Bolus flow and biomechanical properties of the esophageal wall during primary esophageal peristalsis: Effects of bolus viscosity and posture

Abstract

Background:

Studies show that intraluminal impedance recordings of the esophagus allow one to measure the luminal distension during peristalsis, an important parameter for calculation of the biomechanical properties of esophageal wall. The goal was to determine the effect of subject posture and bolus viscosity on the biomechanical properties of esophageal wall, and the rate of bolus flow along the length of the esophagus during primary peristalsis.

Methods:

High-resolution manometry impedance recordings were obtained in 14 normal healthy subjects. Swallows of 10 ml saline and viscous bolus were recorded in the supine and Trendelenburg positions. User identified the region of interest, and a custom-designed software extracted parameters of interest such as bolus flow rate, esophageal wall tension, and esophageal wall distensibility in four equal segments of the esophagus.

Key Results:

Bolus flow rate decreases along the length of the esophagus, being slowest in the distal esophagus. Bolus flow rate is smaller in the Trendelenburg position and with viscous bolus as compared with supine position and saline bolus. Esophageal wall tension is greater in the Trendelenburg position and with viscous bolus as compared with the supine position and saline bolus. The esophageal wall distensibility is larger in the distal as compared with proximal esophagus, which is true for both the saline and viscous bolus.

Conclusions & Inferences:

We report, for the first time, bolus flow rate and biomechanical properties of the esophageal wall during swallow-induced primary peristalsis. Future studies may investigate biomechanical basis of esophageal motility disorders using the methodology described.

1 |. INTRODUCTION

Peristalsis throughout the gastrointestinal tract, including esophagus, consists of two phases, initial inhibition followed by contraction, both of which travel sequentially from the oral to aboral direction.^1–3 Initial inhibition allows esophagus to receive the bolus that results in luminal distension. Studies show that during primary peristalsis, the esophagus distends in the shape of an “American Football” throughout the length of the esophagus as bolus travels from the oral to aboral direction. The latter suggests that like peak contraction in the esophagus, peak distension/relaxation also moves sequentially through the esophagus. Several investigators have studied the effect of bolus volume, bolus viscosity, bolus temperature, and posture on the various parameters (amplitude, duration, and velocity) of the contraction phase of peristalsis. We found that the bolus volume, bolus viscosity and body posture have significant influence on the amplitude of esophageal distension and temporal relationship between luminal distension and contraction phase of primary peristalsis.^4,5 The bolus flow rate (bolus volume/time) in a normal esophagus under physiological conditions and the effects of bolus viscosity and body posture have not been studied before.

Biomechanical properties of the esophageal wall such as wall tension and compliance have an important bearing on the flow of fluid through the esophagus. Manometry,⁶ impedance planimetry,⁷ ultrasound imaging^8,9 and more recently, functional luminal imaging probe (FLIP)¹⁰ have been used to study the biomechanical properties of the esophageal wall. In all of the above techniques, one records the relationship between changes in luminal cross-sectional area and pressure during distension of the esophagus induced by a balloon. The latter, while appropriate under experimental conditions, may not represent the biomechanical properties of esophageal wall under physiological situation of swallow-induced primary peristalsis and bolus transport. One requires parameters of luminal CSA, luminal pressure to determine the biomechanical properties of esophageal wall accurately. Our laboratory has developed and validated the method of measuring luminal CSA from the intraluminal impedance recordings.^11,12 Therefore, for the first time, we have the ability to record intraluminal pressure and luminal CSA values at closely spaced intervals, all along the length of the esophagus using high-resolution manometry impedance (HRMZ) recordings during primary peristalsis. The goal of our study was to measure bolus flow rate and biomechanical properties of esophageal wall along the entire length of esophagus during primary peristalsis in normal healthy subjects.

2 |. METHODS & MATERIALS

2.1 |. Study population

Fourteen healthy subjects (11 female, 3 male), mean age 37 years (range 21– 65) with no history of gastrointestinal disease or surgery were studied. None of these subjects had symptoms pertaining to the esophagus. The study was approved by the Human Research Protection Program of the University of California, San Diego and all subjects signed an informed consent prior to participation in the study (protocol # 091745).

2.2 |. Swallow materials

Two bolus materials were used for this study; (1) saline solution, which was prepared to a concentration of 0.45% NaCl in distilled water (equivalent to 0.5N normal saline). (2) A custom-made novel viscous gel was prepared from a food substitute and diluted with saline to have conductivity of 0.45% NaCl. All solutions were prepared fresh before the study, and warmed to body temperature (37 °C) before ingestion by the subject. Conductivity of all solutions were checked in vitro using an Omega model CDH221 conductivity meter (Omega) prior to each study.

2.3 |. Data acquisition

All subjects were studied using the high-resolution manometry-impedance (HRMZ) catheter, which is 4.2 mm in diameter (Medtronic Inc.). It is equipped with 36 pressure transducers (1 cm apart) and 19 impedance electrodes (2 cm apart). After administration of viscous lidocaine (2% lidocaine hydrochloride topical solution, USP) orally and nasally for local anesthesia, the HRMZ catheter was placed through the nose into the esophagus and stomach. Testing was first performed in the supine position, with the medical stretcher perfectly parallel to the floor, and then in the −15 degrees Trendelenburg position (head down). Eight to ten swallows of 10 ml bolus volumes were performed for the four test conditions; (1) 0.5N saline in supine position (SS), (2) 0.5N saline in Trendelenburg position (ST), (3) 0.5N novel viscous gel in supine position (VS), and (4) 0.5N viscous novel gel in Trendelenburg position (VT). Subjects swallowed 30 s apart and only once each time.

