was seen in 74% of the emptying periods. All emptying periods were followed by a reflux period, which was caused by duodenal contraction in 93% of the periods. The proximal duodenal pressure change was recorded in only one channel in 50% of the periods. The reflux periods in association with two or more duodenal contractions were caused by propagated (42%) and retroperistaltic (58%) duodenal contractions with a mean amplitude of 4.8 (2.4–7.6) kPa.

Relationship between transpyloric emptying and nonoccluding peristaltic contractions.

When nonoccluding peristaltic related flow was seen, this corresponded with pressure waves recorded only in the most distal antral sidehole. The gradients at onset of emptying periods were low. The reflux periods were caused by duodenal contractions with a mean amplitude of 2.4 (2.3–3.5) kPa (Table 3).

Nonperistaltic-related emptying.

Nonperistaltic-related flow sequences could be seen with more alternating emptying–reflux episodes than those associated with peristalsis. The pressure gradients during nonperistaltic-related emptying were significantly lower than during peristaltic-related emptying (nonperistaltic 0.15 (0–0.3) kPa, occluding peristaltic 1.7 (0.2–2) kPa, P < 0.001) (Fig. 3). The reflux periods were related to a duodenal contraction in 75% of the periods, with a mean amplitude of 1.8 (1.3–2.8) kPa. Twenty-five percent of the emptying–reflux–emptying periods were followed by a second reflux period that was related to a duodenal contractionm with a mean amplitude of 3.0 (3–3) kPa (Table 3).


Figure 3.  Antroduodenal pressure gradients (mean [range]) during nonperistaltic-related emptying and peristaltic-related emptying.

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Duration of flow episodes.

In total, the duration of nonperistaltic-related emptying pulses were longer than those seen during peristaltic-related emptying (nonperistaltic 6.5 (3–9) s, occluding peristaltic 4.4 (2–6) s; P=0.05). The reflux episodes were significantly longer during nonperistaltic-related emptying than during peristaltic-related (nonperistaltic 2.6 (2–3) s, peristaltic 2.0 (2–2) s; P < 0.03) (Table 3).


  1. Top of page
  2. Abstract
  8. References

Gastric emptying of a low-calorie liquid meal occurs in sequences of emptying–reflux–emptying pulses. About half of the sequences in our study were peristaltic-related, but both nonoccluding peristaltic-related and nonperistaltic-related emptying sequences also occurred. Nonperistaltic-related flow sequences frequently had more alternating emptying–reflux episodes than those associated with peristalsis, and the duration of nonperistaltic-related emptying and reflux pulses were longer. The pressure gradients for all types of emptying were low and the pressure gradients during nonperistaltic-related emptying were significantly lower than during peristaltic-related emptying.

Transpyloric flow can be classified into flow associated with a local increase in the pressure gradient between the antrum and duodenum (Pa–Pd) due to antral propagating pressure waves, and flow associated with a common cavity pressure difference between the distal antrum and the proximal duodenum, as was observed during nonperistaltic-related flow. The second type of flow is independent of peristalsis and is likely to be caused by changes in gastric tone or by pressure changes outside the stomach, such as aortic pulsation and inspiration.25,26

Flow can only occur in the presence of an open pylorus. The rate of transpyloric flow in time is proportional to (Pa–Pd)/R, where R is pyloric resistance at that time. R is given by μ/D4, where D is the average diameter of the pyloric channel and μ is the gastric fluid viscosity27. Because R is proportional to 1/D4, the rate of transpyloric flow is highly influenced by the pyloric diameter. A large pyloric diameter therefore reflects low pyloric resistance. Consequently, the transpyloric flow can be high in spite of a low pressure gradient between the distal antrum and proximal duodenum (Pa–Pd).

In the present study we found very low antroduodenal pressure gradients (Pa–Pd) and they were significantly lower during nonperistaltic- with peristaltic- related flow. Despite the lower pressure gradients the duration of emptying episodes was significantly longer during nonperistaltic flow, suggesting more emptying during these events. The pyloric channel was therefore probably wide resulting in a low pyloric resistance.

The pressure gradient (Pa–Pd) was significantly higher during antral propagating pressure events than during quiescent periods of antral activity, implying that the pylorus had a higher probability of closure during peristaltic activity in the antrum than during quiescent periods. Unfortunately, it was not possible to measure the pyloric diameter during the Doppler ultrasound because the line indicating the sample volume covered the pyloric channel during recording of transpyloric flow. It was also not possible to assess pyloric pressure reliably using manometry. The sleeve sensor measures pressures over a distance of 6 cm including the distal antrum and proximal duodenum. Furthermore, the pressure recorded from one single side hole does not adequately assess pyloric pressure due to movement of the pylorus and catheter during flow.

