The composition of anal basal pressure. An in vivo and in vitro study in man.

The maximal anal basal pressure (MABP) was measured with probes of 0.3, 1, 2 and 3 cm diameter in 21 subjects, 60 years old, without anal pathology. The components of MABP were analyzed by inducing a maximal internal sphincter (IS) relaxation, taking pressure measurements in the conscious state and during narcosis with curarization. In seven cases pressure measurements were done on isolated anorectum after abdominoperineal rectum amputation. MABP increases with probe diameter before as well as during anaesthesia with curarization. The contribution of the striated sphincter tonic activity is constant within the range of probe diameters used. At rest, i.e. when the 0.3 cm diameter pressure recording probe is used, 30% of MABP is made up by striated sphincter tonic activity, 45% of it is due to nerve induced IS activity, 10% to purely myogenic IS activity and 15% can be attributed to the expansion of the haemorrhoidal plexuses. Although MABP is mainly based on active forces generated by the smooth and striated sphincter apparatus, the presence of the anal cushions is essential for perfect anal continence, as they have to fill the gap within the IS ring to hermetically close the anal canal. The global IS activity, contributing 50-60% of MABP at rest, can completely be inhibited by a maximal rectoanal inhibitory reflex. Stretching of passive elements starts at 1 cm anal distension, but steeply increases thereafter, accounting for 65% of the MABP at 3 cm anal distension. It is deduced that optimal stool diameter is about 2 cm.