2.4 |. Data analysis

Five swallows for each of the four test conditions were used for analysis from each subject (total of 20 swallows). Only those swallows in which prior and analyzed swallow had resulted in complete bolus clearance (determined by impedance recording) were used for the analysis. The HRMZ data were visualized using the Manoview program (Medtronic Inc.) and DPlots (Motilityviz). The data from the selected swallows were exported as text files. Dplots software is a user-interactive program that automatically extracts numerous features from the multi-channel pressure and impedance signals of the HRMZ recordings. User identified the region of interest in these recording. For each selected swallow, the region of interest comprised the esophageal length (between the lower border of upper esophageal sphincter (UES) and the contraction deacceleration point (CDP) of the swallow-induced peristalsis), and time (between the onset of UES relaxation and return of LES pressure back to the baseline after the swallow induced peristalsis), Figure 1. The parameters of interest for this study and their definitions are as follows; (1) Peak distension time (T1): the time from the swallow-induced onset of UES relaxation to the nadir impedance at each location in the esophagus, (2) Peak contraction time (T2): the time from the onset of UES relaxation to the peak contraction at each location in the esophagus, (3) Amplitude of peak esophageal distension: the maximal luminal CSA during distension phase of peristalsis measured using algorithm published previously,^4,13 (4) Sum of distension values falling inside the distension waveform (same as the area under curve), (5) Esophageal pressure at the instance of peak distension/nadir impedance. For each swallow, the pressure values recorded by each transducer at the peak distension were referenced to the average pressure during 5 s prior to the swallow by that transducer, to account for the sensor pressure drift, if any, (6) Distensibility of the esophagus at the instance of peak distension, cross-sectional area at the peak distension divided by the luminal pressure at peak distension, (7) Wall tension at peak distension, which is luminal radius multiplied by luminal pressure at peak distension, and (8) Bolus flow rate: the ratio of ingested bolus volume (10cc) divided by the duration of distension at each location in the esophagus, and the unit of measurement is cubic centimeters or ml/second. The esophagus between the lower edge of UES and CDP of the peristalsis was divided into 4 equal segments. The CDP is the distal most point of the esophagus as recorded by HRM. Segment one (Seg 1) is the proximal esophagus and segment 4 (Seg 4) is the distal most esophageal segment.

FIGURE 1.

Distension-contraction plots of the pressure (green) and CSA (purple) displayed as waveforms in this figure: (A) saline in the supine position, (B) viscous gel in the supine position, (C) saline in the TB position, and (D) gel in the TB position of a normal subject. ROI is shown as white-colored rectangular dashed lines. Note that the amplitude of distension in the supine position is smaller compared with the viscous bolus. Distension waveform is closer to the pressure waveform with gel bolus as compared with saline. Also displayed are the color topography of the pressures during peristalsis adjacent to the distension contraction waveforms

2.5 |. Statistical analysis

Quantitative data are shown as mean ± standard error of the mean (SEM), unless otherwise stated. Shapiro-Wilk test was used to verify normality of data. Levene test was applied to assess the equality of variances. Data were analyzed by two-way repeated-measures ANOVA, with liquid (saline bolus/viscous bolus) and posture (supine/Trendelenburg) as the within-subjects factors. When the main effect of either liquid bolus or posture was significant, post-hoc paired t-tests (among measurements for each liquid type or between postures) was performed. A p-value <0.05 was deemed statistically significant.

3 |. RESULTS

Figure 1 shows the distension contraction waveforms of a single swallow of 10 ml saline in the supine (SS) and Trendelenburg (ST) positions, and with viscous gel bolus in the supine (GS) and Trendelenburg (GT) positions. Figure 2 shows a schematic of the contraction (as pressure heat map) and luminal distension (as esophageal diameter) along the length of the esophagus at various time points during a single swallow. Figure 2 schematic is for the same four iterations shown in Figure 1. All extracted parameters for each swallow are shown in Figure 3 for two normal subjects. Important features of the distension waveforms are that (1) the duration of luminal distension waveform increases from the proximal to distal direction in the esophagus, for both saline and viscous boluses. The above is also true for both supine and Trendelenburg positions. However, there are important differences, especially in the distal esophagus between bolus types and posture. With saline bolus, the duration of distension in the distal esophagus is smaller in the Trendelenburg position as compared with the supine position. The duration of distension in the distal esophagus is smaller with viscous as compared with saline bolus.

FIGURE 2.

Schematic of distension contraction patterns along the length of the esophagus in one subject, (A) saline in the supine position, (B) viscous bolus in the supine position, (C) saline in the TB position, and (D) viscous bolus in the TB position. Note the differences in the temporal relationship between contraction and distension

FIGURE 3.

