image image image

Gastric Acid Secretion

INDICATION

Used in the diagnosis of Zollinger-Ellison syndrome.
Consider syndrome if:

  1. Raised gastrin (>40 pmol/l) in the absence of other causes (e.g. H2 antagonists, PPIs, pernicious anaemia, other causes of achlorhydria, renal failure).
  2. Associated upper gastrointestinal disease, i.e. peptic ulcer disease with poor response to treatment; multiple duodenal or jejunal ulcers; peptic ulcer disease with unexplained diarrhoea; fulminant peptic ulcer disease (perforation, haemorrhage, oesophagitis and stricture); ulcer in upper part of ligament of Treitz.
    Measuring gastric output distinguishes secondary hyper-gastrinaemia (due to achlorhydria) from primary hyper-gastrinaemia. Administration of pentagastrin i.v. does not improve the diagnostic accuracy.

PREPARATION

Book with endoscopy suite since the end couch next to the apparatus will be required.
Liase with Gastro research fellow to ensure that equipment is not being used.
Stop H2 antagonists for 72 hrs and stop PPIs for 2 weeks.
Stop antacids 24 hours before blood sample.
Patient should be fasting.
Check that the autotitrator is available, otherwise you will need to obtain a burette, conical flask, pH meter and 0.1M NaOH.

METHOD

  1. Pass the special double lumen naso-gastric tube (obtained from sister in endoscopy) with plenty of Xylocaine spray to the nose and throat, and lignocaine jelly to the nose. Pass the NG tube as far as the 50cm mark at the nostril.
  2. Ask the patient to drink 50mls of water and then aspirate this via the NG tube to check that it is in the most dependent part of the stomach.
  3. Connect NG tube to the pump and collect four samples of gastric juice, each over 15 min into polystyrene cups. Alternatively, aspirate regularly and periodically with a 50ml syringe to collect gastric juice over each 15 min period.
  4. Measure total volume of each sample. Decant 10mls of each into a fresh polystyrene cup and titrate against 0.1M NaOH with the automated titrating equipment, or carry out a standard neutralisation titration manually.
  5. Calculate the acid production of each 15 min collection:
    A = (N/100) ยด V
    A = mmol of acid production
    N = volume (ml) of 0.1M NaOH solution needed to neutralise 10mls of gastric juice
    V = volume (ml) of gastric juice in 15 min collection
    A sum of the acid production for each 15 min will give the total hourly production.

INTERPRETATION

Spontaneous basal acid outputs of 20 - 25 mmol/hr are almost diagnostic, >10 mmol/hr is suspicious. Post ulcer surgery >5 mmol/hr is indicative.

SENSITIVITY AND SPECIFICITY

Hypergastrinaemia and raised gastric acid are also found with:

  • gastric outlet obstruction: resolves with NG decompression
  • massive small bowel resection: resolves a few months post op
  • antral G cell hyperplasia: excess cells on histochemistry

REFERENCE

Deveney et al., Surg. Clin. N. Amer. 67(2) 411 (1987).