Pelvic bioelectrical impedance measurements to detect rectal filling

Anorectal sensory deficits are an important cause of defecatory disorders a nd are also a reason for evacuatory difficulties in patients undergoing tot al anorectal reconstruction. A method to detect rectal tilling would be ben eficial in such patients. We have investigated the feasibility of detecting rectal filling in vitro and in vivo by measuring changes in pelvic impedan ce. In vitro, a model of the pelvis was constructed using a cylindrical plastic tank filled with an electrolyte solution (conductivity 3 mS cm(-1)). Condu ctive Visking tubing representing the rectum was suspended in the tank and incrementally filled with artificial faeces. An impedance meter detected ch anges in voltage on rectal tilling when an alternating current of 2 mA was passed at eight frequencies (4.8 to 612 kHz). In vivo, changes in pelvic bi oelectrical impedance upon retrograde and antegrade rectal filling with art ificial faeces were evaluated in three pigs, Sour electrodes being implante d in the pelvis. Impedance measurements accurately detected 'rectal' volume changes in vitro (n = 10, p < 0.0001; Kruskal Wallis), but not in vivo (n = 68, p = 0.48: K ruskal Wallis). This was probably due to extreme sensitivity of the detecti ng device to movement, a problem that needs to be resolved before this tech nique could be used in man.