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MII–pH monitoring can be performed with several different catheters (usually no more than 2 mm in diameter) that incorporate a varying number of impedance and pH electrodes in different configurations. The catheter is placed transnasally and is connected to a data logger that the patient carries for 24 h. Ordinarily, the catheter has a pH electrode for placement 5 cm above the LES (similar to conventional pH testing), with the possibility of additional pH sensors in the stomach or proximal esophagus. Catheters may incorporate six or more impedance measuring segments (each composed of two metal electrodes usually spaced 2 cm apart) to detect impedance changes along variable lengths of the esophagus. While there are no published studies comparing the accuracy of different catheter configurations, a catheter with six impedance measuring segments and one pH electrode (Figure 9.10) enables accurate detection of reflux episodes with an assessment of proximal extent of reflux, and it permits distinction between swallowing and reflux episodes; a smaller number of impedance measuring segments may compromise accuracy [29].

Figure 9.10 Schematic representation of the 2 mm diameter multichannel intraluminal impedance (MII)–pH catheter with impedance electrodes (4 mm in length) set in pairs at 2 cm intervals, allowing for six impedance measuring segments, as well as one pH electrode. Once properly positioned, this catheter allows recording of pH at 5 cm above the lower esophageal sphincter (LES) and impedance in six measuring segments, with their centers 3, 5, 7, 9, 15, and 17 cm above the LES.

Source: Vela [9] with permissions of Wolters Kluwer.