Radar chart of eight parameters (i.e., Peak distension time, Peak contraction time, Amplitude of peak distension (CSA), Sum of distension, Pressure at peak distension, Distensibility (DI) at peak distension, Tension at peak distension, and Bolus flow) in two normal subjects under four different test conditions, in four segments of the esophagus, (seg1 is just distal to upper esophageal sphincter and seg4 is most distal)

The saline bolus arrives in the distal esophagus, faster in the supine as compared with the Trendelenburg position (shorter T1). On the other hand, viscous bolus in the supine position travels slowly through the esophagus, in close temporal correlation with the contraction phase of peristalsis (longer T1). The time between the onset of swallow and peak contraction (T2) in the distal esophagus is longer with the viscous bolus as compared with the saline bolus. The amplitude of distension in the distal esophagus is greater with saline bolus in the Trendelenburg position as compared with supine position. On the other hand, with viscous bolus, the amplitude of distension in the distal esophagus was similar for the supine and Trendelenburg positions. Summary of the data on the effect of bolus viscosity and posture, on the amplitude of distension, sum of distension, T1 and T2 are shown in Figures 4 and 5. Detailed per-segment analysis of all features is provided in the Appendix.

FIGURE 4.

Forest plot of the mean and the 95% CI of (A) peak distension time, (B) amplitude of distension, (C) sum of distension, (D) distensibility at peak distension, (E) peak pressure time, (F) Peak contraction pressure, (H) tension at peak distension, and (F) bolus flow

FIGURE 5.

Statistical significance of the parameters of Figure 4 by posture type (supine and TB) and liquid type (saline and viscous gel). For example, in segment 1, the parameter bolus flow had a significant main effect for both liquid and posture. However, in segment 4, bolus flow was only significant by liquid type and not posture

Bolus type and posture have significant influence on the peak distension pressure in the esophagus. It is higher in the Trendelenburg position with the saline bolus as compared with the supine position, both in the proximal and in the distal esophagus. Bolus pressure is higher with the viscous compared with saline bolus in all 4 segments of the esophagus. Posture has significant effects on the bolus pressure in the proximal and distal segments of the esophagus (See Figures 4 and 5).

3.1 |. Effect of viscosity and posture on the bolus flow rate

There is a significant difference in bolus flow rate, both for bolus type and posture in segments 1 and 2, but the interaction between the two is not significant. Viscous bolus swallows had a lower flow rate than the saline (Figure 6). On the other hand, the flow rate is greater in the supine posture as compared with the Trendelenburg position. In segment 3, results of the two-way repeated-measures ANOVA revealed that there was no significant effect of bolus type on flow rate (p > 0.05) or body posture, saline bolus (2.5 ± 0.2 ml/s, 95% CI = 2.126 to 2.839), compared with gel bolus (2.4 ± 0.2 ml/s, 95% CI = 2.096 to 2.746). In segment 4, the flow rate is affected by the bolus type (f(1,12) = 23.91, p < 0.001): saline bolus (2.0 ± 1.0 ml/s, 95% CI = 1.775 to 2.203) flows at a lower rate compared with gel bolus (2.7 ± 0.2 ml/s, 95% CI = 2.3 to 3.1).

FIGURE 6.

Bolus flow across 4 segments of the esophagus in 6 normal subjects under 4 test conditions, Segment 1 is proximal esophagus (just below the UES) and segment 4 is the most distal esophagus (above LES)

3.2 |. Esophageal wall tension at peak distension

For segments 1 and 2, significant difference exists in the tension values at peak distension for two bolus types, although the interaction between the two is not significant (Figure 7). Saline swallows had lower tension values at peak distension compared with the gel bolus. Trendelenburg position has greater tension values compared with their supine counterpart. In segment 3, statistically significant difference in peak tension at peak distension was found for only posture type (f(1,13) = 6.619, p = 0.023). Swallows in the Trendelenburg position (10.3 ± 0.8 N/m, 95% CI = 8.5 to 12.2) had higher tension values compared with supine (6.2 ± 0.5 N/m, 95% CI = 5.3 to 7.2). For segment 4, significant difference was found between liquid type (f(1,13) = 47.4, p < 0.001) and posture (f(1,13) = 45.6, p < 0.001), although the interaction between these terms was not significant (f(1,13) = 0.086, p = 0.774). Saline swallows had lower tension values at peak distension (9.2 ± 0.9 N/m, 95% CI = 7.3 to 11.2) compared with the gel bolus (13.2 ± 1.3 N/m, 95% CI = 10.5 to 16.0). Swallows in the TB position had greater tension values (12.7 ± 1.2 N/m, 95% CI = 10.1 to 15.3) compared with their supine counterpart (9.8 ± 1.0 N/m, 95% CI = 7.7 to 11.8).

FIGURE 7.

Tension topographs of 10cc swallows of (A) saline in the supine position, (B) gel bolus in the supine position, (C) saline in the TB position and (D) gel in the TB position of a normal subject

3.3 |. Esophageal wall distensibility at peak distension

In segments 1, 2 and 3, the results of the two-way repeated-measures ANOVA revealed that there was no significant effect of liquid type and body posture on the distensibility values, and no significant interaction between the two (Figures 4, 5 and 8). On the other hand, in segment 4, there was a significant difference in the distensibility values, by both liquid type (f(1,13) = 5.351, p = 0.038), and posture (f(1,13) = 23.916, p < 0.001), but not their interaction (f(1,13) = .0001, p = 0.991). Saline swallows had higher distensi-bility values at peak distension (20 ± 2.3 mmHg, 95% CI = 14.9 to 24.7) compared with the gel bolus (16.5 ± 1.6 mm²/mmHg, 95% CI = 13.1 to 19.9), independent of posture. Swallows during supine (20.1 ± 2.1 mm²/mmHg, 95% CI = 15.6 to 24.7) also had higher distensibility values compared with the Trendelenburg position (16.1 ± 1.6 mm²/mmHg, 95% CI = 12.7 to 19.6), independent of liquid type.

