SUBJECTS

Studies were performed in 10 healthy subjects (eight males, two females) aged 19–39 years (median 25). Subjects were free of gastrointestinal symptoms, had no history of upper gastrointestinal surgery, and were not taking regular antacids or medications known to influence oesophageal or gastric motor function. Each subject gave written informed consent and the protocol was approved by the Research Ethics Committee of the Royal Adelaide Hospital.

RECORDING METHODS

Oesophageal motility was measured using a multilumen manometric assembly with an outer diameter of 4.2 mm (fig 1). LOS pressure was measured with a reverse perfused sleeve sensor.15 Gastric pressure was recorded using a side hole located 10 mm distal to the sleeve sensor. Side holes, spaced at 30 mm intervals starting at the proximal margin of the sleeve sensor, monitored pressure at seven sites along the oesophageal body. A side hole in the pharynx recorded swallowing. The oesophageal and pharyngeal side holes and sleeve sensor were perfused with degassed distilled water at 0.15 ml/min and the gastric side hole at 0.08 ml/min.

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Figure 1

Diagrammatic illustration of the combined manometric and barostat assembly (left) and a cross sectional view of the assembly (right).

Proximal gastric function was measured with an electronic barostat (Distender Series II G&J Electronics Inc., Willowdale, Ontario, Canada) which consisted of a rigid cylinder that introduced or withdrew air from a polyethylene bag positioned in the proximal stomach. The lumina used by the barostat for delivery of air into the bag and sensing of pressure within the bag were incorporated within the manometric assembly. The polyethylene bag had a capacity of approximately 1100 ml. The proximal portion of the polyethylene bag was tied to the manometric assembly 60 mm distal to the sleeve sensor. The cylinder introduced or withdrew air from the bag at 30 ml per second via an oval channel measuring 1.9×2.4 mm internal diameter (ID) and 1570 mm in length. Pressure in the bag was sensed via a lumen of 0.6 mm ID that opened directly into the bag.

Oesophageal pH was measured with an antimony electrode (Synectics Medical AB, Stockholm, Sweden) positioned 5 cm above the proximal margin of the LOS.

Data were recorded on a personal computer (PowerPC 7100, Apple Computer Inc., Cupertino, California, USA). Manometric data were digitised at 10 Hz using a NBMI016 A-D board (National Instruments, Austin, Texas, USA). Barostat data were acquired at 1 Hz via a serial interface. A custom written program (G Hebbard) using Labview (National Instruments) controlled the barostat and acquired both manometric and barostat data. Data were imported into AcqKnowledge (Biopac Systems, Santa Barbara, California, USA) for subsequent analysis.

ASSESSMENT OF SYMPTOMS

Perceptions of sensations of fullness, nausea, abdominal discomfort, and hunger were quantified using validated 100 mm visual analogue scales.16

STUDY PROTOCOL

Subjects were studied after an overnight fast. A cannula was inserted into the forearm for administration of saline or atropine. The manometric assembly, with the barostat bag folded around it, and the pH electrode were passed via an anaesthetised nostril into the proximal stomach. The barostat bag was initially unfolded by inflation with 500 ml of air under controlled conditions (pressure <20 mm Hg). The bag was then deflated and the manometric assembly positioned so that the sleeve sensor straddled the LOS, thereby positioning the barostat bag in the proximal stomach. The pH electrode was then positioned 5 cm above the proximal margin of the LOS. All studies were performed with subjects sitting upright in an ergonomic chair designed to minimise abdominal compression. Subjects were allowed to accommodate to the manometric assembly for 20 minutes. Intravenous atropine (15 μg/kg bolus, 4 μg/kg/h infusion) or saline was then given in random order on separate days and maintained for the duration of the study. During the infusion period, pulse and blood pressure were recorded every 15 minutes.

The minimum distending pressure (MDP), defined as the first intrabag pressure at which the mean intrabag volume was greater than 30 ml and continuous respiratory fluctuations were first detected, was determined. Pressure controlled (isobaric) and volume controlled (isovolumetric) distensions were then performed in random order separated by a 10 minute rest period. During pressure controlled distensions, intrabag pressure was increased in 1 mm Hg increments in a stepwise manner every two minutes starting at a pressure 2 mm Hg below the previously determined MDP and continued until either an intrabag volume of 800 ml or a pressure of 12 mm Hg above MDP, or the threshold for subject discomfort was reached. During volume controlled distensions, intrabag volume was increased in 100 ml increments in a stepwise fashion every two minutes to either 800 ml or the threshold for discomfort. Sensations of fullness, nausea, abdominal discomfort, and hunger were recorded at the end of each distension step.

Following the distensions, subjects rested for 10 minutes. The barostat was then set to maintain a pressure of 2 mm Hg above MDP and fasting baseline intrabag volume was recorded for 20 minutes. The bag was then deflated and subjects consumed a 1700 kJ (400 kcal), 45% fat, soft mixed nutrient meal, consisting of savoury minced meat, mashed vegetables, milk, and ice cream. The bag was then reinflated to 2 mm Hg above MDP and recordings made for another two hours. Sensations were assessed before and every 15 minutes after the meal.

DATA ANALYSIS

During pressure controlled and volume controlled distensions, mean values for intrabag volume and pressure, respectively, were calculated for the second minute of each distension step, thereby allowing one minute for equilibration. Pressures were expressed as mm Hg above MDP. Measured volume was corrected for the effects of air compressibility using an experimentally derived constant which also included a component related to the internal compliance of the barostat unit. Gastric compliance was calculated as the ratio of change of pressure to change of volume in the barostat bag (dV/dP) for each individual subject during pressure controlled distension.

Fasting tone was defined as the mean intrabag volume over the last 15 minutes of the 20 minute preprandial observation period. Postprandial changes in gastric tone were determined as change in intrabag volume from mean fasting volume. Maximal post- prandial relaxation was defined as the maximal and uniform (variations less than 30 ml) increase in intrabag volume observed after the meal.17

For assessing reflux episodes, acid reflux was defined as a decrease in oesophageal pH to less than 4 for at least four seconds or, if basal oesophageal pH was already below 4, a further decrease in pH of at least 1 pH unit. When determining the occurrence of reflux during transient LOS relaxation, however, a decrease in pH of 1 pH unit was used.18 ,19 For each reflux episode, the mechanism of reflux was determined from the pattern of LOS pressure and oesophageal body activity, the relationship of these variables to swallowing, and the occurrence of abdominal straining.4 ,19 Transient LOS relaxations were defined and counted separately according to criteria published previously.20 The rate of spontaneous swallowing was determined by counting the number of pharyngeal pressure waves.

STATISTICAL ANALYSIS

Data for intrabag volume and pressure, and sensation scores were analysed using repeated measures analysis of variance (SuperAnova, Abacus Concepts Inc., Berkley, California, USA) followed by paired comparisons where appropriate. Data for reflux episodes and transient LOS relaxations were analysed using the Wilcoxon signed rank test and are presented as median (interquartile range). All other data were analysed using the paired t test and are presented as mean (SEM). A p value of <0.05 was accepted as indicating statistical significance.