Comparison of methods used for measurement of rectal compliance

Compliance is defined as the change in volume or cross-sectional area divided by the change in pressure. Pressure-volume measurement during distention with a compliant balloon is the most commonly used method for computation of rectal compliance. However, intraindividual and interindividual variations are large, restricting the usefulness of the method. Other methods such as rectal distention by a large, noncompliant bag and rectal impedance planimetry for assessment of pressure-cross-sectional-area relations have been proposed as alternatives owing to the reduction of errors from elongation of the balloon within the rectal lumen. However, in vivo reproducibility of pressure-volume measurement during distention with a compliant balloon, pressure-volume measurement during rectal distention by a large, noncompliant bag, and rectal impedance planimetry have never been compared.

The aim of this study was to compare in vivo reproducibility of the above-mentioned methods and to study their in vitro reproducibility and validity.

Ten healthy volunteers (six men) aged 21-59 years were randomized to either rectal pressure-volume measurement with a compliant balloon or rectal impedance planimetry. After a one-hour rest, the other procedure was performed. After two weeks, both procedures were again performed in the same order. During rectal impedance planimetry the volume of the bag used (maximum volume 450 ml; secured at both ends to the probe) was continuously registered, measuring pressure-volume relations during rectal distention by a large, noncompliant bag. Reproducibility was tested by comparing the difference divided by the mean for each method at eight pressure steps in the range from 5 to 40 cm H2O. Furthermore, the in vitro reproducibility and validity of the three methods were studied using polyvinyl chloride tubes with known cross-sectional areas.

In vivo reproducibility for pressure-volume measurement with a large, noncompliant bag and rectal impedance planimetry was significantly better than for pressure-volume measurement with a compliant balloon (P = 0.005 and P = 0.019, respectively). No statistically significant difference was found between pressure-volume measurement with a large, noncompliant bag and rectal impedance planimetry (P = 0.20). In vitro reproducibility of pressure-volume measurement with a large, noncompliant bag and rectal impedance planimetry was good, but some elongation occurred, reducing the validity of pressure-volume measurement with a large, noncompliant bag. Coiling and elongation of the balloon within the lumen were major sources of error for pressure-volume measurement with a compliant balloon.

In vivo and in vitro reproducibility of methods used for measurement of rectal compliance can be improved by restricting the effects of elongation within the lumen either by using a large-volume, noncompliant bag or by rectal impedance planimetry. However, pressure-volume measurement will to some degree depend on the properties of the balloons or bags.