FIGURE 8.

Distensibility at peak distension surface heatmaps of 10cc swallows of (A) saline in the supine position, (B) gel bolus in the supine position, (C) saline in the TB position and (D) gel in the TB position of a normal subject

4 |. DISCUSSION

Our previously published studies described the methodology to measure luminal CSA from the intraluminal impedance measurements^11,12 and the effects of posture and bolus viscosity on the distension contraction profiles of swallow-induced peristalsis in normal asymptomatic subjects.^4,5 These studies show that the posture and bolus viscosity have significant effects on the distension and contraction phase of peristalsis. The novel aspect of our present study is that we measured bolus flow rate along the length of the esophagus during primary peristalsis and the effects of posture and viscosity on bolus flow. Distinction should be made between bolus flow rate measured in cubic centimeters or ml/s and bolus velocity that is measured in centimeters per second. The later was not measured, but it can be derived from T1; a shorter T1 implies greater bolus velocity. We also measured the biomechanical properties of esophageal wall, that is, wall tension and distensibility using the luminal CSA and pressure measured by the HRMZ recordings. We studied the effects of posture and bolus viscosity on the esophageal wall tension and distensibility along the length of the esophagus during swallow-induced peristalsis. We focused on the above biomechanical properties during the distension phase of peristalsis and not the contraction phase because (1) luminal diameter is constant during the contraction phase of peristalsis (equal to the diameter of the manometry probe) and therefore pressure alone is a good measure of the wall tension; (2) it is likely that abnormalities in the distension phase of peristalsis, and not the contraction phase, are more likely to be responsible for the genesis of esophageal symptoms such as dysphagia and esophageal pain.¹⁴

The flow in a cylindrical tube can be measured using Poiseuille equation, according to which, the flow rate (Q) depends on fluid viscosity (η), pipe length (L), and the pressure difference between the ends (P) of tube, using the equation Q = πPr⁴/8ηL. We used a simpler approach to measure the flow rate using the assumption that the 10 ml swallowed bolus enters and leaves each location in the esophagus in entirety and both the entry and complete bolus clearance are detected precisely by the impedance recording. Therefore, 10ml divided by the duration of luminal distension is the average flow rate (ml/s) during the period of luminal distension at each location along the length of the esophagus. Hence, duration of luminal distension is an indirect measure of the flow rate at each location in the esophagus. The duration of luminal distension is shorter in the proximal as compared with distal esophagus, and hence, the flow rates is higher in the proximal esophagus. Gravity (posture) and viscosity will be expected to reduce the flow rate which is what we found.

The ability to measure luminal CSA during primary peristalsis allowed us to measure biomechanical properties of the esophageal wall during primary peristalsis for the first time. Earlier studies used impedance planimetry⁷ and FLIP¹⁰ to measure the biomechanical properties of the esophageal wall during balloon-induced distension of the esophagus. These studies show that the distal esophagus is more distensible, and has a higher distensibility than the proximal esophagus.¹⁵ We found the above to be true in our current study of primary peristalsis. Greater distensibility of the distal as compared with proximal esophagus may be explained on the basis of differences in the biomechanical properties of skeletal versus smooth muscles present in the proximal and distal esophagus, respectively. Another relevant factor is that the distal or the smooth muscles of esophagus have greater inhibitory or nitrinergic innervation than the proximal esophagus and the gradient of inhibitory innervation increases distally.^16,17

As intraluminal bolus pressure and luminal CSA are higher in the distal as compared with proximal esophagus, it results in greater wall tension in the distal as compared with proximal esophagus. In the case of saline bolus, the Trendelenburg position increases wall tension in the distal esophagus because it results in an increase in the bolus pressure and the luminal CSA in the distal esophagus. On the other hand, with viscous bolus in the supine position, even though the luminal CSA is not different from the saline bolus in the Trendelenburg in the distal esophagus, the wall tension is higher with the viscous bolus because of a greater luminal pressure. Hence, differences in the wall tension under different conditions may be related to either changes in the luminal CSA or intraluminal pressure or both. Measuring bolus flow rate and biomechanical properties of the esophageal wall is important because it can provide a pathophysiologic basis for the symptoms of dysphagia and esophageal pain. Esophagus is basically a conduit to transfer oropharyngeal contents from the pharynx into the esophagus. The physical principles such as pressure gradients and biomechanical properties can predict efficient/inefficient passage of bolus through the conduit. For example, we recently described that the velocity of bolus flow (not the flow rate) is faster in patients with nutcracker esophagus,¹³ functional dysphagia,¹⁸ and esophagogastric junction outflow obstruction (EGJOO), even though these patients have smaller luminal CSA or a narrower esophagus. One may wonder how the same bolus volume, that is, 10 ml bolus can enter and leave the esophagus completely in the same time duration when the esophagus is narrower in patients as compared with normal subjects. The above can be predicted based on the physical principle of the relationship between the velocity of bolus flow, bolus flow rate, and the diameter of the tube. For a given driving force (pharyngeal pump), a 10-ml bolus will travel faster through a narrower tube as compared with a wider tube (a shorter T1). In other words, even though the esophagus is narrower in patients, an increase in the flow rate can compensate and clear the bolus in the same time duration as otherwise. Many studies have shown abnormalities of the biomechanical properties of the esophageal wall in patients with dysphagia related to esophageal motility disorders⁷ and eosinophilic esophagitis (EOE).¹⁰

