Ano-rectal activity in man during rectal infusion of saline: a dynamic assessment of the anal continence mechanism.

1. Anal and rectal pressures and the external anal sphincter electromyogram were recorded in eighteen normal subjects at rest and during rectal infusion of 1500 ml. saline at a rate of 60 ml./min. 2. During saline infusion, the recordings described a pattern of regular relaxations of the internal sphincter associated with contractions of the rectum and the external anal sphincter. The frequency of ano-rectal pressure fluctuations increased as the infusion progressed. 3. The peaks in rectal pressure were always lower than the lowest anal pressure in all normal subjects tested. 4. External sphincter contractions only occurred in association with 60% of internal anal sphincter relaxations and did not prevent the maximum decrease in anal pressure. These observations suggest that phasic contraction of the external anal sphincter does not play a major role in maintaining continence.