Gastric Tone and the Pyloric Sphincteric Cylinder #

Schulze-Delrieu (l986) pointed out that there was much uncertainty about the concept "gastric tone". It was a term that had been widely used at the beginning of this century and it was thought that tone determined the mechanical responsiveness of the stomach. Radiologists elaborated on this concept by assuming that a long, angulated "fish-hook" stomach reflected a low gastric tone; such a hypotonic stomach was associated with delayed gastric emptying and gastric stasis. On the other hand a short, globular, "steerhorn" stomach indicated a high gastric tone, i.e. it was hypertonic, and was associated with rapid gastric emptying.

Determination of Tone #

In order to determine gastric tone, Stadaas and Aune (l970) recorded intragastric pressure-volume relationships in 27 patients with dyspepsia, the tension generated by the gastric walls being measured over a range of balloon volumes. A thin-walled intragastric plastic balloon was filled stepwise with known volumes of water; after each step the intragastric pressure was recorded. If balloon pressures for all volumes were low, muscular tension of the gastric wall was low, i.e. the stomach was considered to be hypotonic. If balloon pressures were high, muscular tension of the wall was high and the stomach was hypertonic.

In a study of gastric tone Schulze-Delrieu (l983) examined the changes in gastric dimensions that accompanied the accommodation of volume by the stomach. He pointed out that while distension of the stomach was seen during routine radiographic studies, it had not been determined whether all the walls distended to the same degree, whether distension occurred mostly in the transverse axis or in the longitudinal axis, and which muscular structures were primarily concerned with volume accommodation. Excised stomachs of herbivores (rabbits) and carnivores (cats) were immersed in carboxygenated physiological solution and the shape and dimensions of the stomach, the response to filling, the intragastric pressures and the effects of drugs were determined. It was found that in both herbivores and carnivores the length and angulation of the lesser curvature were little affected by filling of the stomach. During volume accommodation gastric distension occurred mostly in the proximal stomach and along the greater curvature. Filling the stomach increased the length of the gastric circumference. These features could be explained on the muscular build of the stomach as a whole as determined by Pernkopf (l929) and Torgersen (l945). Another reason for the sizeable expansion of the greater curvature was that this curvature was concave while the lesser curvature was convex with respect to the lumen.

Normally, according to Schulze-Delrieu (l983), intraluminal pressures rose sharply on filling the stomach and declined thereafter. Carbachol, a known stimulant of gastric smooth muscle, caused contraction with a shortening of the longitudinal axis, as well as tubular contraction of the "antrum" and an increase in intragastric pressure. Isoproterenol, a known inhibitor of gastric smooth muscle, caused a more elongated shape, a flaccid appearance and a decrease in intragastric pressure. The findings provided experimental proof that the degree of gastric muscle tension was reflected in the overall configuration of the stomach. The existence of correlations between the configuration and the state of activity or tone of the musculature was demonstrated, thereby confirming the assumptions of early radiologists, according to Schulze-Delrieu (l983). Consequently measurements of gastric size and configuration provide one method of assessing gastric tone.

Another method is to measure tone by its effects on gastric volume rather than on gastric pressure. This was done by Azpiroz and Malagelada (l985, G229) who had developed an electronic barostat allowing the measurement of tone by recording changes in the intragastric volume of air. The barostat consisted of an intragastric air-filled bag, the pressure in the bag being kept constant electronically. When the stomach relaxed, air was injected into the bag; when it contracted, air was withdrawn. Gastric tone could be monitored from the barostat as variations in intrabag volume. Simultaneously intraluminal pressure activity was recorded by implanted manometric catheters. Tonic changes undetected by manometry were clearly registered by the barostat. In dogs it was found that marked changes in gastric tone were induced by meals. Receptive relaxation during feeding was followed by a low-tone accommodation period; this in turn was followed by a period of sustained high tonicity.

Azpiroz and Malagelada (l985, G501) demonstrated that nutrients in the small bowel could affect gastric tone. Fat infused into the proximal small bowel induced gastric relaxation in canines, whereas protein had only a modest effect and carbohydrate had no effect. In contrast, protein and carbohydrate infused into the distal small bowel markedly reduced gastric tone, whereas fat had no effect. These authors defined gastric tone as sustained muscular contraction of the gastric wall; variations in gastric tone probably mediated important physiological functions such as accommodation to a meal and emptying of liquids.

