Stomach

and Ian A. D. Bouchier²

(1)

Bishop Auckland, UK

(2)

Edinburgh, Midlothian, UK

5.1 Gastric Emptying

5.1.1 Isotope Tests

A standardised protocol based on experience with 123 normal controls is recommended.

After overnight fasting (clear fluids only) and nil by mouth for 2 h the patient is fed a test meal. This is 120 g egg-white omelette labelled with 12 MBq any ^99mtechnetium non –absorbable compound (half-life 6 h), a sandwich of 2 white slices and 30 g jam, and 120 ml water.

Four images of 3 min, anterior and posterior, are made immediately after the meal, and at 30, 60, 120 and 240 min. Emptying is exponential, so half-life is a useful measure.

Normal is 60–90 min. Where liquid emptying is measured at the same time, ¹¹¹indium-labelled DTPA is used (half-life 2.8 days).

5.1.2 Radiology

The time required for a barium meal to clear the stomach is normally 2–3 h. Prolongation of this time may indicate disease, but is an unphysiological test. The presence of gastric barium will complicate any early surgery for obstruction. Solid radio-opaque markers can be used, e.g. 10 mm lengths of 16F nasogastric tube. Most of these should clear the stomach in 4 h and almost all will have left in 6 h.

5.1.3 Ultrasonography

Serial real-time scans parallel to the long axis of the stomach or of the antrum can be used to calculate gastric emptying of physiological meals. This is a useful and completely non-invasive test, though probably unnecessary in children with pyloric stenosis, in whom physical examination and observation confirm the diagnosis. The presence of gastric air often prevents accurate assessment.

5.1.4 Dye Dilution

This requires nasogastric intubation. A test meal of 750 ml of water is drunk, and either before or after ingestion, phenol red 30 ppm is added. After thorough mixing a 7–8 ml aliquot is withdrawn and 20 ml phenol red 500 ppm is added. After further thorough mixing a second aliquot is withdrawn, and from the concentrations gastric volume can be calculated. The procedure is repeated at intervals to determine the rate of reduction of gastric volume. In normal subjects the half-life of gastric volume is 11 min and the emptying time is 22 min.

5.2 Gastric Acid Secretion (Peak Acid Output, PAO)

The estimated output of acids has been a widely used test of gastric function. Various stimulants have been used in the past, but they have been largely superseded by the introduction of pentagastrin. This is a pentapeptide containing the key C-terminal sequence of gastrin, tryptophane-methionine-asparagine-phenylalanine-NH_2. Patients should not be receiving anti-acid medication!

5.2.1 Interpretation

There is a great deal of individual variation in tests and a large overlap between groups with different conditions. Results depend on patients’ build, and height and lean body mass are both important. For adults these are usually neglected, but in children results should be expressed as mmol/kg/hour.

Men secrete more gastric acid than women, and secretion falls off with advancing age. Race is also a factor, but the data are conflicting. To assist understanding, Table 5.1 shows the results of the test outlined above during I year in patients who also underwent a full upper digestive endoscopy within 6 weeks.

Table 5.1

Short pentagastrin test: PAO (mmol/hour)

	n	Men mean	Range	n	Women mean	Range
Normal	41	30	0.6–56.6	26	20	0–45.6
Duodenal ulcer (no duodenitis)	61	42**	21.7–72.9	38	35***	6.6–52.2
Duodenitis (no duodenal ulcer)	63	36*	0–84.8	18	28**	12.5–51.1
Oesophagitis	77	34*	1.6–84.8	50	28**	1–52.2
Gastritis and gastric erosions	63	33	2.4–84.8	36	28**	1–51.1
Pyloric and prepyloric ulcer	23	33	9.9–60.2	15	26	0–39.7
Benign gastric ulcer	10	29	1.6–41.9	12	19	10–35.7
Vagotomy and/or drainage (no gastrectomy)	47	26	0.1–47.7	22	19	0–35.7

Differences calculated versus normal endoscopy

*p < 0.05; **p < 0.01; ***p < 0.001

The distribution of values is not parametric. However, for an individual the results are highly reproducible, with a coefficient of variation of 4.6%.

Normal acid secretion is usually taken to be 10–30 mmol/h for women and 15–40 mmol/h for men. Studies of endoscopy-normal dyspeptic patients and apparently healthy volunteers show that values often fall outside these ranges.

