Multichannel intraluminal impedance (MII) is a new technique designed to detect intraluminal bolus movement without the use of radiation. It is generally performed in combination with manometry or pH testing. When combined with manometry, it provides information on the functional (ie, bolus transit) component of manometrically detected contractions. When combined with pH testing, it allows for detection of gastroesophageal reflux independent of pH (ie, both acid and non-acid reflux).
PRINCIPLES OF MII
Impedance testing depends upon measurement of changes in resistance (in Ohms) to alternating electrical current when a bolus passes by a pair of metallic rings mounted on a catheter. In an empty tubular organ (ie, esophagus or small intestine) the electrical current between the two rings is conducted by the few ions present in and on the esophageal mucosa. Liquid containing boluses with an increased number of ions have a higher conductivity and when entering the impedance measuring segment will lower the impedance to a nadir value.
The impedance stays at its nadir as long as the bolus is present in the segment, returning to baseline once the bolus is cleared by a contraction. The contraction produces a slight increase in impedance above the baseline due to a decrease in luminal cross-section. Gas passing transiently by the impedance measuring segments will produce a rapid rise in the impedance since it has poor electrical conductance.
Measuring impedance at multiple sites (multichannel) allows for determination of direction of bolus movement based upon temporal differences in bolus entry and exit (ie, bolus entries progressing from proximal to distal indicate antegrade bolus movement while bolus entries progressing from distal to proximal indicate retrograde bolus movement) (waveform 1). Combined with manometry (multichannel intraluminal impedance and manometry; MII-EM) it provides information about both pressures and bolus transit within the esophagus. Combined with pH (multichannel intraluminal impedance and pH; MII-pH) it permits detection of both acid and non-acid gastroesophageal reflux.
The United States Food and Drug Administration approved MII to detect intraluminal bolus movement in July 2002. Combined MII-EM is approved for esophageal function testing and combined MII-pH for gastroesophageal reflux monitoring (Sandhill Scientific Inc, Highlands Ranch, CO, USA).