Estimating umbilical cord flow resistance from measurements of the whole cord.

Measuring umbilical blood pressure in utero is challenging and for this reason non-invasive methods are required. However, the total vessel blood pressure drop can be estimated using numerical and empirical results by studying the mechanics of fluids in coiled and straight tubes. Two key findings emerge from such an analysis. Firstly, the total pressure drop along a vessel at a given blood flow-rate depends on both the tightness of the coils and the total cord length. Relatively short and straight cords exhibit low pressure, while long, tightly coiled cords with large width exhibit high pressure. It follows that an estimate of the pressure requires three measurements: the full cord length, its average width and number of coils. Using this result we propose two prototype indices for clinical testing that estimate umbilical cord flow resistance. The umbilical pressure index (PX) and flow index (QX) quantify the deviation of a cord geometry from defined typical conditions by considering the steady pressure drop and flow-rate, respectively. These indices can be quickly calculated, and require only a single additional measurement to the conventional umbilical coiling index (UCI); namely the cord coiling width. Unlike the UCI, these indices are derived from blood-flow properties and provide a measure of the relative flow-resistance inherent to a cord geometry. Furthermore, the pressure index can be applied to irregularities, including loose true knots, which we show must be accounted for.

Pressure and flow in the umbilical cord.

A fluid dynamic study of blood flow within the umbilical vessels of the human maternal-fetal circulatory system is considered. It is found that the umbilical coiling index (UCI) is unable to distinguish between cords of significantly varying pressure and flow characteristics, which are typically determined by the vessel curvature, torsion and length. Larger scale geometric non-uniformities superposed over the inherent coiling, including cords exhibiting width and/or local UCI variations as well as loose true knots, typically produce a small effect on the total pressure drop. Crucially, this implies that a helical geometry of mean coiling may be used to determine the steady vessel pressure drop through a more complex cord. The presence of vessel constriction, however, drastically increases the steady pressure drop and alters the flow profile. For pulsatile-flow within the arteries, the steady pressure approximates the time-averaged value with high accuracy over a wide range of cords. Furthermore, the relative peak systolic pressure measured over the period is virtually constant and approximately 25% below the equivalent straight-pipe value for a large range of non-straight vessels. Interestingly, this suggests that the presence of vessel helicity dampens extreme pressures within the arterial cycle and may provide another possible evolutionary benefit to the coiled structure of the cord.