Spontaneous anorectal pressure activity. Evidence of internal anal sphincter contractions in response to rectal pressure waves.

To characterize spontaneous anorectal pressure activity and a possible relation between the activity in the rectum and the anal canal, 11 healthy female volunteers were investigated. Resting activities were obtained during 1-h recordings with a multi-channel perfused catheter measuring the pressure 1, 2, 3, 7, and 8 cm from the anal verge. In five subjects sequences of rhythmic rectal pressure waves with amplitudes exceeding the maximal anal resting pressure coincided with a similar internal anal sphincter activity, preventing rectal pressure from exceeding the anal pressure at any point. The mean frequency was 5 x min-1 (range, 3-6 x min-1). This may well be a reflex mechanism by which the internal anal sphincter prevents incontinence in the resting state. Low-frequency pressure waves, not previously described, were detected in four women. These pressure waves were attributed to the internal anal sphincter and were named ultra-slow waves type II. The mean frequency and amplitude were 0.16 x min-1 (range, 0.15-0.17 x min-1) and 24 cm H2O (range, 11-41 cm H2O), respectively. The function of these pressure waves is unknown.