Effect of rectal distension on rectal electromechanical activity

The rectum possesses electric activity in the form of pacesetter potentials (PPs) and action potentials (APs). The latter are associated with rectal p ressure elevation and share in the rectal motile activity. A recent study h as shown that electric waves are transmitted by the longitudinal but not th e circular rectal muscle fibers. Rectal motile activity under normal physio logic conditions was suggested to be induced by the electric waves, that ef fect longitudinal muscle contraction, as well as by circular muscle stretch resulting from rectal distension. The current study investigated the effec t of rectal overdistension on the rectal electromechanical activity aiming at assessing the effect of stool accumulation in the rectum on rectal motil e activity. Under general anesthesia, the abdomen of 16 mongrel dogs was op ened, the rectum exposed, and 3 electrodes were sutured to the rectal seros a. The rectal pressure was measured by a 10-F catheter connected to a press ure transducer. Rectal distension was achieved by a balloon inflated with c arbon dioxide (CO2). Simultaneous recording of the electric activity and re ctal pressure was performed during rectal inflation in increments of 10 mL CO2. There was significant increase of rectal pressure as well as of freque ncy, amplitude, and conduction velocity of PPs and APs on rectal distension . The more the rectal balloon was distended, the more was the increase in r ectal pressure and waves variables; the increase was maximal just before ba lloon expulsion at 40 mL distension. Upon rectal overdistension (50 and 60 mL), no PPs or APs were recorded and the rectal pressure was 0; no balloon expulsion occurred. Rectal overdistension (pathologic distension) appears t o abort the electromechanical activity of the rectum and lead to failure of the rectum to expel the balloon. This effect is suggested to be due to ove rstretch of rectal musculature with a resulting loss of the rectal electric waves and noncontraction of the muscle fibers. These findings appear to ex plain the cause of rectal atony, which occurs in rectal inertia and leads t o constipation.