Effect of Rectal Distension on Rectal Electromechanical Activity

Abstract

The rectum possesses electric activity in the form of pacesetter potentials (PPs) and action potentials (APs). The latter are associated with rectal pressure elevation and share in the rectal motile activity. A recent study has shown that electric waves are transmitted by the longitudinal but not the circular rectal muscle fibers. Rectal motile activity under normal physiologic conditions was suggested to be induced by the electric waves, that effect longitudinal muscle contraction, as well as by circular muscle stretch resulting from rectal distension. The current study investigated the effect of rectal overdistension on the rectal electromechanical activity aiming at assessing the effect of stool accumulation in the rectum on rectal motile activity. Under general anesthesia, the abdomen of 16 mongrel dogs was opened, the rectum exposed, and 3 electrodes were sutured to the rectal serosa. The rectal pressure was measured by a 10-F catheter connected to a pressure transducer. Rectal distension was achieved by a balloon inflated with carbon dioxide (CO 2 ). Simultaneous recording of the electric activity and rectal pressure was performed during rectal inflation in increments of 10 mL CO 2 . There was significant increase of rectal pressure as well as of frequency, amplitude, and conduction velocity of PPs and APs on rectal distension. The more the rectal balloon was distended, the more was the increase in rectal pressure and waves variables; the increase was maximal just before balloon 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 to 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 overstretch of rectal musculature with a resulting loss of the rectal electric waves and noncontraction of the muscle fibers. These findings appear to explain the cause of rectal atony, which occurs in rectal inertia and leads to constipation.

• Action Potentials Motility Pacesetter Potentials Rectal Pressure Slow Waves