Schulze-Delrieu (l986) stated that physiologists working with isolated strips of gastric musculature abhorred the term gastric tone, as it was imprecise and reflected a judgment on gastric muscle tension as gained from purely visual or radiographic inspection. However, more precise measurements of gastric tone had been made with the use of gastric balloons. In his view gastric tone referred to the tension generated by the gastric walls. It affected the ability of the stomach to accommodate volume and to generate the driving force for gastric emptying. Experimental studies (Schulze-Delrieu l983) provided proof that the degree of gastric muscle tension development was reflected in the overall configuration of the stomach. If it was stimulated, the stomach was short and globular; if it was inhibited, the stomach was long and flaccid. The longitudinal musculature of the proximal greater curvature had a pivotal role in the overall control of gastric tone. According to Azpiroz and Malagelada (l990) tonic muscular contraction of the stomach (i.e. gastric tone), determines the balance between gastric accommodation and emptying. Tone is finely regulated by multiple interacting mechanisms; reflexes arising from different regions of the upper gastrointestinal tract may modulate the gastric emptying process (and hence presumably gastric tone).

An unusual but well-documented case of gastric atony was described by Telander et al (l978). This concerned a 5 months old child who presented with massive gastric distension and intractable gastric stasis. No evidence of organic narrowing or occlusion was found at the pylorus during 3 separate exploratory operations. Biopsy of the gastric wall revealed no abnormality of the ganglion cells in the myenteric plexuses and the smooth muscle cells of the tunica muscularis were normal. A gastric pressure-volume response obtained through a gastrostomy revealed no concomitant increase of intragastric pressure during stepwise increments of volume, indicating a complete lack of gastric tone, i.e. gastric atony. The cause was considered to be absence of electrical potential normally coupling electrical and mechanical activity, diminished sensitivity of the gastric smooth muscle to excitatory stimuli and probably an ectopic antral pacemaker. (Chap.16).

Discussion #

Hypotonicity #

Mainly as a result of the investigations of Stadaas and Aune (l97O), Schulze-Delrieu (l983, l986) and Azpiroz and Malagelada (l985) it may now be accepted that the radiographic image of a long, angulated, "fish-hook" stomach with a sagging greater curvature is due to hypotonicity of the gastric musculature. The hypotonic stomach is associated with decreased or absent peristalsis, absent cyclical contractions of the pyloric sphincteric cylinder and delayed emptying of liquid barium in the erect position (Chap. 13, 20). Changing the position of the subject to the left anterior oblique recumbent position, causes immediate, gravitational emptying of liquid barium, showing that pylorospasm is not a factor in these cases (Chap. 20).

In most instances gastric hypotonicity is of idiopathic origin and presumably of little clinical significance. More severe degrees, sometimes progressing to acute gastric dilatation, may occur in a variety of conditions e.g. post-operatively, after severe trauma and in electrolyte disturbances.

Hypertonicity #

A short, transversely situated, "steerhorn" stomach on the other hand, is now known to be the result of gastric hypertonicity. In these cases immediate emptying of liquid barium usually commences in the erect position, before the onset of peristalsis or cyclical contractions of the pyloric sphincteric cylinder (Chap. 13).

References #

1. Azpiroz F, Malagelada JR. Physiologic variations in canine gastric tone measured by an electronic barostat. Amer J Physiol l985, 248, G229 - G237.
2. Azpiroz F, Malagelada JR. Intestinal control of gastric tone. Amer J Physiol l985, 249, G501 - G509.
3. Azpiroz F, Malagelada JR. Perception and reflex relaxation of the stomach in response to gut distension. Gastroenterology l990, 98, ll93 - ll98.
4. Pernkopf E. Beiträge zur vergleichenden Anatomie des Vertebratenmagens. Zeitschr gesamte Anat l929, 91, 329 - 362.
5. Schulze-Delrieu K. Volume accommodation by distension of gastric fundus (rabbit) and gastric corpus (cat). Dig Dis Sci l983, 28, 625 - 632.
6. Schulze-Delrieu K. Selected Summaries: Gastric Tone. Gastroenterology l986, 90, 1298 - 1299.
7. Stadaas J, Aune S. Intragastric pressure-volume relationship before and after vagotomy. Acta Chir Scand l970, 136, 611 - 615.
8. Telander RL, Morgan KG, Kreulen DL, et al. Human gastric atony with tachygastria and gastric retention. Gastroenterology l978, 75, 497 - 501.
9. Torgersen J. The muscular build and movements of the stomach and duodenal bulb. Acta Rad l945, Suppl 45, 1 - 191.