Incremental elastic modulus--a challenge to compliance.

Rectal compliance has theoretical and practical drawbacks resulting in wide variation in the reported normal values. Slope of stress-strain relation, a measure of rectal wall stiffness (Incremental elastic modulus; IEM) may be an effective alternative. The aim of this study was to compare IEM with rectal compliance during ramp inflation. In a group of 15 normal adults [Median age 51.5 years (range 31 to 74); 11 female and 4 male], these parameters were assessed at three rates of inflation, using a proctometrogram-catheter-balloon (PCB) complex and endorectal ultrasound scanner (7 Mhz; B & K) to measure intrarectal pressure and radius respectively. IEM had a linear relation with strain (Slope 33.55; R2 = 0.9815) in contrast to compliance (R2 = 0.0088). Series elastic component (SEC), a measure of passive viscoelasticity, was rate independent elasticity [Slope 1.02 (0.98 to 1.15); P = ns] and a rate dependent viscosity (P = 0.004; One way ANOVA). Both IEM and SEC were reproducible (R = 0.985; P < 0.01). This study emphasises the importance of rectal wall viscoelastic evaluation based on stress and strain rather than compliance.