Pressure reflectometry: in vitro recordings with a new technique for simultaneous measurement of cross-sectional area and pressure in a collapsible tube

Abstract

Pressure reflectometry is a new and easily applicable technique for simultaneous measurement of cross-sectional area and pressure in a collapsible biological tube. Related values of pressure and cross-sectional area are relevant for assessment of the mechanical properties of a biological tube. A very thin and highly flexible 6 cm long polyurethane bag with a diameter of 5 mm when expanded was introduced into test models. The cross-sectional area of the first 6 cm of the cavities up to 16 mm2 (depending of the size of the bag) can be measured at the same time. The cross-sectional area was measured by acoustic reflectometry while pressures of 10–200 cmH2O were applied by a pump. The echoes from the cavity were analyzed and displayed by a computer technique. The aim of the study was to evaluate the technical capabilities and limitations of this new method. The accuracy and reproducibility were examined in eight different models with known cross-sectional areas in the interval of 4–16 mm2. Two of the models had two successive constrictions reducing the cavity to 50% and 25% respectively. Measurements were reliable from about 1 to 5 cm within the cavity and at pressures from 10 to 200 cmH2O. The absolute error in models without constrictions did not exceed 0.9 mm2 and at the first constriction the error did not exceed 1.2 mm2. In cavities without constrictions, the coefficients of variation (CVs) were 0.2–11%. In models with constrictions the CVs were 6–25% at the first constriction when different pressures were applied to the plastic bag. The reproducibility was not influenced significantly by change in background noise, temperature, catheters, geometries of the cavities or new calibration. In conclusion, the accuracy of this new technique depends on the size of the measured cavity, on the pressure in the models and whether the model has constrictions or not. The measurements seem reliable for clinical use in the range of 4–16 mm2 and at pressures from 10 to 200 cmH2O.