Costa et al., based on the relationship between pressure and luminal diameter, described 12 possible neuromechanical states of the esophageal wall during peristalsis.¹⁹ These neuromechanical states may have relevance to the genesis of ascending contraction and descending relaxation phases of peristalsis. Lin et al. described four phases of bolus transit through the esophagus and biomechanical properties of esophageal wall during peristalsis using simultaneous high-resolution manometry impedance and fluoroscopy.²⁰ Esophageal diameter was measured in their studies using fluoroscopic images. They speculated that various neuromechanical states may have relevance to the genesis of esophageal motility disorders and symptoms in patients with esophageal motility disorders. Cock et al. found differences in certain neuromechanical states of the esophageal wall among normal subjects with and without conscious awareness of the swallowed solid bolus.¹⁴ The latter was more often seen in the presence of higher bolus pressure and larger luminal CSA, the two parameters that determine wall tension. Tension receptors located on the vagal and spinal afferent nerve ending in the esophageal wall are important for eliciting physiologic reflexes that mediate peristalsis and also pain (nociception). A greater wall tension is likely to be important in the genesis of esophageal pain and possibly pulsion diverticula of the esophagus. High bolus pressure in the presence of narrow luminal area is indicative of a low compliance/distensibility of the esophageal wall, which has relevance to bolus flow through the esophagus. Compliance or the distensibility of the esophageal wall is lower in patients with EOE as compared with healthy subjects.¹⁰ Our methodology allows one to study bolus flow rate, bolus velocity, esophageal wall tension, and compliance under physiologic condition of swallow-induced primary peristalsis. The HRMZ studies are performed routinely in clinical practice to assess esophageal motility and bolus clearance. Using minor modification of the recording protocol and computer software such as DPlots, future studies can determine if these biomechanical properties of esophageal wall are important in the genesis of esophageal symptoms.

The CDP, an easily identifiable landmark in HRMZ recording in normal subjects, represents the distal end of the esophagus. The vestibule, seen only during primary peristalsis, is located below the CDP and may represent a stretched lower esophageal sphincter. We used CDP as the distal most location for our analysis, and therefore did not study flow across the LES and esophagogastric junction in our measurements, which can be considered as a limitation of our study. It is possible, however, that vestibule represents the proximal or cardia of the stomach and not a stretched LES. If the above were true, our methodology to measure luminal CSA may not be valid in the vestibule because stomach is lined with columnar mucosa, which has different baseline impedance values (required in our algorithm to measure luminal CSA) than the squamous-lined esophagus. Future studies need to clarify the true nature of vestibule to measure its biomechanical properties and bolus flow in the vestibule.

Key Points.

• Biomechanical properties of the esophageal wall have important bearing on the flow of fluid through the esophagus. We measured bolus flow rate and biomechanical properties of esophageal wall during primary peristalsis.

• Results indicate that bolus flow rate decreases along the length of the esophagus, affected by subject posture and viscosity of the bolus.

• Abnormalities in the distension phase of peristalsis are likely to be responsible for the genesis of esophageal symptoms such as dysphagia and esophageal pain.

APPENDIX

DETAILED RESULTS

The results of the two-way repeated-measures ANOVA revealed that:

Peak pressure time

In segment 1, we found a statistically significant difference in average Peak Pressure Times (T2), only by Posture (f(1,13) = 32.938, p < 0.001). Supine swallows (3.497 ± 0.091 s, 95% CI = 3.301 to 3.693) had faster times compared with the gel bolus (3.84 ± 0.128 s, 95% CI = 3.563 to 4.118).

In segment 2, similar to segment 1, there was a statistically significant difference in average Peak Pressure Times (T2), only by Posture (f(1,13) = 44.465, p < 0.001). Supine swallows (5.474 ± 0.127 s, 95% CI = 5.199 to 5.749) had faster times compared with the gel bolus (6.049 ± 0.163 s, 95% CI = 5.697 to 6.401).

In segment 3, similar to the two previous segments, there was a statistically significant difference in average Peak Pressure Times(T2), only by Posture (f(1,13) = 28.166, p < 0.001). Supine swallows (6.788 ± 0.165 s, 95% CI = 6.43 to 7.145) had faster times compared with the gel bolus (7.617 ± 0.271 s, 95% CI = 7.032 to 8.202).

Finally, in segment 4, similar to the three previous segments, there was a statistically significant difference in average Peak Pressure Times (T2), only by Posture (f(1,13) = 24.845, p < 0.001). Supine swallows (8.336 ± 0.224 s, 95% CI = 7.852 to 8.819) had faster times compared with the gel bolus (9.357 ± 0.35 s, 95% CI = 8.602 to 10.113).

Peak distention time

In segment 1, we found a statistically significant difference in average Peak Distension Times (T1), by Liquid type (f(1,13) = 35.71, p < 0.001). Saline swallows (1.628 ± 0.063 s, 95% CI = 1.491 to 1.765) had faster times compared with the gel bolus (2.042 ± 0.0858 s, 95% CI = 1.86 to 2.227).

