The trigger for rectal filling sensation.

In order to evaluate whether rectal volume, weight or pressure is the main trigger for rectal sensation, their respective values were determined at each of the rectal filling sensation thresholds (first, constant, urge, maximum) in 12 adult control subjects. The rectal balloon was filled at 60 ml/min in sitting position using water (twice), air and mercury consecutively. Pressure values were corrected for the elastic properties of the balloon, while the volume of inflated air was recalculated taking into account the prevalent rectal pressure and temperature. The results obtained using water, air and mercury demonstrated a constant relationship between a given rectal sensation level and the pressure recorded in the distending balloon, but not its volume or weight. Pressure values recorded at each sensation level were constant during repeated determinations of rectal sensation, the volume of rectal distension increased, probably because the rectum had already been dilated by previous testing. Balloon distension using air with the patient in the lateral position were found to be most practical for routine evaluation of rectal sensation. It is therefore concluded that any disturbance of rectal sensation will be reflected by changes in pressure and not by changes in the volume needed to produce a given sensation level. The location of the receptors involved has to be elucidated, but it seems that the pelvic floor can be excluded since the weight of the rectal contents was not related to sensation.