The stomach accommodates a rapidly ingested, heterogeneous meal, and delivers into the small bowel homogenised chyme at a rate adapted to the processing capability of the intestine. The stomach is emptied by a finely modulated, sustained contraction of the proximal part of the stomach, i.e. the corpus and fundus regions. This tonic contraction is known as gastric tone. Grinding of solids occurs in the distal part, i.e. the antrum. The pyloric region is able to discriminate between solids and liquids and retains in the antrum particles larger than 1 mm, which need to be ground before they can be evacuated into the duodenum.
In the fasted state the stomach is virtually empty and is contracted. During ingestion the stomach relaxes to accommodate the meal without increasing the tension of its walls. Once ingestion has finished, the stomach progressively contracts, i.e. regains tone, and this tonic contraction gently forces intragastric chyme distally, loading the antral pump and facilitating gastric emptying. Liquid chyme is squeezed through the pyloric sphincter, which determines the final gastric outflow, while solid material is retained and ground in the antrum by phasic contractions. This process is precisely regulated so that the delivery of nutrients is matched to the capacity of the intestine to process them.
Impaired gastric contraction
Muscular activity of the stomach depends on the balance of excitatory and inhibitory neural pathways. Alteration of the excitatory pathways impairs the contractile ability of the stomach—a condition known as gastroparesis—and this results in delayed gastric emptying (Fig. 2). In severe cases the stomach becomes flaccid and unable to empty its contents, leading to nausea and vomiting of retained food. This condition is particularly problematic in patients with diabetes because it interferes with the absorption of sugars and medication and therefore contributes to poor control of glycaemia .
Impaired gastric relaxation
Alteration of the inhibitory neural pathways results in impaired relaxation of the stomach in response to a meal. If this occurs, meal ingestion increases the tension of the wall of the stomach. Gastric wall tension receptors are linked to afferent nerve pathways that send the afferent signal up to the brain cortex, inducing conscious perception. Increased wall tension causes early satiation, inability to finish a meal, fullness and nausea . In contrast to gastroparesis, gastric emptying is normal.
Measurement of gastric emptying
Gastric emptying can be evaluated by different techniques.
Radionuclide gastric emptying test
Radioscintigraphy measurement of the gastric emptying rate remains the gold standard for quantitative assessment of the emptying function of the stomach. To perform this test, patients are administered a standarised meal in which one component (or several) has been labelled with a radioisotopic γ-emitting marker. An external γ-camera quantifies the radioactivity in the area of the stomach in sequential images over 4 h. The rate of disappearance of the radioisotope corresponds to the gastric emptying rate . Solids and liquids are emptied from the stomach at different rates and can be independently evaluated by labelling each component with a different radioisotope (Fig. 2).
13C breath test
Gastric emptying can also be evaluated indirectly by labelling the solid or liquid phase of a meal with a 13C-containing substrate (octanoic acid, acetic acid, glycin or spirulina). When the meal enters the intestine, the 13C-containing substrate is rapidly absorbed and metabolised in the liver to 13CO2, which is then excreted through the breath. The amount of 13CO2 in successive breath samples is directly proportional to the gastric emptying rate . This test has the advantages that the label is non-radioactive and the test is simple to perform. However, the test is not well standardised, and therefore is not considered the gold standard for the diagnosis of gastroparesis.
Gastric ultrasound can be used to quantify gastric emptying of liquids. It is performed by consecutive measurements of the gastric antral diameter after ingestion of a liquid meal . Gastric emptying is considered complete when the antral diameter returns to its basal value. Even though this tool is used for experimental purposes, it is not widely used in clinical settings because of its operator dependence.
Measurement of gastric tonic contraction
The tonic contraction of the stomach can be measured by means of a barostat, which is a computerised air pump connected via a thin tube to a bag. The bag is introduced into the stomach and the pump is programmed to maintain a constant pressure within the bag. When the stomach relaxes, the system injects air, and when the stomach contracts, air is aspirated. Hence, the volume of air within the bag reflects the degree of contraction of the stomach: at a constant pressure, a large volume reflects a gastric relaxation and a small volume represents a contraction. Patients with gastroparesis exhibit a flaccid, weak stomach with a large volume of air in the intragastric bag at relatively low intragastric pressures . Gastric relaxation after a meal is normally detected as a volume increment at constant pressure, and this response is blunted in patients with impaired gastric accommodation .