In segment 2, we found a statistically significant difference in average Peak Distension Times, by Liquid type (f(1,13 = 137.713, p < 0.001), and by Posture (f(1,13) = 7.194, p = 0.019), although the interaction between these terms was not significant (f(1,13) = 3.611, p = 0.08). Saline swallows (2.748 ± 0.115 s, 95% CI = 2.501 to 2.996) had faster T1 arrival times compared with the gel bolus (3.043 ± 0.125 s, 95% CI = 2.773 to 3.313). Moreover, supine postures (2.212 ± .063, 95% CI = 2.075 to 2.349) had faster arrival times compared with TB (3.579 ± .16, 95% CI = 3.235 to 3.924).

In segment 3, we found a statistically significant difference in average Peak Distension Times, by Liquid type (f(1,13) = 132.724, p < 0.001), and by Posture (f(1,13) = 9.053, p = 0.010), with significant interaction between these terms (f(1,13) = 14.671, p = 0.002). Saline swallows (3.765 ± 0.186 s, 95% CI = 3.364 to 4.166) had faster T1 arrival times compared with the gel bolus (5.404 ± 0.16 s, 95% CI = 5.059 to 5.750). Post-hoc comparisons revealed that supine swallows (4.346 ± 0.199 s, 95% CI = 3.915 to 4.776) had faster T1 arrival times compared with the Trendelenburg position (4.824 ± 0.151 s, 95% CI = 4.497 to 5.151). Moreover, there was a significant interaction between Liquid Type and Posture such that subjects T1s were fastest firstly, when swallowing saline in the supine position compared with saline in the TB position (p = 0.01), and secondly, saline swallows compared with bolus gel, independent of posture (p < 0.001).

In segment 4, the pattern was similar to the previous segment; we found a statistically significant difference in average Peak Distension Times by Liquid type (f(1,13) = 74.975, p < 0.001) and by Posture (f(1,13) = 11.189, p = 0.005), with significant interaction between these terms (f(1,13) = 13.732, p = 0.003). Saline swallows (5.199 ± 0.242 s, 95% CI = 4.675 to 5.722) had faster T1 arrival times compared with the gel bolus (6.841 ± 0.22 s, 95% CI = 6.366 to 7.317). Post-hoc results show that supine swallows (5.74 ± 0.232 s, 95% CI = 5.239 to 6.24) had faster T1 arrival times compared with the Trendelenburg position (6.3 ± 0.222 s, 95% CI = 5.82 to 6.781). Moreover, there was a significant interaction between Liquid Type and Posture such that subjects T1s were fastest firstly, when swallowing saline in the supine position compared with saline in the TB position (p < 0.001), and secondly, saline swallows compared with bolus gel, independent of posture(p < 0.001).

Peak distension

In segment 1, we found a statistically significant difference in average Peak Distension values both by Liquid type (f(1,13) = 107.929, p < 0.001) and by Posture (f(1,13) = 40.731, p < 0.001), although the interaction between these terms was not significant (f(1,13) = 3.601, p = 0.08). Saline swallows had lower peak distension values (85.149 ± 4.421 mm², 95% CI = 75.598 to 94.701) compared with the gel bolus (138.263 ± 3.783 mm², 95% CI = 130.091 to 146.436). Swallows in the TB position were greater in magnitude (119.16 ± 8.59 mm², 95% CI = 110.823 to 127.498) compared with their supine counterpart (104.252 ± 2.336 mm², 95% CI = 97.908 to 110.596).

In segment 2, similar to segment 1, we found a statistically significant difference in average Peak Distension values both by Liquid type (f(1,13) = 135.518, p < 0.001) and by Posture (f(1,13) = 13.18, p = 0.003), although the interaction between these terms was not significant (f(1,13) = 4.234, p = 0.06). Saline swallows had lower peak distension values (98.157 ± 4.296 mm², 95% CI = 88.877 to 107.437) compared with the gel bolus (140.795 ± 4.204 mm², 95% CI = 131.713 to 149.877). Swallows in the TB position were greater in magnitude (113.045 ± 3.521 mm², 95% CI = 105.439 to 120.651) compared with their supine counterpart (125.907 ± 4.827 mm², 95% CI = 115.48 to 136.334).

In segment 3, similar to segments 1 and 2, we found a statistically significant difference in average Peak Distension values both by Liquid type (f(1,13) = 69.196, p < 0.001) and by Posture (f(1,13) = 9.968, p = 0.008), although the interaction between these terms was not significant (f(1,13) = 3.977, p = 0.068). Saline swallows had lower peak distension values (117.37 ± 4.835 mm², 95% CI = 106.925 to 127.816) compared with the gel bolus (149.185 ± 4.245 mm², 95% CI = 140.014 to 158.356). Swallows in the supine position were smaller in magnitude (128.009 ± 4.347 mm², 95% CI = 118.618 to 137.399) compared with their TB counterpart (138.547 ± 4.556 mm², 95% CI = 128.703 to 148.39).

In segment 4, the results of the two-way repeated-measures ANOVA revealed that there was no significant main effect of Liquid type on the subjects’ peak distension (f(1,13) = 4.395, p = 0.056). They performed similarly using saline swallows (167.909 ± 6.152 mm², 95% CI = 154.62 to 181.199) and gel swallows (182.141 ± 5.683, 95% CI = 169.864 to 194.419). In contrast, there was a significant main effect of body Posture on the subjects’ peak distension values (f(1,13) = 16.549, p = 0.001) such that normal subjects had higher distensions in the TB position (183.843 ± 5.793 mm², %95 = 171.327 to 196.359) as compared with supine (166.208 ± 4.788 mm², %95 CI = 155.863 to 176.552).