Our results show that gastric emptying of a low caloric liquid meal occurs both during peristaltic and nonperistaltic antral activity, probably primarily through a mechanism controlled by the gastric and duodenal tone and by pyloric opening during periods of relative quiescence of the antrum. The findings are consistent with recent observations in dogs,28 and in humans using MRI27.

Most of the duodenogastric reflux episodes were associated with duodenal contractions that were both propagated and retroperistaltic. Castedal et al.29 have recently demonstrated the importance of the duodenum as a retroperistalic pump that can push duodenal contents to the stomach. In the present study duodenal pressures were recorded from four side holes (1.5 cm apart) in the duodenal bulb and the most proximal part of the duodenum. Pressure rise in the most proximal duodenal side hole close to the open pylorus mediates flow in both the antegrade and retrograde direction and may explain why propagated duodenal contractions are associated with duodenogastric reflux.

Duodenogastric refluxes that followed gastric emptying episodes were significantly shorter than the emptying periods despite the higher pressure gradients associated with reflux. For peristaltic-related flow, this can be explained by the fact that the reflux episodes occurred shortly before closure of the pyloric channel,8 during which the pyloric resistance was high. A physiological modulation of the width of the pyloric channel, and thereby the pyloric resistance at the end of a nonperistaltic emptying episode, may similarly explain the relatively shorter reflux episode.

It should be stressed that the results of the present study may be specific to gastric emptying of a low-caloric liquid meal. Stimulation of the duodenum with a higher calorie meal increases the antroduodenal resistance and this seems to be consistent with previous findings of a decrease in pyloric diameter after a nutrient meal.

At present, there is no reliable method to quantify the stroke volumes of transpyloric flow in humans. Based on electromagnetic flowmeter measurements in dogs,12,13 force transducers applied in isolated cat stomachs,14 or continuous collection and weighing of all effluent from the open duodenal cannula in dogs,30,31 the emptying stroke volumes have been found to vary between 0.1 and 75 mL. Using the Doppler technique, flow volume can be estimated by assuming a constant diameter of the human pylorus and calculating the mean velocity within the sample volume averaged over the reflux period. The flow volume of a single gush of duodenogastric reflux of a liquid meal has been estimated to be approximately 1.8 mL.8 However, because the pyloric size, geometric shape, spatial flow profile and Doppler angle vary during the reflux episode, accurate calculation of transpyloric flow is difficult. These limitations of the Doppler technique restrict its clinical applications in calculating transpyloric flow where irregularly shaped flow passage, nonparabolic velocity profiles and ambiguous Doppler angles are expected.

Despite the fact that we set a rather low threshold for detection of pressure events, ultrasound seems to be superior to manometry in recording antral motility during ingestion and the initial postprandial period when using a low-calorie liquid meal. In total, significantly more antral contractions were detected on ultrasound. Perfused manometric systems need lumen-occluding contractions to detect rises in pressure and therefore only pick up very few of the non-lumen-occluding contractions. These results are consistent with previous findings.32 The amplitudes of antral contractions were significantly higher during fasting than during the following ingestion and postprandial period. With regard to the differences between lumen-occlusive and non-lumen occlusive contractions, we found that of the non-lumen-occluding contractions, only 22% were detected on manometry, and that they occurred mostly at the end of the 10-min recording period. Accordingly, the non-lumen-occlusive contractions were probably of lower amplitudes than those associated with lumen-occlusive contractions.

In conclusion, emptying of a low-calorie liquid meal was seen during both peristaltic and nonperistaltic antral activity. In spite of lower antroduodenal pressure-gradients during emptying unrelated to peristalsis, the emptying episodes were longer. An open pylorus and low pyloric resistance may explain considerable transpyloric flow even with a low pressure-gradient across the pylorus. Considerable flow appears to occur without peristalsis during gastric emptying of a low-calorie liquid meal in humans.


  1. Top of page
  2. Abstract
  8. References

This work was supported by Haukeland University Strategic, Research Program and by the Royal Netherlands Academy of Arts and Sciences.


  1. Top of page
  2. Abstract
  8. References
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