The fasting patient’s fullest cooperation should be obtained. If the patient is taking any drugs which might influence the test to be undertaken these should be discontinued.

Before the tube is introduced it is advisable to check that all connections are correct, that the syringe fits snugly on the end of the tube and whether an adaptor is required. The tube is moistened by soaking in water, or lubricated. Many patients cannot, or will not, tolerate the sensation of a tube in the pharyngeal region and a small quantity of local anaesthetic such as 2% lignocaine hydrochloride sprayed on the fauces will prevent a great deal of distress and discomfort. The hazards are minimal and there is no evidence that the agent has any influence on the results of the investigation if it is used sparingly. On the other hand some gastrointestinal functions may be profoundly influenced when the introduction of the tube is accompanied by much hawking, heaving and emotional distress. Acid secretion may be inhibited by a technically difficult intubation.

Although most of the tubes are traditionally introduced via the nose, many patients find it more comfortable to swallow by mouth. Sipping water and sucking ice have also been recommended but this may not always be advisable as the aspirate will be diluted and contaminated, and water may be inhaled if the fauces have been anaesthetized. Most patients find it easier to swallow the tube sitting up: leaning slightly forward assists passage. It is impossible to swallow when the neck is extended, and flexing the neck guides the tube into the oesophagus.

The passage of tubes through the nose may be difficult and unpleasant for the patient, and it is generally better to introduce the tube via the mouth. The nasal passages may be deviated or narrowed and many tubes have firm metal ends. Making nasal introduction awkward, painful and traumatic at times. The firm indications for introducing a diagnostic tube via the nose include the unconscious patient; the patient who cannot voluntarily coordinate swallowing, where sipping water encourages a nasal tube to enter the oesophagus; the patient who refuses to open his mouth; and the patient who persists in biting the tube. The use of the term ‘nasogastric’ is retained even though the tubes are introduced via the mouth.

If a nasogastric tube enters a bronchus the patient usually coughs, wheezes, or becomes cyanosed. Methods to check the position of the tube include holding the end of the tube against the cheek to feel if air is being exhaled, injecting air via the tube and auscultating over the stomach for a bubbling sound, and testing the aspirated material for acidity. The problem of the position of a tube need never arise when it is passed for diagnostic purposes since all small intestinal tubes should be positioned under radiological control. The correct positioning of a tube is usually essential for the accuracy of a diagnostic test: failure to do this generally invalidates the result.

Ideally the position of the tube should be checked radiologically during and at the end of a procedure. The best arrangement is for the procedure room to be equipped with an X-ray image intensifier so that the patient can remain there for the duration of the test. Premenopausal women should ideally be examined within 10 days of the onset of menstruation because the radiation dose administered is considerable.

For gastric secretory studies the tube may be positioned by passing a length of 50–60 cm and then giving the patient 20 ml water to drink. This is aspirated, the tube withdrawn by 2.5 cm, and the procedure repeated. This is continued until the highest level at which water can be aspirated is found, and tube taped in position.

This method avoids the need for radiological monitoring of tube position.

The patient can be made more comfortable if the tube is taped to the side of the cheek, avoiding the hair, eyebrows and the nose. It also adds to the comfort if the external connection is pinned to the garment or pillows so that the tube does not pull on the tape attached to the skin.