Distension sum

In segment 1, we found a statistically significant difference in average Sum of Distension by Liquid type (f(1,13) = 56.214, p < 0.001). Saline swallows (563.771 ± 31.677 mm², 95% CI = 495.338 to 632.204) had lower values compared with the gel bolus (794.887 ± 48.809 mm², 95% CI = 689.442 to 900.332).

In segment 2, we found a statistically significant difference in average Sum of Distension values both by Liquid type (f(1,13) = 252.969, p < 0.001) and by Posture (f(1,13) = 9.598, p < 0.008), although the interaction between these terms was not significant (f(1,13) = 2.163, p = 0.165). Saline swallows had lower distension sum values (713.13 ± 35.797 mm², 95% CI = 635.795 to 790.466) compared with the gel bolus (1047.621 ± 43.065 mm², 95% CI = 954.584 to 1140.658). Swallows in the TB position were greater in sum magnitude (924.669 ± 46.063 mm², 95% CI = 825.156 to 1024.182), compared with their supine counterpart (836.082 ± 34.669 mm², 95% CI = 761.184 to 910.979).

In segment 3, we found a statistically significant difference in average Sum of Distension by Liquid type (f(1,13) = 79.915, p < 0.001). Saline swallows (863.073 ± 44.313 mm², 95% CI = 767.341 to 958.806) had lower values compared with the gel bolus (1112.848 ± 42.289 mm², 95% CI = 1021.489 to 1204.207).

In segment 4, the results of the two-way repeated-measures ANOVA revealed that there was no significant main effect of liquid type on participants’ Sum of Distension values (f(1,13) = .006, p > 0.05) or body posture (f(1,13) = 2.852, p > 0.05).

Pressure at peak distension

In segment 1, we found a statistically significant difference in average Pressure at Peak Distension both by Liquid type (f(1,13) = 61.758, p < 0.001) and by Posture (f(1,13) = 11.529, p = 0.005), although the interaction between these terms was not significant (f(1,13) = 3.599, p = 0.08). Saline swallows had lower pressure values at peak distension (9.961 ± .896 mmHg, 95% CI = 8.025 to 11.896) compared with the gel bolus (15.617 ± 1.33 mmHg, 95% CI = 12.745 to 18.49). Swallows in the TB position had greater pressure magnitudes (13.955 ± 1.226 mmHg, 95% CI = 11.306 to 16.603) compared with their supine counterpart (11.623 ± 1.022 mmHg, 95% CI = 9.416 to 13.831).

In segment 2, we found a statistically significant difference in average pressure at peak Distension, by Liquid type (f(1,13) = 19.678, p < 0.001). Saline swallows (8.447 ± .681 mmHg, 95% CI = 76.976 to 9.917) had lower values compared with the gel bolus (11.593 ± .944 mmHg, 95% CI = 9.553 to 13.634).

In segment 3, similar to segment 2, there was a statistically significant difference in average pressure at peak Distension by Liquid type (f(1,13) = 22.835, p < 0.001). Saline swallows (6.42 ± .599 mmHg, 95% CI = 5.125 to 7.716) had lower values compared with the gel bolus (9.727 ± .789 mmHg, 95% CI = 8.023 to 11.43).

Finally, in segment 4, we found a statistically significant difference in average pressure at peak Distension values both by Liquid type (f(1,13) = 42.377, p < 0.001) and by Posture (f(1,13) = 45.219, p < 0.001), although the interaction between these terms was not significant (f(1,13) = 0.041, p = 0.843). Saline swallows had lower pressure values at peak distension (9.938 ± 0.925 mmHg, 95% CI = 7.94 to 11.936) compared with the gel bolus (12.458 ± 1.1 mmHg, 95% CI = 10.081 to 14.834). Swallows in the TB position were greater in pressure values (12.912 ± 1.154 mmHg, 95% CI = 10.419 to 115.404) compared with their supine counterpart (9.484 ± .888 mmHg, 95% CI = 7.565 to 11.403).

Esophageal wall distensibility at peak distension

In segment 1, the results of the two-way repeated-measures ANOVA revealed that there was no significant main effect of liquid type on participants’ distensibility values (f(1,13) = .524, p = 0.482) or body posture (f(1,13) 1.271, p = 0.28) nor their interaction (f(1,13) = 0.084, p = 0.776).

In segment 2 the pattern was similar to segment 1, and there was no significant main effect of liquid type on participants’ distensibility values (f(1,13) = 0.245, p = 0.629), but not body posture (f(1,13) = 1.254, p = 0.283) or their interaction (f(1,12) = 2.272, p = 0.156).

In segment 3, the results of the two-way repeated-measures ANOVA revealed that there was no significant main effect of liquid type on participants’ Distensibility values (f(1,13) = 4.064, p = 0.065) or body posture (f(1,13) = 0.119, p = 0.736), or their interaction (f(1,13) = 0.008, p = 0.932).

Finally, in segment 4, we found a statistically significant difference in Distensibility during distension both by Liquid type (f(1,13) = 5.351, p = 0.038) and by posture (f(1,13) = 23.916, p < 0.001), but not their interaction (f(1,13) = 0.0001, p = 0.991). Saline swallows had higher distensibility values at peak distension (19.785 ± 2.254 mmHg, 95% CI = 14.916 to 24.655) compared with the gel bolus (16.504 ± 1.554 mm²/mmHg, 95% CI = 13.147 to 19.86) independent of posture. Swallows during supine (20.144 ± 2.082 mm²/mmHg, 95% CI = 15.647 to 24.641) also had higher distensibility values compared with the Trendelenburg position (16.145 ± 1.578 mm²/mmHg, 95% CI = 12.736 to 19.554) independent of liquid type.

