Presented by Dr Philip Woodland, Consultant Gastroenterologist, Royal London Hospital, Honorary Senior Lecturer, Queen Mary University of London.

The concept of impedance can be used to complement ambulatory reflux measurements in investigation of gastro-oesophageal reflux symptoms and belching disorders. It enables detection of directionality of material movement (swallowed versus refluxed), detection of non-acidic reflux, detection of gas movement (including distinguishing gastric from supragastric belching) and measurement of nocturnal baseline impedance.

In ambulatory reflux studies, the impedance segments (usually 6 along the length, measuring from proximal to distal oesophagus) are integrated into the catheter along with pH measurement (most commonly with the pH probe measuring in the distal oesophagus). Each impedance segment is a pair of electrodes. They are able to measure the impedance (inverse of conductance) to current flow between them. The current is unable to pass directly through the catheter so is conducted by the material directly outside the catheter. When this material is liquid it is a good conductor of current and the impedance measured at this segment is low. A reflux event will be detected as a fall in impedance that moves upwards from the distal oesophagus towards the proximal oesophagus. Where this is accompanied by a fall in pH detected by the pH probe this will represent an acid reflux event. If there is no fall in pH then a non-acid reflux event can be inferred.

A liquid swallow can be represented by a fall in impedance that progresses in the direction from proximal to distal oesophagus. In some cases, this allows us to discount the swallowing of an acidic drink (e.g. Coca Cola) from contributing to overall acid exposure scores.

Gas is a poor conductor, so a belch will be seen as a sharp increase (spike) in impedance. A gastric belch (gas appears first in the distal impedance segment and moves proximally) can be distinguished from a behavioural, supragastric belch (gas appears first in the proximal oesophagus as air is drawn in, then moves distally to the lower oesophagus before being expelled again).

In the resting state, where there is no liquid or gas movement in the oesophagus, the oesophagus is collapsed against the pH-impedance catheter. In this situation it is the oesophageal mucosa that connects the impedance electrode pairs. This ‘baseline impedance’ is most reliably seen at night-time, when swallowing and belching is at a minimum. Repeated exposure to acid reflux causes an increased permeability of the oesophageal mucosa. This increased permeability also applies to ionic movement, so a more permeable oesophageal mucosa offers lower impedance to current flow. Thus, an oesophageal mucosa that has experienced greater chronic exposure to acid is associated with a lower ‘baseline impedance’ on ambulatory studies. Whilst this phenomenon is most marked in cases of Barrett’s oesophagus (due to reduced barrier function of columnar epithelium compared to squamous) or severe reflux oesophagitis, it can also be seen in non-erosive reflux disease and can add an additional piece of evidence to the diagnostic process. This may be particularly helpful where there is a ‘grey-area’ result on the pH testing, where total acid exposure may be between 4% and 6%. The presence of a low baseline impedance in this situation may suggest that long-term acid exposure is high and may help clinch the diagnosis of pathological reflux.