Impedance Planimetry: Clinical Impedance Planimetry

Abstract

Rectal dynamics traditionally have been assessed using the construct of “rectal compliance” measurement, assessing the change in volume with changes in rectal pressure. Various studies have reported “normal control” results over a wide range of measurements (1–7), reflecting the relative lack of reproducibility of the technique where there are differing amounts and speeds of rectal distension and variable biomechanical rectal properties (Figure 2.5(ii).1).This reported range has effectively limited the clinical and research usefulness of the technique. Impedance planimetry [determination of cross-sectional area (CSA) of a hollow viscus] in relation to pressure variation with distension offers a more reliable investigation of rectal dynamics, and although its measurement would mirror that of standard rectal compliance assessment, in theory its recording would have several clinical advantages. Impedance planimetry (IP) is a technique first described for determining CSA measurement, contraction velocity, and bolus shape (8). It is based on the field gradient principle and is performed using a thin probe with four electrodes attached. The technique was developed further in 1976, being performed using a probe with two electrodes attached for the investigation of ureteric peristalsis and urine flow in pigs (9,10). Further developments allowed the addition of manometry recording devices, and in the 1990s, IP was used in the human esophagus for bolus velocity and clearance assessments employing eight and twenty electrode systems, respectively (11). In 1983, Colstrup et al. developed the technique for the measurement of active forces by the addition of an expanding, non-conducting latex balloon to the probe (12). This enabled the measurement of passive biomechanical wall properties because the current that facilitated the recording was restricted to the balloon. This method has since been applied in esophageal and duodenal (13) studies and in limited anorectal work (14). The system originally was designed for use in small diameter lumens, so it was necessary to develop a system capable of recording larger diameters