Esophageal wall tension at peak distension

In segment 1, we found a statistically significant difference in average Peak Tension at Peak Distension both by Liquid type (f(1,13) = 20.754, p < 0.001) and by Posture (f(1,13) = 83.523, p<0.001), although the interaction between these terms was not significant (f(1,13) = 3.024, p = 0.189). Saline swallows had lower tension values at peak distension (9.086 ± .812 N/m, 95% CI = 7.331 to 10.841) compared with the gel bolus (11.656 ± 1.062 N/m, 95% CI = 9.362 to 13.951). Swallows in the TB position had greater tension values (13.824 ± 1.217 N/m, 95% CI = 11.196 to 16.453) compared with their supine counterpart (6.918 ± .659 N/m, 95% CI = 5.494 to 8.342).

In segment 2, we found a statistically significant difference in average Peak Tension at Peak Distension both by Liquid type (f(1,13) = 6.619, p = 0.023) and by Posture (f(1,13) = 49.738, p < 0.001), although the interaction between these terms was not significant (f(1,13) = 0.773, p = 0.395). Saline swallows had lower tension values at peak distens ion (7.654 ± 71 N/m, 95% CI = 6.121 to 9.187) compared with the gel bolus (8.9222 ± .606 N/m, 95% CI = 7.613 to 10.23). Swallows in the TB position had greater tension values (10.341 ± 0.844 N/m, 95% CI = 8.519 to 12.164) compared with their supine counterpart (6.235 ± 0.455 N/m, 95% CI = 5.252 to 7.217).

In segment 3, we found a statistically significant difference in average peak Tension at Peak Distension not by Liquid type (f(1,13) = 0.053, p = 0.821) but by Posture (f(1,13) = 34.395, p < 0.001). Moreover, the interaction between these terms was not significant (f(1,13) = .478, p = 0.502). Swallows in the TB position had greater tension values (8.961 ± 0.76 N/m, 95% CI = 7.32 to 10.603) compared with their supine counterpart (5.283 ± 0.543 N/m, 95% CI = 4.111 to 6.455).

Finally, in segment 4, we found a statistically significant difference in average peak Tension at Peak Distension both by Liquid type (f(1,13) = 47.373, p < 0.001) and by Posture (f(1,13) = 45.589, p < 0.001), although the interaction between these terms was not significant (f(1,13) = .086, p = 0.774). Saline swallows had lower tension values at peak distension (9.217 ± 0.908 N/m, 95% CI = 7.256 to 11.178) compared with the gel bolus (13.233 ± 1.28 N/m, 95% CI = 10.468 to 15.997). Swallows in the TB position had greater tension values (12.7 ± 1.211 N/m, 95% CI = 10.085 to 15.316) compared with their supine counterpart (9.749 ± .96 N/m, 95% CI = 7.676 to 11.823).

Bolus flow

In segment 1, we found a statistically significant difference in average Bolus Flow Rate both by Liquid type (f(1,13) = 13.239, p = 0.003) and by Posture (f(1,13) = 11.859, p = 0.004), although the interaction between these terms was not significant (f(1,13) = 1.412, p = 0.256). Gel bolus swallows had lower flow rates (4.837 ± .203 ml/s, 95% CI = 4.399 to 5.276) compared with the saline bolus (6.725 ± .45 ml/s, 95% CI = 5.752 to 7.698). Swallows in the supine position had greater flow rates (6.356 ± .312 ml/s, 95% CI = 5.683 to 7.030) compared with their TB counterpart (5.206 ± .261 ml/s, 95% CI = 4.643 to 5.769).

In segment 2, similar to segment 1, we found a statistically significant difference in the average Bolus Flow Rate both by Liquid type (f(1,13) = 9.6, p = 0.008) and by Posture (f(1,13) = 7.685, p = 0.016), although the interaction between these terms was not significant (f(1,13) = 2.797, p = 0.118). Gel bolus swallows had lower flow rates (2.79 ± .145 ml/s, 95% CI = 2.476 to 3.103) compared with the saline bolus (3.814 ± .354 ml/s, 95% CI = 3.05 to 4.579). Swallows in the supine position had greater flow rates (3.702 ± .334 ml/s, 95% CI = 2.979 to 4.425) compared with their TB counterpart (2.902 ± .146 ml/s, 95% CI = 2.586 to 3.218).

In segment 3, results of the two-way repeated-measures ANOVA revealed that there was no significant main effect of liquid type on participants’ flow rate (p > 0.05) or body posture in any of the four segments (p > 0.05), with mean saline swallows (2.482 ± 0.165 ml/s, 95% CI = 2.126 to 2.839) compared with gel bolus (2.421 ± 0.15 ml/s, 95% CI = 2.096 to 2.746).

In segment 4, we found a statistically significant difference in the flow rate by Liquid type (f(1,12) = 23.91, p < 0.001). This time, saline swallows (1.989 ± .098 ml/s, 95% CI = 1.775 to 2.203) had lower flow rate values compared with the gel bolus (2.702 ± .171ml/s, 95% CI = 2.329 to 3.075).