Younger subjects have rather higher values, and over the age of 50 years the difference between the sexes becomes less marked. Any disease apart from pernicious anaemia may be found in the presence of normal acid secretion.

Benign gastric ulcer is in general associated with normal acid secretion. However, the more proximal the ulcer the lower the acid output; conversely, patients with pyloric and pre-pyloric ulcers tend to be hypersecretors.

Achlorhydria

The absence of any titratable acid in a stomach whose contents have a pH of 6 or more after an adequate pentagastrin test, in a patient who has not undergone gastric resection or cholecystectomy is undoubtedly achlorhydria. If these conditions are not met the definition becomes arbitrary and the expression should not be used.

Achlorhydria may occur in apparently normal individuals and becomes more common with ageing. It is also found in the autoimmune gastritis of pernicious anaemia, in iron deficiency, in atrophic gastritis and in 18% of patients with gastric cancer. Benign peptic ulcer very rarely occurs in achlorhydria.

Reduced acid secretion

A PAO of less than 10 mmol/h in women and less than 15 mmol/h in men virtually excludes active spontaneous duodenal ulcer. This may be important where radiology shows only deformity of the duodenal cap and endoscopy does not identify an active ulcer in a patient with dyspepsia. A low acid output is characteristic of gastric cancer, but is certainly not pathognomonic and is not always associated with the condition.

Increased acid secretion

This is characteristically found in duodenal ulcer, though half of these patients have a normal acid output. It is occasionally caused by gastrinoma or hypercalcaemia. The hypersecretion tends to be more marked in patients with duodenal ulcer complications.

5.3 Basal Acid Output

This yields variable results and does not add to the diagnostic usefulness of the PAO in duodenal ulcer. The test should only be performed alone in special circumstances, e.g. when gastrinoma is suspected.

5.3.1 Method

The patient is intubated after an overnight fast. The overnight juice is aspirated and its volume, pH and titratable acidity are measured. The stomach is then aspirated for 1 h without any stimulation and the volume, pH and titratable acidity are measured. To ensure the most reliable results the collection should be fractionated into 4 × 15 min periods and the results of the analyses summated. The coefficient of variation between fractions is about 50%, but at least it provides some indication that the test is adequately performed. The basal acid output (BAO) is expressed in mmol/hour. Pentagastrin-stimulated PAO should then be measured to obtain the maximum useful information.

5.4 Indications for Gastric Acid Studies

1.

Diagnosis of pernicious anaemia. Achlorhydria is obligatory, and PAO is nil. The test is not often used now because there are other direct means of diagnosing pernicious anaemia.
2.

Diagnosis of hypersecretion secondary to hypergastrinaemia as in gastrinoma (Zollinger-Ellison syndrome), G-cell hyperplasia, retained antrum after gastric surgery, hyercalcaemia and short bowel syndrome. The basal hour volume is usually >200 ml, BAO >15 mmol/h, BAO/PAO >60% and PAO usually >50 mmol/h.
3.

Pre- and post-operatively in peptic ulcer. Little elective surgery is now performed in the age of powerful anti-acid drugs and H.pylori eradication. A record of the change in acid output with surgery is a useful measure of the completeness of any vagotomy and risk of recurrence.
4.

Assessment of response to anti-acid treatment. The dose of drugs in an unresponsive duodenal ulcer and in hypergastrinaemia-induced hyper-secretion can be titrated to reduce PAO to subnormal levels.
5.

Selection of operation for oesophageal reflux. It has been claimed that hyper-secretors with oesophagitis do well after vagotomy, without the need for major plastic procedures to the cardia.
6.

Occasionally in the differentiation of benign from malignant gastric ulcer. If a gastric ulcer exists in the presence of achlorhydria it should be regarded as malignant. In practice the PAO is of little help in management because all gastric ulcers should have a biopsy taken at the time of gastroscopy.

5.5 Alternative Techniques

Acid secretion may be stimulated by the technique of sham-feeding, where food is chewed and then spat out without swallowing. Though attractively simple and harmless, the procedure is unaesthetic and has not gained wide popularity. The concept of testing gastric pH intra-operatively during pentagastrin infusion to ensure completeness of vagotomy has appeal, but it prolongs time in theatre and increases morbidity.

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