Chapter 18 (page 85)

The Pyloric Sphincteric Cylinder in Health and Disease

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Chapter 18 (page 85)

Read et al. (l986) determined gastric emptying time by labelling a solid test meal with ^99mTechnetium sulphur colloid; the meal included labelled mashed potato mixed in water but was of homogeneous consistency. In normal subjects the stomach emptied exponentially with a half time of 1.2 ± 0.3 hours. A residue of up to 12 percent of the food (mean 6 percent) was present in the stomach 4 hours ater eating the meal. Although the radionuclide was mixed with liquid which reconstituted the mashed potato, they found no difference in the rate of emptying of the meal whether the marker was in the water used for reconstitution or whether it was incorporated in chicken liver. This supported the results of Hinder and Kelley (l977), who had shown that the solid and liquid components emptied at the same rate from meals of homogeneous consistency, but at different rates from meals composed of well separated solid and liquid components (such as the steak and water meal used by Meyer et al. in l976).

Camilleri et al. (l986) studied "antral" motility and gastric emptying in patients with clinically suspected upper gastrointestinal dysmotility. For the motility studies manometry was performed with a multiple-lumen perfusion assembly, and for gastric emptying a mixed solid and liquid meal was employed, ¹³¹I-Fibre being incorporated into the meal as a solid, nondigestible marker, and ^99mTc-DTPA as the aqueous marker. Solid emptying was found to be significantly prolonged in patients with "antral" hypomotility (but not in those with intestinal dysmotility). The prolongation was characterised both by an increase in the duration of the lag phase and a slower emptying rate in the postlag emptying phase (the lag phase being defined as the interval between the ingestion of the meal and the first appearance of ¹³¹I in the proximal small intestine). The half-times for liquid emptying were significantly prolonged in antral hypomotility as well as in intestinal dysmotility, but it was more marked in the former. These authors came to the conclusion that radionuclide gastric emptying and manometric studies provided closely interrelated physiologic information and could be regarded as complementary diagnostic tools.

Velchik et al. (l989) pointed out that while gastric empyting was influenced by meal weight and composition (the relative percentages of carbohydrate, protein and fat), the effect of meal energy content had not been thoroughly investigated. By means of radioinuclide techniques these authors showed that gastric emptying was progressively delayed by increasing the energy or caloric content of a standard meal.

Discussion

The rate and pattern of gastric emptying is influenced by multiple factors, e.g. the composition, volume, osmolarity, pH and fluidity of a meal, the specific gravity, viscosity, digestibility and size of the more solid components, the posture of the subject (Heading et al. l974; Sheiner l975), and emotional states (Velchik et al. l989).

The advantages of radionuclide studies of gastric emptying are that they are simple, noninvasive, do not interfere with normal physiological processes and allow simultaneous labelling of solid and liquid components of a meal (Horowitz et al. l982; Velchik et al. l989).

The observations of Heading et al. (l974), Meyer et al. (l976) and Horowitz et al (l982) indicate that the stomach handles liquids and solids by different processes. The liquid component of a meal leaves the stomach more rapidly than solids, and is emptied in an exponential or first order pattern (Heading et al. l974; Meyer et al. l976). The pressure gradient across the gastro-duodenal junction is a major factor influencing emptying of liquids, and is largely controlled by the tone of the fundus (Horowitz et al. l982). During early emptying of liquids (occurring in the lag phase for solids), there is no relationship between "distal antral" motility and liquid emptying, as shown by simultaneous manometric and radioscintigraphic studies (the "distal antrum" was defined as the region 1.0 cm proximal to the gastro-duodenal junction) (Camilleri et al. l985).

Although it is true that upper gastrointestinal radiography is not quantitative and not suitable for analyzing the complex process of gastric emptying in its entirety, one agrees with Velchik et al. (l989) that it exquisitely displays anatomic detail. As contractions of the walls are clearly visible (Chap. 13), much information can be gained about the early stages of emptying of a liquid barium suspension (Chap 13). The influence of gastric tone (Chap. 19) and the posture of the subject (Chap. 20) is of major importance in the emptying of liquid barium. During the early stages emptying of the suspension usually occurs in the absence of both gastric peristaltic waves and contractions of the pyloric sphincteric cylinder, although shallow ripples of contraction of the gastric walls may be evident (Chap. 13). During later stages regular, cyclical contractions of the sphincteric cylinder occur at a rate of 3 per minute (Chaps. 13, 15). In the barium studies it is not possible to determine unequivocally whether individual contractions of the cylinder are associated with propulsion of liquid contents into the duodenum, or with retropulsion into the stomach, or with both (Chap. 13). However, progressive filling of the duodenum occurs in association with regular contractions of the cylinder, which suggests that one of the functions is propulsion.

Solids are emptied at a slower rate than liquids and in a linear or zero-order pattern (Heading et al l974; Meyer et al. l976). Hinder and Kelly (l977) showed that a distinction had to be made between the emptying of digestible and indigestible solids, and that larger solid particles were retained for reduction to a smaller size before emptying. The volume of solids may be decreased by solubilization and partial digestion in the stomach (Malagelada l977). Solids only empty after an initial delay or lag phase (Horowitz et al. l982; Collins et al. l983).

The "antropyloric muscle" is of major importance in controlling emptying of solid food (Horowitz et al. l982); a positive correlation exists between "antral" motility and emptying of solids (Camilleri et al. l986). This was confirmed by Velchik et al. (l989), who stated that the emptying of solids was largely determined by the "antrum and duodenopyloric mechanism".