Barostat measurement of rectal compliance and capacity

Fecal continence requires relaxation of the rectal wall and a reservoir of adequate capacity. Rectal compliance provides an assessment of rectal wall stiffness; however, compliance is also affected by rectal capacity. We developed and validated a barostat measurement of rectal capacity. By accounting for variation in rectal capacity, we aimed to improve the inconsistent relationship between rectal compliance, sensation, and continence reported in the literature.

Barostat measurements of rectal compliance and capacity were validated in 41 healthy, continent subjects. Slow staircase (0-40 mmHg) and rapid phasic (12-40 mmHg) distentions were performed on two separate days, filling sensations were assessed by visual analog score. A stool substitute retention test of rectal filling sensation and continence was performed.

Variance of volume measurements decreased with pressure comparing conditioning vs. index distentions, staircase vs. phasic distentions, and measurements on different days (all P < 0.001). Correction for rectal capacity measured at 40 mmHg reduced the "normal range" of compliance measurements (P < 0.01) but not vice versa. Compared with unadjusted volume measurements, normalized rectal volume (percentage filling relative to rectal capacity) improved the description of rectal sensation visual analog score (P < 0.01). Rectal capacity correlated with filling sensations and the volume retained on retention testing (P < 0.01).

Barostat measurements of rectal capacity at 40 mmHg are highly reproducible and not affected by distention protocol. The assessment of rectal capacity complements that of rectal compliance. Correction for rectal capacity provides an assessment of rectal wall stiffness independent of rectal geometry and improves the association of barostat volume measurements with rectal sensitivity and continence.